Weed Management in Direct-Seeded Rice

Abstract

Rice (Oryza sativa L.) is a principal source of food for more than half of the world population, especially in South and Southeast Asia and Latin America. Elsewhere, it represents a high‐value commodity crop. Change in the method of crop establishment from traditional manual transplanting of seedlings to direct‐seeding has occurred in many Asian countries in the last two decades in response to rising production costs, especially for labor and water. Direct‐seeding of rice (DSR) may involve sowing pregerminated seed onto a puddled soil surface (wet‐seeding) or into shallow standing water (waterseeding), or dry seed into a prepared seedbed (dry‐seeding). In Europe, Australia, and the United States, direct‐seeding is highly mechanized. The risk of crop yield loss due to competition from weeds by all seeding methods is higher than for transplanted rice because of the absence of the size differential between the crop and weeds and the suppressive eVect of standing water on
weed growth at crop establishment.
Of 1800 species reported as weeds of rice, those of the Cyperaceae and
Poaceae are predominant. The adoption of direct‐seeding has resulted in a
change in the relative abundance of weed species in rice crops. In particular,
Echinochloa spp., Ischaemum rugosum, Cyperus diVormis, and Fimbristylis
miliacea are widely adapted to conditions of DSR. Species exhibit variability
in germination and establishment response to the water regime postsowing,
which is a major factor in interspecifically selecting constituents of the weed
flora. The relatively rapid emergence of ‘‘weedy’’ (red) rice, rice phenotypically
similar to cultivars but exhibiting undesirable agronomic traits, has
been observed in several Asian countries practicing DSR, and this poses a
severe threat to the sustainability of the production system.
Stale seedbeds, tillage practices for land leveling, choice of competitive rice
cultivars, mechanical weeders, herbicides, and associated water management
are component technologies essential to the control of weeds in DSR.
Herbicides in particular are an important tool of weed management, but
hand weeding is either partially or extensively practiced in countries of Asia,
Africa, and Latin America. Though yet to be globally commercialized,
transgenic rice varieties engineered for herbicide resistance are a potential
means of weed control. The release of herbicide‐resistant rice for red rice
control in the United States has indicated the need to critically examine
mitigation methods for the control of gene flow. Integrating preventive and
interventional methods of weed control remains essential in managing weed
communities in DSR, both to prohibit the evolution of herbicide resistance
and to maximize the relative contributions of individual components where
herbicides are not widely used. There remains a need to further develop
understanding of the mechanisms and dynamics of rice weed competition
and of the community dynamics of weed populations in DSR to underpin
sustainable weed management practices.

Full text

WEED MANAGEMENT IN
DIRECT‐SEEDED RICE
A. N. Rao,
1
D. E. Johnson,
2
B. Sivaprasad,
1
J. K. Ladha
1
and A. M. Mortimer
3
1
International Rice Research Institute (IRRI), IRRI‐India Office,
National Agriculture Science Center (NASC) Complex, New Delhi 110012, India
2
International Rice Research Institute (IRRI), Crop, Soil, and
Water Sciences Division, Metro Manila, Philippines
3
Integrative Biology Research Division, School of Biological Sciences,
The University of Liverpool, Liverpool L69 3BX, United Kingdom
I. Introduction
A. Direct‐Seeding of Rice
B. Yield Loss Due to Weeds in Direct‐Seeded Rice
II. Weeds, Weed Competition, and Ecology in Direct‐Seeded Rice
A. Occurrence of Major Weeds in DiVerent Methods of Direct‐Seeding
Across the World
B. Crop–Weed Competition in Direct‐Seeded Rice
C. Weed Species Shifts and Weed Population Dynamics Due to
Changes in the Methods of Rice Establishment
III. Integrating Weed Management Practices in Direct‐Seeded Rice
A. Preventive Methods of Weed Control
B. Intervention Methods of Weed Control
C. Developing Weed Management for Direct‐Seeded Rice
IV. Future Research Needs
Acknowledgments
References
Rice (Oryza sativa L.) is a principal source of food for more than half of the
world population, especially in South and Southeast Asia and Latin America.
Elsewhere, it represents a high‐value commodity crop. Change in the method
of crop establishment from traditional manual transplanting of seedlings
to direct‐seeding has occurred in many Asian countries in the last two decades
in response to rising production costs, especially for labor and water.
Direct‐seeding of rice (DSR) may involve sowing pregerminated seed onto
a puddled soil surface (wet‐seeding) or into shallow standing water (water‐
seeding), or dry seed into a prepared seedbed (dry‐seeding). In Europe,
Australia, and the United States, direct‐seeding is highly mechanized. The
risk of crop yield loss due to competition from weeds by all seeding methods is
higher than for transplanted rice because of the absence of the size diVerential
153
Advances in Agronomy, Volume 93
Copyright 2007, Elsevier Inc. All rights reserved.
0065-2113/07 $35.00
DOI: 10.1016/S0065-2113(06)93004-1

between the crop and weeds and the suppressive eVect of standing water on
weed growth at crop establishment.
Of 1800 species reported as weeds of rice, those of the Cyperaceae and
Poaceae are predominant. The adoption of direct‐seeding has resulted in a
change in the relative abundance of weed species in rice crops. In particular,
Echinochloa spp., Ischaemum rugosum, Cyperus diVormis, and Fimbristylis
miliacea are widely adapted to conditions of DSR. Species exhibit variability
in germination and establishment response to the water regime postsowing,
which is a major factor in interspecifically selecting constituents of the weed
flora. The relatively rapid emergence of ‘‘weedy’’ (red) rice, rice phenotypi-
cally similar to cultivars but exhibiting undesirable agronomic traits, has
been observed in several Asian countries practicing DSR, and this poses a
severe threat to the sustainability of the production system.
Stale seedbeds, tillage practices for land leveling, choice of competitive rice
cultivars, mechanical weeders, herbicides, and associated water management
are component technologies essential to the control of weeds in DSR.
Herbicides in particular are an important tool of weed management, but
hand weeding is either partially or extensively practiced in countries of Asia,
Africa, and Latin America. Though yet to be globally commercialized,
transgenic rice varieties engineered for herbicide resistance are a potential
means of weed control. The release of herbicide‐resistant rice for red rice
control in the United States has indicated the need to critically examine
mitigation methods for the control of gene flow. Integrating preventive and
interventional methods of weed control remains essential in managing weed
communities in DSR, both to prohibit the evolution of herbicide resistance
and to maximize the relative contributions of individual components where
herbicides are not widely used. There remains a need to further develop
understanding of the mechanisms and dynamics of rice weed competition
and of the community dynamics of weed populations in DSR to underpin
sustainable weed management practices. #2007, Elsevier Inc.
I. INTRODUCTION
Over 1800 plant species have been reported as weeds of rice in South and
Southeast Asia (Moody, 1989), and there is an enormous diversity of taxa
considered to be weeds of rice (Soerjani et al., 1987). There are two major
reasons for this. The first is that rice is grown over a range of agroecosystems,
characterized by the presence or absence of water (from dry land to fully
flooded land) for all or parts of its growing season, which generates highly
diverse weed floras. The second is that, in many developing countries, rice
farming relies on manual labor, and removal of weeds is ineYcient, leading to
their persistence. Reviews of rice yield loss due to the presence of weeds
equally indicate considerable variability and, in part for the same underlying
154 A. N. RAO ET AL.

reasons, a diversity of weed species across diVerent ecosystems (Karim et al.,
2004; Oerke and Dehne, 2004; Oerke et al., 1994; Sanint et al., 1998;
Widawsky and O’Toole, 1996; Yaduraju and Mishra, 2004). It is widely
accepted that, in the absence of chemical weed control, rice that is trans-
planted into standing water that is subsequently maintained for much of the
growing season will suVer less competition from weeds and consequentially
have less yield loss than rice established by other ways. The underlying
ecological reason for this is the inherent size and growth advantage of rice,
which derives from transplanting seedlings into standing water under which
weed species must initially germinate, establish, and subsequently compete
for limiting resources (principally light) during growth in a dense crop
monoculture. Where geographically large flood plains exist (e.g., the Mekong
Delta), transplanting rice has been time honored, and labor for crop estab-
lishment and subsequent manual weeding has been one mainstay of the
sustainability of the system. Contrastingly, before the advent of the Green
Revolution and adoption of irrigation, rainfed rice was often broadcast into
moist soil (Pandey and Velasco, 2002, 2005) and yields were low, variable,
and highly prone to weed competition, as is still experienced today, particu-
larly in upland rice (Roder et al., 2001). There is now evidence that water
scarcity prevails in rice‐growing areas (Tuong et al., 2005), and societal
demands for water from the urban and commercial sectors will continue to
increase. Direct‐seeding of rice, in place of transplanting, provides oppor-
tunities for water savings but at the expense of the absence of the suppres-
sive eVects of standing water on weed growth. Hence, the direct‐seeding
of rice (DSR) crop faces severe challenges from weeds, and eVective
weed management is essential for cropping of DSR.
Weed management in DSR grown in both tropical and temperate regions
has been assessed previously by many authors (De Datta, 1986; De Datta
et al., 1989; Hill et al., 1994; Ho, 1996; Moody, 1981, 1983, 1993, 1995; Moody
and Cordova, 1985; Sankaran and De Datta, 1985). However, given the
anticipated shortages of labor and water, there will likely be a continuing
shift from transplanting to direct‐seeding and greater reliance will be placed
on direct‐seeded systems for food security. This chapter reviews the man-
agement of weeds in DSR from the background of three perspectives. First,
rice is a principal source of food for more than half of the world’s popula-
tion, and rice cultivation underpins the livelihoods of hundreds of millions
of households around the globe. Moreover, several countries of Asia and
Africa are highly dependent on rice as a source of foreign exchange and govern-
ment revenue (www.fao.org/rice2004/en/world.htm). Rice is planted on
153 million hectares annually, of which 134 million hectares are planted in
Asia. Whereas rice production increased at 3.0% and 2.5% per annum during
the 1970s and 1980s, respectively, to meet demographic demand, the increase
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 155

was only 1.5% during the 1990s (Dawe and Dobermann, 1999). Globally,
rice production must increase by 36% by 2025 to feed an estimated 4 billion
rice consumers (Pinstrup‐Anderson et al., 1997). Reducing yield losses due
to weed competition will contribute to that increase. Second, the global area
planted to rice is declining because of pressure of urbanization and industri-
alization through land loss, and water and labor shortages (Nelson‐Smith,
1995). As Khush (2005) and others have argued, new technologies to increase
rice productivity will be needed to meet the challenge of enhancing rice
production under such constraints. Improved weed management, particularly
in DSR, will contribute in this respect. The final perspective is one of sustain-
ability. Historically, in weed management, overreliance on a single manage-
ment technology has resulted in both ecological and evolutionary responses in
the target weed flora. Successful weed management is concerned with mini-
mizing the impacts of weeds in the short term and simultaneously ensuring
that yield losses will not increase in the long term as a result of practices that
are implemented.
In this chapter, we (1) assess the current status of direct‐seeded rice, includ-
ing the extent of yield losses due to weeds, (2) examine changes in weed species
composition, (3) document and critically evaluate the available weed manage-
ment options, and (4) discuss future research needs and strategies to continue
to manage weeds eVectively and economically, in a sustainable manner.
A. DIRECT‐SEEDING OF RICE
1. Methods of Direct‐Seeding
Direct‐seeding refers to the process of establishing a rice crop from seeds
sown in the field rather than by transplanting. Once germination and seed-
ling establishment are complete, the crop can then be sequentially flooded
and water regimes maintained as for transplanted rice. Alternatively, the
crop can remain rainfed, the upper surface soil layers fluctuating from
aerobic to nonaerobic conditions. Direct‐seeding is the oldest method of rice
establishment and, prior to the late 1950s, direct‐seeding was the major method
used in developing countries (Grigg, 1974; Pandey and Velasco, 2005).
DSR systems are classified into (1) dry‐seeded rice, (2) wet‐seeded rice, and
(3) water‐seeded rice based on the physical condition of the seedbed and the
seed/seedling environment at germination and establishment (Table I).
Dry‐seeded rice is a traditional practice developed by farmers to suit the
agroecological conditions in systems ranging from shifting cultivation in
the humid forest zones to intensive cultivation in the rainfed lowlands
(Fujisaka et al., 1993; Johnson et al., 1991; My et al., 1995; Roder, 2001;
156 A. N. RAO ET AL.

Table I
Direct‐Seeded Rice Systems of the World
Direct‐seeding
method
Seedbed
condition
Land preparation
procedure
Soil microtopography
in relation to land
preparation and
leveling method,
data are standard
deviations (cm)
a
Methods and
pattern of seeding
Typical seed
germination
environment
Typical water
regimes
0–14 days
after sowing
Agricultural system/
region where method
is practiced
Dry‐seeded
rice
Dry (un‐saturated)/
moist soil
Plowing and
harrowing
once suYcient
soil moisture
is present
Lithao (manual
or animal
drawn): 15 cm
Random: hand
broadcasting
Dry seed
buried (2–5 cm)
in aerobic soil
No standing water
until rice has
reached about
the 3‐leaf stage
(i) Rainfed upland or dry
land, (ii) some areas
of rainfed lowland and
deep‐water agricultural
systems of Asia, Africa,
Latin America, and the
Caribbean
Mechanized seed
drill: 8–10 cm
Rows: drilled
or manually
sown
Agricultural systems with
controlled irrigation
systems in America,
Europe, and Australia
Harrowed seed
bed: 5 –11 cm
Wet‐seeded
rice
Puddled soil Plowed, flooded,
puddled
and leveled
Planking by power
tiller: 4–12 cm
1. Broadcast
by hand
or motorized
blower onto
a puddled soil
surface; after
drainage
Aerobic wet
seeding:
pregerminated
seed onto a
saturated mostly
aerobic soil surface
Initially a saturated
soil with 0–0.3 cm
surface water
Irrigated and favorable
rainfed agricultural
systems in Asia, Africa,
Latin America, and the
Caribbean
(continued)
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 157

Leveling may be soil
movement by
tractor‐mounted
bucket and scraper,
by ‘‘planking’’:
drawing a flat
plank behind a
tractor or power
tiller or by laser
leveling with
precision‐grading
mechanical
operations
Mechanized
bucket and
scraper: 4–7 cm
Laser leveled
(Asia): 3–5 cm
2. Row seeded by a
drum‐seeder
either hand
drawn or
attached to
power tiller
Anaerobic wet
seeding:
pregerminated
seed sown into
the surface
layers (5–10 cm
of a saturated
soil). Seed may
be coated with
oxygenating
agents
(e.g., calcium
peroxide).
Irrigation applied
to ensure soil
surface does not
crack
Drainage as needed
to prohibit
premature
flooding
Sequential irrigation
to a depth of
3–5 cm depending
on rate of rice
shoot elongation
Water‐seeded
rice
On puddled or
unpuddled soil
in standing
water to a
depth of
3–5 cm
As for wet‐seeded
rice
Soil ridged before
flooding to
prevent
aggregation
of settled seed;
laser‐leveled
(USA): 1–2 cm
Broadcast into
standing
water manually,
by motorized
blower or
from the air
Anaerobic Standing water
of 5–10 cm
Irrigated
agricultural
systems of
Malaysia,
America, Asia,
Australia,
and Europe
a
Data from IRRI (unpublished); Fujii and Cho (1996).
Developed from Balasubramanian and Hill (2002).
Table I (continued )
Direct‐seeding
method
Seedbed
condition
Land preparation
procedure
Soil microtopography
in relation to land
preparation and
leveling method,
data are standard
deviations (cm)
a
Methods and
pattern of seeding
Typical seed
germination
environment
Typical water
regimes
0–14 days
after sowing
Agricultural system/
region where method
is practiced
158 A. N. RAO ET AL.

Watters, 1971) in Asia, Africa, and Central and South America. Dry seed is
sown at the beginning of the rainy season after either minimum or zero
tillage in the shifting cultivation systems or into prepared seedbeds in more
intensive systems. Hand broadcasting or dibbling seeds into furrows or
drill‐seeding in rows by machine is used for seeding at shallow depths into
moist, aerobic soil (Hill et al., 1991). Subsequently, rice is raised as a dryland
crop or the field is kept flooded during much of the season depending on the
soil and climatic conditions. In the United States, Europe, and Australia, to
some extent rice is drilled in rows and then irrigated.
In contrast, wet‐seeding involves sowing pregerminated seed, with a radicle
varying in size from 1 to 3 cm, on or into puddled soil. Where motorized
broadcasting is used, the pregermination period is shortened to ensure short
radicles for ease of handling and to minimize damage, as is the case when
drum‐seeders for row‐seeding are employed (Balasubramanian and Hill,
2002). Land preparation is critical to successful crop establishment under
wet‐seeding since seedbeds need to be leveled to prohibit premature seedling
death by flooding and to ensure that pregerminated seeds remain at or near
the surface of the soil. Where seeds sink into the upper layers of the soil
surface, anaerobic conditions prevail during seedling establishment and seed
coatings to improve oxygenation in the immediate vicinity of the developing
seedling may be employed. Achieving precision leveling requires considerable
land preparation and maintenance, and variability is always likely to occur
not least because of machinery passage on small fields but also due to biotic
factors, including wading birds. Kawasaki (1989) argued that a standard
deviation of 2 cm in soil surface height across a field was acceptable as
this rarely led to large gaps in DSR and modern laser‐leveling techniques
can achieve greater precision. Simple leveling of a field by drawing a plank
behind a power tiller or tractor in contrast imparts much greater variability
(Table I). The significance of this variability is that it generates spatial varia-
tion in water regimes in the field that in turn influences weed seed germination
and recruitment into the weed flora.
Water‐seeded rice can be established by traditional means where preger-
minated seeds are sown into standing water that may recede with time.
Seeds must be heavy enough to sink below standing water to enable anchor-
age at the soil surface. Traditionally in Asia, water‐seeding is resorted to
only when early flooding occurs, and water cannot be drained from the field.
Only a few traditional rice varieties are used for this method of water‐seeding in
Thailand, Indonesia, and Vietnam. Modern techniques of water‐seeding
include aerial sowing using aircraft in the United States and Australia,
seed broadcasting using tractor‐mounted seeders in Italy, or broadcasting
using motorized blowers in Malaysia. Aerial water‐seeding is the most
common seeding method used in temperate rice zones (Hill et al., 1991).
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 159

2. Global Distribution of Direct‐Seeded Rice
Rice cropping varies widely across countries and regions. Table II
summarizes the extent of global direct‐seeding of rice. Currently, 23% of
the rice is direct‐seeded in the world. Rice is planted either in a dry‐seeded or
water‐seeded system in the United States (Gianessi et al., 2002), Australia
(Pratley et al., 2004), and Europe (Ferrero and Nguyen, 2004; Gianessi et al.,
2003; Ntanos, 2001). The vast majority of Australian rice (more than 90%) is
aerially sown into water (Pratley et al., 2004). Pandey and Velasco (2002)
noted that direct‐seeding was increasing rapidly in Asia, with 21–22% of the
total rice area being dry‐or wet‐seeded. Broadcasting and dibbling are
common seeding practices for upland rice in Africa (Ampong‐Nyarko,
1996) and, on dry soils in Latin American countries, drill‐seeding is used.
However, direct‐seeding on saturated soil has been widely adopted in south-
ern Brazil, Chile, Venezuela, Cuba, some Caribbean countries, and in certain
areas of Colombia (Fischer and Antigua, 1996).
In India, dry‐seeding is extensively practiced in rainfed lowlands, uplands,
and flood‐prone areas, while wet‐seeding remains a common practice in
irrigated areas (Misra et al., 2005). In southern Vietnam, especially in the
Mekong Delta, rice is wet‐seeded by broadcasting (Luat, 2000). Kim et al.
(2001) reported that 11% of the total rice area in Korea was under direct‐
seeding, dry‐seeding being 6.4% and wet‐seeding 4.7%. In the northern
provinces of Cambodia, in a typical wet season, 80–90% of rice fields are
dry‐seeded, whereas, in the southern provinces, both wet‐and water‐seeding
occur (CIAP, 1998). In the Philippines, both dry‐and wet‐seeding are used,
with wet‐seeding being the most common practice and dry‐seeding again
occurring in areas with higher elevation and slope, non‐irrigated flat lands
and irrigated lowlands with medium‐to light‐textured soils (de Dios et al.,
2005). In Thailand, the majority of rainfed rice area is dry‐seeded, whereas,
under irrigation in the Central Plain, rice is predominantly wet‐seeded (Azmi
et al., 2005). In Malaysia, wet‐seeding is favored but is encouraged only where
eVective water management is possible (Azmi et al., 2005). Rice, seeded
directly into aerobic nonpuddled and nonflooded soils (‘‘aerobic rice’’) in
China, is mainly distributed in the northern and northeastern regions, with
80,000 and 60,000 ha, respectively (Xie et al., 2005).
B. YIELD LOSS DUE TO WEEDS IN DIRECT‐SEEDED RICE
Weeds are a major yield‐limiting factor in rice production (Bastiaans
et al., 1997), and the literature reporting yield losses is numerous. Globally,
actual rice yield losses due to pests have been estimated at 40%, of which
weeds have the highest loss potential (32%). The worldwide estimated loss in
160 A. N. RAO ET AL.

Table II
Estimated Direct‐Seeded Rice (DSR) Area in DiVerent Rice‐Growing Countries
Continent/country
Total rice
area
a
(000 ha)
Estimated DSR
area (000 ha)
DSR area
(% of total area)
Asia 134,544 27,186 20.2
Bangladesh 11,000 2090 19
b
Cambodia 2300 230 10
b
China 29,420 1471–2648 5–9
b
India 42,500 11,900 28
b
Indonesia 11,753 2116 18
b
Iran 570 28 5
c
Japan 1650 0 0
b
Korea 990 89 9
b
Laos 820 271 33
b
Malaysia 670 476 71
b
Myanmar 6000 540 9
b
Nepal 1550 0 0
b
Pakistan 2210 0 0
b
Philippines 4000 1680 42
b
Sri Lanka 756 582 77
b
Thailand 9800 3332 34
b
Turkey 80 72 90–100
c
Vietnam 7400 2309–3478 39–47
b
South America 5799 3176 55
Argentina 172 155 90–100
c
Bolivia 142 128 90–100
c
Brazil 3732 1866 50
c
Colombia 517 465 90–100
c
Ecuador 350 70 20
c
Guyana 130 117 90–100
c
Paraguay 28 25 90–100
c
Peru 318 57 18
c
Uruguay 190 171 90–100
c
Venezuela 135 122 90–100
c
North and Central America 2026 1628 80
Cuba 205 144 70
c
Dominican Republic 107 43 40
c
Mexico 51 36 70
c
Nicaragua 94 56 60
c
United States 1349 1349 100
c
Africa 10,220 2226 22
Burkina Faso 51 1 2
c
Chad 80 4 5
c
Congo Democratic Republic 415 332 80
c
Co
ˆte d’Ivoire 510 357 70
c
Egypt 630 126–158 20–25
d
Ghana 125 6 5
c
Guinea 525 52 10
c
(continued)
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 161

rice yield from weeds is around 10% of the total production (Oerke and
Dehne, 2004). However, accurate estimates of yield loss are limited, and
much of the information on yield loss is often derived from basic trials
assessing pesticide eYcacy, which tend to overestimate yield loss and do
not take into account the natural variation and extremes that occur in
farmers’ fields. A superficial extension of this observation is that variability
in rice yield losses due to weeds might be greater in rainfed environments
than in irrigated environments because of the lack of a managed water
supply that suppresses weeds. In a detailed analysis of the injury profiles
(due to pathogens, insects, and weeds) of lowland rice in farmers’ fields,
Savary et al. (2000a,b) argued that specific injury profiles could be associated
with particular cropping patterns, distinguishing those of intensive trans-
planted rice from direct‐seeded rice–rice rotations. In both, however, weed
infestations were a common constraint and, whether weed species at maturi-
ty grew above or below average rice canopy height, yield losses due solely to
weeds were about 20% of attainable yield.
Table II (continued)
Continent/country
Total rice
area
a
(000 ha)
Estimated DSR
area (000 ha)
DSR area
(% of total area)
Guinea Bissau 65 6 10
c
Liberia 120 12 10
c
Madagascar 1219 244 20
c
Mauritania 17 9 50
c
Mozambique 179 13 7
c
Nigeria 4900 980 20
c
Senegal 88 26 30
c
Sierra Leone 200 20 10
c
Tanzania 330 33 10
c
Europe 594 534 90
c
France 21 19 90–100
c
Greece 22 20 90–100
c
Italy 215 194 90–100
c
Portugal 26 23 90–100
c
Russian Federation 143 109 90–100
c
Spain 121 109 90–100
c
Oceania 73 73 100
c
Australia 65 65 100
c
World 153,257 34,823 22.7
a
Source: FAO (2005).
b
Pandey and Velasco (2002).
c
Based on FAO (2002), Ampong‐Nyarko (1996), and other papers quoted in this chapter.
d
Hassan and Rao (1996).
Note: When a range was given in the source, minimum percent area was taken for the continent/
country estimate.
162 A. N. RAO ET AL.

Figures 1–3 illustrate the scales of yield loss reported for DSR in the last
decade. Ranges for yield loss by country (Fig. 1) indicate considerable
variability and are a reflection of both site‐specific conditions and the meth-
odologies used in estimation. However, they do serve to reinforce the severity
of losses that may occur due to a variety of potential, and often additive,
consequences of weed infestations. These include (1) the diversity of the
weed flora as a consequence of crop establishment practices, (2) water man-
agement during the life of the crop, (3) associated pests and diseases whose
abundance may be influenced by companion weeds of rice, and (4) the impact
on harvest and postharvest operations, including grain cleaning and drying
(Hill et al., 1994; Smith and Hill, 1990). Widawsky and O’Toole (1996)
estimated that weeds in eastern India were the second largest damaging
constraints in both rainfed lowlands and uplands, although the magnitude
of yield losses in upland rice was greater than in lowland environments
(Fig. 2). Weeding was considered as inadequate in both environments for a
number of socioeconomic and technical reasons.
Of greater importance is a measure and understanding of the scale of yield
loss that is experienced at the farm level. Figure 3 illustrates the variability
of loss in transplanted rice and DSR. Where rainfed DSR is grown on
terraced land, the weed flora may diVer substantially because of the position
on the toposequence (Pane et al., 2005). Nevertheless, the range of yield
losses due to weeds may be similar (Fig. 3) and diVer little from that
0 102030405060708090100
Range of yield loss (%)
Australia
Egypt
Europe
India
Korea
Malaysia
Sri Lanka
Thailand
USA
Vietnam
West Africa
Country
Figure 1 Reported range of yield loss in direct‐seeded rice due to weed competition in the
absence of any control measures [Sources: Australia, Pratley et al. (2004); Egypt, Hassan and
Rao (1993, 1996); Europe, Oerke et al. (1994); Gianessi et al. (2003); India, Yaduraju and Mishra
(2004); Korea, Kim and Ha (2005); Malaysia, Karim et al. (2004); Sri Lanka, Herath Banda et al.
(1998); Abeysekera (2001); Thailand, Meenakanit and Vongsaroj (1997); Tomita et al. (2003a);
USA, Hill et al. (1994); Vietnam, Chin et al. (2000a); West Africa, Akobundu and Fagade (1978);
Ampong‐Nyarko (1996); Fofana and Rauber (2000)].
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 163

experienced by farmers under transplanted conditions. In all instances,
distributions of loss tend to be skewed, with the highest variability being
evident in drill‐seeded rice. Understanding the causes of this variation and
determining points and means of interventions in the context of whole‐farm
economics and farmer livelihoods are discussed in Section II.
II. WEEDS, WEED COMPETITION, AND ECOLOGY IN
DIRECT‐SEEDED RICE
A. OCCURRENCE OF MAJOR WEEDS IN DIFFERENT METHODS OF
DIRECT‐SEEDING ACROSS THE WORLD
Globally, weed communities of DSR are floristically diverse because they
(1) span temperate and tropical regions, (2) reflect diVerent agroecosystems
in a region, (3) may vary in relation to seasonal crop management patterns at
the farm level, and (4) may diVer because of spatial heterogeneity that is
20
40
60
80
100
120
140
160
180
0
200
RLR
(DRSR)
RL (TPR) IWS (TPR) IDS (TPR)RUR
(DRSR)
Eastern
India
Agricultural system
Yield loss due to weeds (kg ha−1)
Figure 2 Estimated yield losses due to weeds in diVerent rice‐growing environments of
eastern India. (RUR, rainfed upland ecosystem; RLR, rainfed lowland ecosystem; RL, rainfed
deepwater ecosystem; IWS, irrigated ecosystem, wet season; IDS, irrigated ecosystem, dry
season; DRSR, dry‐seeded rice; TPR, transplanted rice) [Source: Widawsky and O’Toole
(1996)].
164 A. N. RAO ET AL.

often linked to patterns of flooding and drainage and soil nutrition at the
field level (Moody, 1995; Mortimer and Johnson, 2005). They can vary
further in relation to the eYcacy of weed management practices. Some
weed species, however, show a ubiquity of distribution that elevates them
to the status of ‘‘the world’s worst weeds’’ (Holm et al., 1977) and particular
sedge (Terry, 2001) and grass (Mortimer, 2001) species feature prominently
in DSR, including Cyperus rotundus, Echinochloa crus‐galli, Echinochloa
colona,andSorghum halepense.
Table III summarizes the weed species that have been reported as the most
common in DSR and that are considered to be of economic importance—
Africa (Ampong‐Nyarko, 1996; Johnson, 1997), Asia (Caton et al., 2004;
Galinato et al., 1999), Bangladesh (Sarkar et al., 2002), Bhutan (Parker,
1992), Brazil (Anon, 1997), Cambodia (John et al., 1996), China (Wang,
1990), Egypt (Hassan and Rao, 1993, 1996), Greece (Ntanos, 2001), India
(Singh, 2005; Tadulingam et al., 1955), Indonesia (Pane et al., 2000), Italy
(Bocchi et al., 2005), Japan (Morita, 1997), Korea (Kim and Ha, 2005), Latin
America and the Caribbean (Fischer and Antigua, 1996), Malaysia (Ho,
1996; Itoh, 1991), Myanmar (Morris and Waterhouse, 2001), Pakistan
0.0
0.2
0.4
0.8
0.0
02 6 24462
46
0.2
0.4
0.6
0.8
ABC
DE
Proportion
0.6
F
Yield loss (ton ha−1)
00
Figure 3 Distribution of yield loss due to weeds under farm management practices, data being
derived by paired plot comparisons in on‐farm and researcher‐managed trials of the impact
of additional weeding on farmers’ current weeding practices. (A) Bangladesh—dry‐seeded, upper
toposequence position; (B) Bangladesh—dry‐seeded, mid toposequence position; (C) Bangladesh—
dry‐seeded, low toposequence position; (D) Bangladesh—transplanted irrigated rice; (E) India—
wet‐seeded rice; (F) India—dry‐seeded rice. For all distributions, n>50, data being collected post
2000 [Sources: Mazid et al. (2001); Singh et al. (2001)].
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 165

Table III
Weed Species Reported to be Most Commonly Associated with Direct‐Seeded
Rice in DiVerent Countries
a
Weed species Family
Type of direct‐seeding
b
DRSR WTSR WRSR
Aeschynomene aspera L. Fabaceae
c
IND, THA, IDO, NEP, PHI, SRI, VIE MAL
Aeschynomene indica L. Fabaceae BAN, BRA, CAM, CHI, IND, JPN, KOR,
LAO, LAC, MAL, MYA, NEP, PAK, PHI,
SRI, THA, USA (ARK), VIE
MAL, PHI, SRI, THA USA (ARK, LOU, MIS)
Aeschynomene rudis Benth. Fabaceae BRA, USA (ARK)
Aeschynomene
virginica (L.)
Britton, Sterns
& Poggenby
Fabaceae LAC, USA (MSO, ARK), NEP LAC, IND USA (ARK, LOU, MIS, MSO)
Aeschynomene
denticulata Rudd
Fabaceae BRA
Ageratum conyzoides L. Asteraceae AFR, BAN, BHU, CHI, COT, IND, IDO, JPN,
PHI, LAO, MAL, MYA, NEP, NGR, SRI,
THA, VIE, CHI, LAC, MAL
SRI
Alisma
lanceolatum
Withering
Alismataceae ITL, AUS
Alisma plantago‐
aquatica L.
Alismataceae IND JPN ITL, EUR, TUR, AUS
Alternanthera
philoxeroides
(Mart.) Griseb.
Amaranthaceae CHI, USA (TX, FLO), IND, IDO, JPN, THA,
BAN, LAO, MYA
THA USA (ARK, LOU, MIS, TX)
Alternanthera
sessilis (L.)
R. Br. ex DC.
Amaranthaceae AFR, BAN, BHU, CAM, CHI, IND, IDO,
LAO, MAL, MYA, NEP, PHI, SRI, THA,
VIE
BAN, IND, PHI, THA, VIE
Amaranthus
retroflexus L.
Amaranthaceae USA (FLO), CHI, MYA
Amaranthus
spinosus L.
Amaranthaceae AFR, BAN, BHU, CHI, IND, IDO, LAO,
MAL, MYA, NEP, PAK, PHI, SRI, THA,
VIE
166 A. N. RAO ET AL.

Amaranthus viridis L. Amaranthaceae COT, IND, IDO, JPN, PHI, SRI, THA, VIE,
AFR, BHU, MAL, MYA, PAK
IND
Ammannia
baccifera L.
Lythraceae AFR, IDO, IND, SRI, THA, VIE, BAN, LAO
CAM, MYA, NEP, PAK
IND, BAN, NEP, SRI
Ammannia
coccinea Rottb.
Lythraceae USA JPN, PHI USA
Bacopa rotundifolia
(Michx.) Wettst.
Scrophulariaceae USA (ARK), IND, JPN, SRI, IDO JPN, MAL USA (ARK, CAL, LOU, MIS)
Bolboschoenus
maritimus (L.)
Palla ¼Scirpus
maritimus L.
Cyperaceae ITL AFR ITL, EUR
Brachiaria
plantaginea (Link)
Poaceae BRA, LAC
Brachiaria platyphylla
(Munro ex C. Wright)
Nash
Poaceae BRA, JPN, USA IND
Butomus umbellatus L. Butomaceae IND JPN TUR, ITL
Caesulia axillaris Roxb. Asteraceae IND, BAN, SRI, NEP IND, PAK
Caperonia palustris (L.)
St. Hil.
Euphorbiaceae USA (FLO, MSO, TX) USA (MSO, TX, ARK, LOU,
MIS)
Celosia
argentea L.
Amaranthaceae BAN, IND, IDO, LAO, MYA, PHI, SRI, THA,
VIE, NEP
Chara sp. (algae) Characeae SRI USA (CAL)
Chloris
pilosa Schumach.
Poaceae COT, AFR
Commelina
benghalensis L.
Commelinaceae AFR, BAN, BHU, IND, IDO, KOR, LAO,
MYA, NEP, NGR, PAK, PHI, SRI, THA,
VIE
IND, SRI
Commelina
communis L.
Commelinaceae IND, JPN, USA (ARK) USA (ARK, LOU)
Commelina
diVusa Burm. f.
Commelinaceae AFR, BAN, BHU, BRA, IND, IDO, JPN,
KOR, LAC, LAO, MAL, MYA, NEP, PHI,
SRI, THA, USA, VIE
AFR, IND, PHI, SRI
Corchorus
aestuans L.
Tiliaceae BAN, IND, PHI, SRI, THA IND
(continued)
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 167

Cynodon
dactylon
(L.) Pers.
Poaceae AFR, BAN, BHU, CAM, CHI, CHL, IND,
IDO, JPN, LAO, MYA, NEP, PAK, PHI,
SRI, THA, VIE
IND, PHI, SRI
Cyperus
brevifolius
(Rottb.)
Endl. ex Hassk.
Cyperaceae CAM, IND, IDO, LAO, MYA, NEP, PHI,
SRI, THA, VIE,
SRI, VIE
Cyperus
diVormis L.
Cyperaceae AFR, AUS, BAN, BRA, CAM, IND, IDO,
KOR, LAC, LAO, MAL, NGR, PAK, PHI,
SRI, THA, VIE, NEP
AFR, BAN, BHU, CHI, EGY,
IND, JPN, NGR, POR,
LAC, MAL, MYA, NEP,
PHI, SRI, THA, VIE
AUS, ITL, TUR, USA (CAL)
Cyperus
esculentus L.
Cyperaceae AFR, BRA, IND, IDO, JPN, LAC, MAL,
NEP, NGR, SRI, THA, USA (ARK, FLO,
MIS)
IND, LAC, SRI USA (ARK)
Cyperus
ferax Rich. ¼Diclidium
ferax (Rich.) Schrad. ex
Nees
Cyperaceae BRA, LAC
Cyperus haspan L. Cyperaceae AFR, BAN, CHL, IDO, IND, PHI, SRI, VIE,
CAM, LAO, MYA, NEP, PAK, THA
CHI, IDO, IND, MAL, SRI,
VIE
Cyperus iria L. Cyperaceae AFR, BAN, BHU, BRA, CAM, CHI, IND,
IDO, KOR, LAO, LAC, MAL, MXC, MYA,
NEP, PAK, PHI, SRI, THA, USA, VIE
CHI, EGY, IDO, IND, LAC,
MAL, MYA, PHI, SRI,
THA, VIE
USA (ARK, LOU, MIS)
Cyperus laetus J. Presl and
C. Presl
Cyperaceae BRA
Cyperus odoratus L. ¼
Diclidium odoratum (L.)
Schrad. ex Nees
Cyperaceae USA (ARK), BAN, CAM, IND, LAO, MAL,
MYA, PHI, SRI, THA, VIE
USA (ARK)
Table III (continued )
Weed species Family
Type of direct‐seeding
b
DRSR WTSR WRSR
168 A. N. RAO ET AL.

Cyperus rotundus L. Cyperaceae AFR, BAN, BHU, CHI, COT, IDO, IND, JPN,
KOR, LAO, LAC, MAL, MXC, MYA,
NEP, NGR, PAK, PHI, SRI, THA, VIE
BAN, IND, PHI, SRI, THA
Cyperus serotinus Rottb. ¼
Juncellus serotinus
(Rottb.) C. B. Clarke
Cyperaceae KOR, ITL, BAN, CAM, LAO, MAL, MYA,
NEP, PAK, SRI, THA, VIE
JPN, KOR KOR
Dactyloctenium aegyptium
(L.) Willd.
Poaceae AFR, BAN, CHI, IND, IDO, KOR, LAO,
MAL, MYA, NEP, PAK, PHI, SRI, THA,
VIE
IND, PHI
Damasonium minus (R. Br.)
Buch.
Alismataceae AUS AUS
Digitaria ciliaris (Retz.)
Koel.
Poaceae AFR, BAN, BHU, BRA, CAM, CHI, IND,
IDO, JPN, KOR, LAO, MAL, MYA, NEP,
PAK, PHI, SRI, THA, VIE
MAL, PHI, SRI
Digitaria horizontalis Willd.
¼Digitaria sanguinalis
var. horizontalis (Willd.)
Rendle
Poaceae AFR, BRA, COT, NGR
Digitaria sanguinalis (L.)
Scop.
Poaceae AFR, BAN, BHU, IND, IDO, ITL, KOR,
LAC, MAL, MYA, NGR, PHI, THA,
USA (ARK)
IND USA (ARK)
Echinochloa colona (L.)
Link
Poaceae AFR, BAN, BHU, BRA, CAM, CHI, COT,
IDO, IND, JPN, KOR, LAC, LAO, MAL,
MYA, MXC, NEP, NGR, PAK, PHI, SRI,
THA, USA (FLO, TX), VIE
AFR, BAN, EGY, IND, IDO,
LAC, MAL, MYA, PHI,
SRI, THA, VIE
ITL, TUR, USA (TX)
Echinochloa crus‐galli (L.)
P. Beauv.
Poaceae AUS, BAN, BHU, BRA, CAM, CHI, CUB,
IDO, IND, JPN, KOR, LAC, LAO, MAL,
MYA, NEP, PAK, PHI, SRI, THA,
USA (CAL, FLO, ARK, MIS, MSO, TX),
VIE
AFR, BAN, CHI, EGY, IND,
JPN, KOR, LAC, MAL,
MYA, NEP, PHI, SRI,
THA, VIE
AUS, ITL, KOR, TUR,
USA (ARK, LOU, MIS,
MSO, TX, CAL)
Echinochloa glabrescens
Munro ex Hook. F.
Poaceae CAM, LAO, NEP, PAK, PHI IND, PHI, SRI
Echinochloa oryzoides
(Ard.) Fritsch
Poaceae MAL, MYA, NEP, PAK, SRI, THA PHI TUR, USA (CAL)
Echinochloa phyllopogon
(Stapf) Stapf ex Koss
Poaceae MYA, NEP, VIE MAL, VIE ITL, USA (CAL)
Echinochloa picta (J. Konig)
P.W. Michael
Poaceae MYA, THA, MAL, PHI
(continued)
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 169

Table III (continued )
Weed species Family
Type of direct‐seeding
b
DRSR WTSR WRSR
Echinochloa spp. Poaceae RUS, ITL, USA PHI, MAL, THA, VIE ITL, TUR, EUR, USA
Echinochloa stagnina (Retz.)
P. Beauv.
Poaceae BAN, IND, MYA, NEP, PHI, SRI, THA MAL, PHI, SRI
Eclipta prostrata (L.) L. Asteraceae AFR, BAN, BHU, BRA, CAM, CHI, CHL,
IND, IDO, JPN, LAC, KOR, LAO, MAL,
MYA, NEP, PAK, PHI, SRI, THA,
USA (ARK), VIE
AFR, IND, MAL, PHI, SRI,
THA, VIE
USA (ARK, LOU, MIS)
Eichhornia crassipes (Mart.)
Solms
Pontederiaceae BAN, CAM, IND, LAO, MYA, NEP, THA,
VIE
CHI, IND, MAL, SRI
Eleocharis acuta R. Br. Cyperaceae AUS
Eleocharis kuroguwai Ohwi Cyperaceae JPN, KOR JPN, KOR KOR
Eleusine indica (L.) Gaertn. Poaceae AFR, BAN, BHU, CAM, CHI, COT, IND,
IDO, KOR, LAC, LAO, MAL, MYA, NEP,
NGR, PAK, PHI, SRI, THA, VIE
IND, PHI
Euphorbia heterophylla L. Euphorbiaceae AFR, BHU, IND, IDO, COT, PHI, SRI, THA,
VIE
Fimbristylis annua (All.)
Roem & Schult.
Cyperaceae LAC LAC
Fimbristylis dichotoma (L.)
Vahl
Cyperaceae BAN, CHI, IND, IDO, KOR, LAO, MAL,
MYA, NEP, PAK, PHI, SRI, THA, VIE
BAN, IND, JPN, LAC, MYA,
PHI, SRI, THA, VIE
Fimbristylis miliacea (L.)
Vahl
Cyperaceae AFR, BAN, BHU, BRA, CAM, CHI, GUY,
IND, IDO, JPN, KOR, LAC, LAO, MAL,
MYA, NEP, PAK, PHI, SRI, THA, VIE
AFR, BAN, CHI, GUY, IND,
IDO, JPN, LAC, MAL, MYA,
NEP, PHI, SRI, THA, VIE
Heliotropium indicum L. Boraginaceae BAN, IND, IDO, LAO, MYA, PHI, SRI, THA,
VIE
Heteranthera limosa (Sw.)
Willd.
Pontederiaceae IND, JPN, USA (ARK, MIS) LAC ITL, USA (ARK, LOU, MIS,
CAL)
Heteranthera reniformis
Ruiz & Pav.
Pontederiaceae BRA, IND LAC ITL
170 A. N. RAO ET AL.

Hymenachne acutigluma
(Steud.) Gilliland
Poaceae BAN, IDO, IND, THA MAL, MYA
Imperata cylindrica (L.)
Raeuschel
Poaceae AFR, BAN, BHU, CHI, IND, IDO, JPN,
KOR, LAO, MAL, MYA, NEP, PAK, PHI,
SRI, THA, VIE
Ipomoea grandifolia
(Dammer) O’Donell
Convolvulaceae BRA, SRI
Ipomoea lacunose L. Convolvulaceae JPN, USA (MIS)
Ipomoea wrightii A.Gray Convolvulaceae USA (ARK, LOU, MIS) USA (ARK, LOU, MIS)
Ischaemum rugosum Salisb. Poaceae AFR, BAN, BRA, CAM, CUB, GUY, IND,
IDO, JPN, KOR, LAC, LAO, MAL, MYA,
NEP, PAK, PHI, SRI, THA, VIE
GUY, IND, JPN, LAC, MAL,
PHI, THA, VIE, SRI
Leersia hexandra Sw. Poaceae AFR, BAN, CAM, BRA, IND, LAO, MAL,
NEP, PAK, SRI, THA, VIE
AFR, BAN, CHI, MAL, MYA,
PHI, VIE
Leersia japonica (Makino ex
Honda) Honda
Poaceae CHI KOR
Leersia oryzoides (L.) Sw. Poaceae ITL, JPN, MAL, THA POR, ITL
Leptochloa chinensis (L.)
Nees
Poaceae CAM, CHI, IND, IDO, JPN, KOR, LAO,
MAL, PHI, SRI, THA, VIE, BAN, PAK,
IND, LAO
IND, MAL, MYA, PHI, SRI,
THA, VIE
Leptochloa caerulescens
Steud.
Poaceae NGR AFR
Leptochloa dubia (Kunth)
Nees
Poaceae USA (MSO, TX) USA (MSO, TX)
Leptochloa fascicularis
(Lam.) A. Gray.
Poaceae CUB, IND, ITL, USA USA
Leptochloa fusca subsp.
fascicularis (Lam.) N. W.
Snow
Poaceae AUS, USA (ARK, MIS) AUS, USA (ARK, LOU, MIS,
MSO, TX)
Leptochloa panicea (Retz.)
Ohwi
Poaceae CHI, LAO, MAL, SRI, USA (ARK, MSO,
TX), VIE
USA (ARK, MIS, TX)
Leptochloa panicoides
(J. Presl) Hitchc.
Poaceae USA IND USA (ARK, LOU, MIS, MSO,
TX)
Limnocharis flava (L.)
Buchenau
Butomaceae JPN, LAO, MAL, MYA, VIE, SRI BAN, JPN, LAC, MAL, MYA,
SRI, THA, VIE
(continued)
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 171

Limnophila aromatica
(Lam.) Merr.
Scrophulariaceae IND, IDO, MAL, SRI, THA, VIE
Lindernia antipoda (L.)
Alston
Scrophulariaceae BAN, BHU, CHI, IDO, IND, SRI, THA, VIE PHI
Lindernia ciliata (Colsm.)
Pennell
Scrophulariaceae BAN, CAM, IND, IDO, LAO, MYA, NEP,
PHI, SRI, THA, VIE
IND
Lindernia dubia (L.) Pennell Scrophulariaceae JPN, USA (ARK) JPN USA (ARK, LOU, MIS)
Lindernia procumbens
(Krock.) Borbas
Scrophulariaceae BAN, BHU, IDO, JPN, KOR, PAK, VIE BHU, JPN, KOR, SRI, VIE KOR
Ludwigia adscendens (L.) H.
Hara
Onagraceae BAN, CAM, IDO, IND, LAO, NEP, THA, VIE AFR, CHI, IND, MAL, MYA,
PHI, THA, VIE
Ludwigia hyssopifolia (G.
Don) Exell
Onagraceae BAN, CAM, CHI, IDO, LAO, MAL, THA AFR, MAL, PHI, SRI, MYA,
THA
Ludwigia octovalvis (Jacq.)
P. H. Raven
Onagraceae AFR, BAN, CAM, IDO, LAO, MAL, MYA,
NEP, PHI, SRI, THA, VIE
AFR, BAN, IND, MAL, MYA,
PHI, SRI, THA, VIE
Ludwigia perennis L. Onagraceae BAN, CAM, IND, LAO, MAL, MYA, NEP,
PAK, SRI
IND, MYA, PHI, SRI
Ludwigia prostrata Roxb. Onagraceae CAM, CHI, KOR, NEP, SRI, VIE KOR, BAN, IND, MAL, NEP,
SRI
KOR
Luziola subintegra Swallen Poaceae LAC
Luziola peruviana Juss.
ex J. F. Gmel.
Poaceae BRA
Malachra fasciata Jacq. Malvaceae MXC, PHI
Marsilea minuta L. Marsileaceae BAN, CAM, LAO, MAL, PAK, SRI, THA,
VIE
AFR, BAN, BHU, IND, IDO,
MAL, MYA, PHI, THA,
VIE
Marsilea quadrifolia L. Marsileaceae CAM, IDO, IND, LAO, MAL, NEP, PAK,
PHI
BAN, CHI, IND, JPN, MYA,
NEP, SRI, VIE
Table III (continued )
Weed species Family
Type of direct‐seeding
b
DRSR WTSR WRSR
172 A. N. RAO ET AL.

Melochia corchorifolia L. Sterculiaceae AFR, BAN, CAM, IND, IDO, MAL, NEP,
PHI, THA, VIE
IND, MAL, PHI, THA
Monochoria vaginalis
(Burm. f.) C. Presl
Pontederiaceae CHI, IDO, IND, JPN, KOR, PHI, THA, VIE,
LAO, NEP, PAK
BAN, BHU, CHI, IDO, IND,
JPN, KOR, MAL, MYA,
NEP, NGR, PHI, SRI, THA,
VIE
KOR, USA (CAL)
Murdannia keisak (Hassk.)
Hand.‐Mass.
Commelinaceae IDO, JPN, KOR, MAL KOR
Murdannia nudiflora (L.)
Brenan
Commelinaceae BAN, LAC, IND, IDO, PHI, THA, VIE BAN, SRI
Oryza longistaminata A.
Chev & Roehr.
Poaceae AFR, MAL AFR, SRI
Oryza sativa L. (weedy rice,
red rice)
Poaceae AFR, BRA, EUR, GUY, IND, JPN, KOR,
LAC, MYA, USA (ARK, MIS, MSO, TX)
AFR, GUY, IND, JPN, LAC,
MAL, MYA, KOR, SRI,
THA, VIE
EUR, ITL, POR, USA (ARK,
LOU, MIS, TX)
Panicum repens L. Poaceae AFR, BAN, BRA, CAM, CHI, IND, IDO, ITL,
KOR, LAO, MAL, MYA, NEP, PAK, PHI,
SRI, THA, USA (ARK), VIE
IND, BAN, MAL, MYA, PHI,
SRI
USA (ARK, LOU, MIS)
Paspalum distichum L. Poaceae BAN, BHU, CHI, IND, IDO, JPN, KOR,
LAO, MAL, MYA, NEP, PAK, PHI, SRI,
THA, VIE
BAN, CHI, IND, MYA, PHI,
SRI, VIE
ITL
Paspalum modestum Mez Poaceae BRA USA (LOU)
Paspalum scrobiculatum L. Poaceae AFR, BAN, CAM, CHI, IND, IDO, KOR
LAO, MAL, MYA, NEP, PAK, PHI, SRI,
THA, VIE
PHI
Phaseolus lathyroides L. ¼
Macroptilium lathyroides
(L.) Urb.
Fabaceae CAM, IDO, LAC, LAO, MAL, PHI, SRI, VIE PHI
Phyllanthus fraternus G. L.
Webster
Euphorbiaceae IDO, IND, NEP, PHI, THA, VIE, NEP IND, PHI
Physalis angulata L. Solanaceae BRA, USA (ARK) USA (ARK, LOU, MIS)
Pistia stratiotes L. Araceae BAN, CAM, LAO, NEP, THA, VIE AFR, CHI, IND, MAL, MYA,
PHI, SRI
(continued)
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 173

Polygonum hydropiperoides
Michx. ¼Persicaria
hydropiperoides (Michx.)
Small
Polygonaceae BAN, BHU, BRA, CHI, IND, IDO, KOR,
LAC, MAL, NEP, THA
KOR
Polygonum spp. (incl. P.
pennsylvanicum L.)
Polygonaceae ITL, USA (ARK) SRI USA (ARK, LOU, MIS)
Portulaca oleracea L. Portulacaceae AFR, BAN, BHU, CHI, COT, IND, IDO, JPN,
KOR, MAL, MYA, NGR, PAK, PHI, SRI,
THA, VIE
IND, PHI, SRI,
Rotala indica (Willd.)
Koehne
Lythraceae AFR, BAN, CAM, CHI,IDO, IND, KOR, LAO,
MYA, NEP, PAK, PHI, SRI, THA, VIE
CHI, KOR, MAL, SRI, THA,
VIE
Rottboellia cochinchinensis
(Lour.) Clayton
Poaceae AFR, CHI, IND, IDO, JPN, KOR, LAO, LAC,
MAL, MYA, NEP, PAK, PHI, SRI, THA,
VIE
Sagittaria guayanensis
Kunth
Alismataceae BRA, CAM, GUY, IDO, LAO, MYA, NEP,
PAK, SRI
GUY, LAC, MAL, THA, VIE
Sagittaria longiloba Engelm.
ex J.G. Sm.
Alismataceae USA (CAL)
Sagittaria montevidensis
Cham. & Schltd. ¼
Sagittaria pugioniformis
var. montevidensis
(Cham. & Schultz)
Kuntze
Alismataceae BRA, AUS AUS, USA (CAL)
Sagittaria pygmaea Miq. Alismataceae CAM, CHI, IND, LAO, MAL, MYA, THA,
VIE
CHI, JPN, KOR KOR
Sagittaria trifolia L. Alismataceae BAN, CAM, IDO, IND, LAO, MAL, MYA,
NEP, PAK
BAN, JPN, KOR, IND, NEP,
SRI, THA, VIE
Table III (continued )
Weed species Family
Type of direct‐seeding
b
DRSR WTSR WRSR
174 A. N. RAO ET AL.

Scirpus fluviatilis (Torr.) A.
Gray.
Cyperaceae USA (CAL)
Scirpus juncoides Roxb. ¼
Schoenoplectus juncoides
(Roxb.) Palla
Cyperaceae BAN, BHU, CAM, IND, LAO, NEP, PAK,
SRI, THA
BAN, BHU, CHI, EGY, IND,
KOR, MYA, MAL, NEP,
SRI, THA
KOR
Scirpus mucronatus L. ¼
Schoenoplectus
mucronatus (L.) Palla
Cyperaceae CAM, IND, ITL, JPN, LAO, MYA, NEP,
PAK, PHI, SRI
IND, JPN, MAL, VIE EUR, ITL, TKY, USA (CAL)
Scleria setuloso‐
ciliata Boeck.
Cyperaceae MXC
Senna obtusifolia (L.) H. S.
Irwin & Barneby
Fabaceae AFR, IND, LAC, MYA, PHI, VIE
Sesbania bispinosa (Jacq.)
W. Wight
Fabaceae BAN, VIE
Sesbania exaltata (Raf.)
Cory
Fabaceae IND, JPN, USA, LAC LAC
Sesbania herbacea (Mill.)
McVaugh
Fabaceae USA (ARK, MSO) USA (ARK, LOU, MIS, MSO)
Setaria glauca (L.) P. Beauv Poaceae IND, BAN, JPN, NEP, SRI IND, SRI
Setaria viridis (L.) P. Beauv. Poaceae IND, KOR, PHI, VIE IND
Sorghum aethiopicum
(Hack.) Rupr. ex Stapf.
Poaceae NGR
Sorghum halepense
(L.) Pers.
Poaceae IND, ITL, JPN, PHI, USA USA (ARK, LOU, MIS)
Sphenoclea zeylanica
Gaertn.
Campanulaceae AFR, BAN, CAM, IND, IDO, JPN, LAO,
MAL, MYA, NEP, NGR, PAK, PHI, SRI,
THA, USA (ARK), VIE
AFR, BAN, IND, JPN, KOR,
LAC, MAL, MYA, NEP,
NGR, PHI, SRI, THA, VIE
USA (ARK, LOU, MIS)
Spilanthes filicaulis
(Schumach. & Thonn.)
C. D. Adams
Asteraceae AFR, COT, MYA
Stenotaphrum secundatum
(Walter) Kuntze
Poaceae LAC
Striga asiatica L. Kuntze Scrophulariaceae AFR
Striga aspera (Willd.)
Benth.
Scrophulariaceae AFR
(continued)
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 175

Trianthema portulacastrum
L.
Aizoaceae AFR, BAN, CAM, COT, IDO, IND, LAO,
MAL, MYA, NEP, PAK, PHI, SRI, THA,
VIE
IND
Urochloa platyphylla
(Munro ex C. Wright)
R. D. Webster
Poaceae USA (ARK, MSO, MIS, TX) USA (ARK, LOU, MIS, MSO,
TX)
Xanthium strumarium L. Asteraceae BHU, IND, JPN, MYA, PAK, THA,
USA (ARK, LOU, MIS)
USA (ARK)
a
Based on the large number of references cited in this review and personal communications.
b
DRSR, dry‐seeded rice; WTSR, wet‐seeded rice; WRSR, water‐seeded rice.
c
AFR, Africa; AUS, Australia; BAN, Bangladesh; BHU, Bhutan; BRA, Brazil; CAM, Cambodia; CHI, China; CHL, Chile; COT, Co
ˆte d’Ivoire; CUB,
Cuba; EGY, Egypt; EUR, Europe; GUY, Guyana; IDO, Indonesia, IND, India; ITL, Italy; JPN, Japan, KOR, Korean peninsula; LAC, Latin America
and Caribbean; LAO, Lao PDR; MAL, Malaysia; MYA, Myanmar; MXC, Mexico; NEP, Nepal; NGR, Nigeria; PAK, Pakistan; PHI, Philippines;
POR, Portugal; RUS, Russia; SRI, Sri Lanka; THA, Thailand; TUR, Turkey; USA, United States of America (ARK, Arkansas; CAL, California;
FLO, Florida; LOU, Louisiana; MIS, Mississippi; MSO, Missouri, TX, Texas); VIE, Vietnam.
Table III (continued )
Weed species Family
Type of direct‐seeding
b
DRSR WTSR WRSR
176 A. N. RAO ET AL.

(Khalid, 1995), Philippines (De Datta and Baltazar, 1996; Rao and Moody,
1994), South Asia (Malik and Moorthy, 1996), Sri Lanka (Abeysekera, 1999;
Haigh, 1951; Sangakkara et al., 2004), Thailand (Noda et al., 1984; Tomita
et al., 2003a,b; Vongsaroj, 1995); the United States (Gianessi et al., 2002;
Hill et al., 1994), and Vietnam (Chin et al., 2002; Koo et al., 2005; Mai et al.,
2000; Tan et al., 2000). Tables such as this, arising out of weed surveys,
commonly lead to the assertion that, even though a large number of species
are known to be weeds of rice (Galinato et al., 1999; Moody, 1989), it is often
only a small number of species that are considered to be economically impor-
tant within a country. Such species are considered major as (1) they are
competitive with rice and individually may cause severe yield loss (Ampong‐
Nyarko, 1996; Chin et al., 2002; Hassan and Rao, 1996; Kim and Ha, 2005;
Tomita et al., 2003a,b), (2) they are usually abundant (Caton et al., 2004;
Galinato et al., 1999), (3) they exhibit high colonization rates (Bocchi et al.,
2005; Tan et al., 2000), and (4) they are resilient to current and past control
measures.
Table III includes over 140 species covering 27 families in which members
of the Cyperaceae and Poaceae are the most dominant. Equally, the species
listed present a diversity of life forms and life histories, including obligate
aquatic and terrestrial species and ‘‘semi‐aquatics,’’ with some species being
recorded under all three methods of direct‐seeding. Such lists need to be
considered cautiously when asserting the importance of an individual spe-
cies. Ranking the comparative damage done by a single species is fraught
with diYculty given the interaction of factors and processes in weed–crop
interference in the field that ultimately leads to yield loss in the crop or
economic damage in general. Cousens and Mokhtari (1998) and Jasieniuk
et al. (2001) in examining competitiveness and yield loss in selected weed
species concluded that there was little correlation of competitiveness across
seasons, and yield losses in response to weed density varied considerably.
However, such lists reflect the persistence of a species under the suite of crop
and weed management practices practiced in the recent past.
1. Dry‐Seeded Rice
More than 50 weed species infest irrigated DSR and cause major yield
losses in the United States (Dilday et al., 2000). Dry drill‐seeded, flush‐
irrigated rice in the southern states of the United States is infested with
dryland weeds (e.g., Urochloa platyphylla) early in the season, followed by
aquatic weeds (e.g., Heteranthera limosa) after the permanent flood. Red rice
(O. sativa L.) remains the most troublesome weed in rice production of
Louisiana, Arkansas, and Missouri (Gianessi et al., 2002) but is excluded
from California. Echinochloa spp. constitute the major weed species in all
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 177

rice‐growing areas of the United States (Echinochloa crus‐galli, in Mississippi
and Texas and Echinochloa crus‐galli, Echinochloa phyllopogon,andEchi-
nochloa oryzoides in California). Similarly, in Italian rice production, pre-
dominantly terrestrial weeds are recruited into the flora initially and followed
by semi‐aquatic and aquatic species. Bocchi et al. (2005) recorded that
species emerging in the initial dry phase prior to flooding were Echinochloa
spp., Panicum dichotomiflorum (L.) Michx, Cyperus serotinus, Bidens spp.,
Digitaria sanguinalis, Sorghum halepense, and Polygonum spp., with a second
less competitive group of weeds emerging later (Bolboschoenus maritimus,
Schoenoplectus mucronatus, and Leersia oryzoides). In the Krasnodar region
(Russia), aquatic perennial weeds B. maritimus and Bolboschoenus compactus
(HoVm.) Drobow, wild rice varieties (O. sativa var. ferruquinoza), and Echi-
nochloa spp. were recorded as strong competitors with rice (Dobermann,
1992). A total of 351 weed species were identified in diVerent rice‐growing
areas of Cuba, with the largest yield reduction occurring with Echinochloa
crus‐galli (Antigua, 1993).
In Asia, lowland DSR fields have a more species‐rich vegetation and
greater diversity in the weed flora than transplanted rice (Tomita et al.,
2003a). Ludwigia hyssopifolia (G. Don) Exell and F. miliacea L. have been
observed season long in both DSR fields and transplanted fields, irrespective
of water level (Tomita et al., 2003a,b). Cynodon dactylon (L.) Pers., Panicum
repens L., Paspalum scrobiculatum L., Melochia corchorifolia L., and Digi-
taria elongata Trin Spreng. infested DSR fields, whereas Ludwigia adscen-
dens (L.) Hara was mainly observed in transplanted fields. Panicum repens
L., Paspalum scrobiculatum L., and Digitaria elongata especially grew under
poor‐(flooding for 0–60 days) and medium‐water (flooded for 60–120 days)
conditions (Tomita et al., 2003a,b). In Indonesia, 56 weed species covering
18 families were recorded in dry‐seeded bunded rice, which is raised on
sloping lands (Pane et al., 2000). Weed communities remaining after farmer
weeding at upper‐and mid‐positions of the toposequence were similar in
species composition (Lindernia spp., Echinochloa colona, F. miliacea,and
Murdannia nudiflora). These diVered from those at the base of the topo-
sequence, which was dominated by Ammannia baccifera, Echinochloa colona,
F. miliacea, and Leptochloa chinensis.
Weedy rice (O. sativa) ecotypes have become a major cause of yield loss in
irrigated DSR in Malaysia (Azmi et al., 2003), Thailand, and Vietnam,
particularly under dry‐seeding conditions (Chin, 1997). Kim and Ha (2005)
also recorded weedy rice along with annual grasses, such as Echinochloa
crus‐galli, Digitaria ciliaris, Leptochloa chinensis, and Setaria viridis, as the
most predominant weeds of dry‐seeded rice in Korea.
In Co
ˆte d’Ivoire, West Africa, weed species in upland rice diVered
between forest and savannah zones, with Chromolaena odorata (L.) R. M.
King and Robinson being common in the former and Platosoma africanum
178 A. N. RAO ET AL.

P. Beauv. and Mariscus cylindristachus in the latter (Kent et al., 2001). These
authors also observed that Bacopa decumbens, Fimbristylis littoralis, Sphe-
noclea zeylanica,andEchinochloa colona were common in both forest and
savannah lowlands, whereas the sedges Cyperus diVormis and Cyperus iria
were particularly abundant in the savannah. Echinochloa spp. were the most
common weeds in fully irrigated systems, and Panicum laxum Sw. was more
common in imperfectly irrigated fields (Becker and Johnson, 1999).
Roder et al. (1997) recorded the major weed species of upland dry‐seeded
rice of Laos in terms of cover and frequency in farmers’ fields. They were
Chromolaena odorata, Ageratum conyzoides, Commelina spp., Lygodium
flexuosum, Panicum trichoides, Corchorus spp., Pueraria thomsonii, Panicum
cambogiense, and Imperata cylindrica. Although C. odorata was the most
abundant weed, farmers generally did not consider it as a serious weed
because of ease of control, reflecting a belief in zero opportunity cost of
manual weeding. In Vietnam, the major weeds in farmers’ dry‐seeded rice
fields were Echinochloa crus‐galli, Echinochloa glabrescens,andF. miliacea
(My et al., 1995). Weed infestations of upland rice are one of the main issues
confronting the sustainability of cropping systems in North Vietnam.
Stevoux et al. (2002) recorded Ageratum conyzoides, Crassocephalum crepi-
dioides, Paspalum conjugatum, Eleusine indica, and Imperata cylindrica
together with adventive species from forest area (Lygodium flexuosum,
Trema angustifolia, and Melastoma sp.). In dry‐seeded bunded rice in
Indonesia, weed species commonly observed after farm weeding were
Eclipta alba, Echinochloa crus‐galli, Echinochloa colona, F. miliacea, Cyperus
diVormis,andC. rotundus (Pane et al., 2000). The existence of grass weeds
such as Echinochloa crus‐galli, Leptochloa chinensis, and I. rugosum, and of
Cyperus diVormis clearly poses a significant threat to rice intensification and
underlines the importance of eVective weed control.
2. Wet‐and Water‐Seeded Rice
Wet‐and water‐seeding provide an aquatic environment for the germina-
tion of weed species adapted to germinate in an anaerobic and partially
anaerobic environment. As a consequence, there is a tendency for annual
aquatic weeds to predominate. Water‐seeded rice in Australia tends to be
infested with Cyperus diVormis, Damasonium minus, Sagittaria monteviden-
sis, Alisma plantago‐aquatica, and Alisma lanceolatum (Skinner and Taylor,
2002). Alisma plantago‐aquatica and Scirpus mucronatus have been identified
as troublesome weed species in rice of Europe (Weber and Gut, 2005).
Water‐seeded rice in California has experienced an increase in the impor-
tance of Echinochloa oryzoides and Echinochloa phyllopogon with the adop-
tion of continuous flooding to suppress Echinochloa crus‐galli. Echinochloa
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 179

oryzoides and Echinochloa phyllopogon in dense infestations continue to
cause yield loss (Fischer et al., 2000b). Leptochloa spp. have also been
recorded in water‐seeded rice (Hill et al., 1994). A recent survey of the
occurrence of weeds in water‐seeded rice in the Red River Delta of Vietnam
recorded 60 weed species from 19 families, the most important families again
being the Poaceae and Cyperaceae. The most widespread species, occurring
at >50% sites in spring and summer, was Rotala indica, followed by Echino-
chloa crus‐galli and Cyperus diVormis (Tan et al., 2000). The dominant weeds
of wet‐seeded rice in the Mekong Delta of Vietnam included the latter two
species and Leptochloa chinensis, F. miliacea,andC. iria (Chin and Mortimer,
2002). Kim and Ha (2005) concluded that Echinochloa crus‐galli, Murdannia
keisak (Aneilema keisak), Ludwigia prostrata,andLeersia japonica were the
dominant weeds of wet‐seeded rice in Korea.
B. CROP–WEED COMPETITION IN DIRECT‐SEEDED RICE
The outcome of the process of interspecific competition among plant
species is the diVerential acquisition of resources for growth and yield.
At the population level, resource acquisition is density dependent and it is
only when there is spatial and or temporal niche diVerentiation that two
species can coexist. In plant species, the most important competition is usually
preemptive competition that occurs among seedlings, since it is almost
impossible for a seedling of one species to outcompete an established adult
of another species. The inherent size diVerence between rice seedlings and
emerging weed species that confers a competitive advantage to transplanted
rice is removed when direct‐seeding is practiced. Relative outcomes of com-
petition between a weed species and direct‐seeded rice are, however, influenced
by weed species and rice cultivar, weed and crop density, and competition
duration as governed by the selective removal of the weed species, along with
associated agronomic and cultural management factors.
Although rice is sown at a seed rate to ensure an optimal dense plant
stand, infestations of weed species may result in a near total loss of yield.
Gibson et al. (2001) recorded a grain yield reduction of up to 99% with a
mixed infestation of Echinochloa oryzoides and Echinochloa phyllopogon in
wet‐seeded rice. Ferrero and Nguyen (2004) reported similar findings for
European rice weeds. The literature contains numerous examples of yield
losses in response to the density of individual weed species (Marambe, 2002;
Mishra, 2000; Smith, 1988), but interspecific comparisons of their impact
provide little value in assessing relative competitiveness. This is because
studies are usually conducted over too narrow a density range and rarely
carried out concurrently under the same experimental conditions. Nevertheless,
180 A. N. RAO ET AL.

this has not reduced the desire to develop weed management tools based on the
concepts of critical periods of crop competition and economic thresholds.
Cousens (1987) argued that the use of short‐term economic thresholds as a
within‐season management tool has little value for two reasons: (1) wide varia-
tion is seen in yield response to the same weed density when intersite compar-
isons are made and (2) it is diYcult to accurately anticipate the future market
value of the crop. Long‐term economic thresholds that take into account how
weed control in one year aVects future weed infestation densities and how
financial decisions made in the present year impact on future expenditures for
weed control appear to oVer a more sound basis on which to develop a weed
management plan. To be eVective, these decisions require a detailed under-
standing of the population dynamics of the weed species under control mea-
sures (Wallinga, 1998). However, Jones and Medd (2000) have suggested that
the integration of control tactics using dynamic optimal decision rules with a
long‐term planning horizon can both maximize farm returns and lead to the
exhaustion of weed seed banks and that this is superior to the use of long‐term
economic thresholds. Such approaches, however, necessarily focus on a single
weed species, which merits attention because of diYculty to control.
The concept of a critical period of crop–weed competition (the crop‐
growth period measured empirically during which yield is reduced by
weed competition, Zimdahl, 1999), introduced by Nieto et al. (1968), holds
an attraction in that it draws attention to the critical time period when
weeding should occur. It suVers criticism, however, for many of the same
reasons that may be leveled at the use of thresholds, and its methodological
empiricism underlines its limitations. The weed flora in toto is presumed to
be injurious, and no distinction is made between the nature of competitive
processes that may be occurring. For instance, in DSR, preemptive (exploit-
ative) competition for light initially may be most important but give way
later to interference competition for other resources. While Gibson et al.
(2001) showed that complete suppression of Echinochloa phyllopogon
occurred if emergence was delayed for 30 days after rice seeding (DAS) or
longer, the shape of the yield loss curve resulting from periods of earlier
introduced competition diVered among seasons. Similarly, in field trials of
dry‐seeded irrigated rice, 95% of a weed‐free rice yield was obtained by
controlling weeds until 32 DAS in the wet season and until 83 DAS in the
dry season in the Senegal River delta (Johnson et al., 2004). The authors
argued that the occurrence of diVerent weed species and lower growth
temperatures in the early dry season accounted for the diVerences in the
critical period for competition. The time after which weeding may be sus-
pended often coincides with periods of maximum tillering of the crop
and consequent canopy closure, which may well explain the maximum
days in the ranges reported in dry‐(15–45 DAS; Singh et al., 1999;
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 181

Yaduraju and Mishra, 2004) and wet‐seeded (15–30 DAS; Azmi, 1991) rice.
However, there is evidence that both above‐and below‐ground competition
plays a major role in governing competitive outcomes in rice–weed mixtures
and that it may be that the seasonal variation seen in many studies is a
reflection of temperature‐mediated diVerential resource responses. Such
observations have led to several authors emphasizing the importance of
mechanistic modeling of rice–weed interactions (Caton et al., 1999a;
KropVand Lotz, 1993; KropVet al., 1993; Lindquist and KropV, 1996).
The general conclusion that emerges from studies of rice–weed competition
for DSR is that competitive processes occur earlier in the life of the crop
and reemphasize the oft‐quoted need for early weed control. This also
underlines the importance of land preparation for seedbeds and the use of
early postemergence chemical weed control.
C. WEED SPECIES SHIFTS AND WEED POPULATION DYNAMICS DUE TO
CHANGES IN THE METHODS OF RICE ESTABLISHMENT
1. Weed Species Shifts
Preadaptation, evolution, and alien immigration are the three principal,
but not mutually exclusive, means by which weeds species become in-
corporated into a weed flora (Mortimer, 1990). While natural selection giving
rise to herbicide resistance (Maxwell and Mortimer, 1994) and evolution
by crop mimicry (Hirosue et al., 2000) together with alien immigration
often through seed importation (Hodkinson and Thompson, 1997; Rao
and Moody, 1990; Yan and Yin, 1994) have led to new or resurgent weed
infestations in many ecosystems, preadaptation is perhaps the most common
means. Preadapted weed species are those that are resident in an agroecosy-
tem within dispersal distance of a crop and come to predominate through a
change in management practice. The soil seed banks of irrigated and rain-
fed rice fields may exhibit high floristic diversity for two reasons: (1) in Asia,
seasonal switching between a diverse terrestrial vegetation in dry‐season crops
and a much altered aquatic and semi‐aquatic flora in the wet season on the
same area of land is common practice (Moody, 1983) and (2) additionally, the
landscape of rice agriculture is fragmented within a season since it simulta-
neously oVers terrestrial (bunds), aquatic (irrigation channels), and semi-
aquatic (drainage channels) habitats for weed growth. Moreover, typically
high seed densities (in excess of 10
7
propagules per m
3
) have been reported
(Sahid et al., 1995) in rice field soils. The weed species shift evidenced
with the switch from transplanting to direct‐seeding is a prime example of
preadaptation.
182 A. N. RAO ET AL.

Ho (1991) reported on changes in the weed flora of Malaysian rice with
the use of direct‐seeding as a result of the adoption of double cropping of
rice in the late 1970s. Double cropping for food security was made feasible
with the introduction of irrigation and drainage schemes and with increased
use of mechanization for land leveling. Adoption of both dry and wet direct‐
seeding in place of transplanting resulted in an increase in floristic diversity
and a change in the relative dominance of the major weed species (Table IV).
In a longer and more detailed study covering the transition from trans-
planting to direct‐seeding and the continued use of direct‐seeding in Malaysia,
Azmi and Mashor (1995) and Azmi (personal communication) assessed the
dominance structure of the weed flora present at 60 DAS/days after trans-
planting (DAT) in fields after farm weeding had been completed. Figure 4
illustrates the changes in the weed flora from 1989 to 2001 when two rice
crops were grown in a year. A total of 46 species were present in transplanted
rice in 1989, logarithmically distributed in terms of abundance, with the flora
being dominated by Sagittaria guayenensis, Monochoria vaginalis, Limno-
charis flava,andF. miliacea. After a further six seasons of cropping and
substantive adoption of wet‐seeding, this dominance had changed, with the
principal weeds being graminaceous, including Echinochloa spp., Ischaemum
rugosum, and Leersia hexandra, and 21 new species being added to the flora.
By 2001, ‘‘weedy’’ rice was the most dominant weed, followed by Echino-
chloa spp., Leptochloa chinensis, and Ischaemum rugosum. A species that
persisted throughout this study period was F. miliacea. Although diVe-
rences in weed management practices were not recorded at the field level,
variation in herbicide use among farmers has been reported (Azmi and
Baki, 1995). The shift in dominance of weed species from dicotyledonous
Table IV
DiVerences in Weed Flora in Relation to Crop Establishment Method
Weed flora
Method of establishment and year
Transplanted (1979) Dry‐seeded (1987) Wet‐seeded (1989)
Number of species 21 50 57
Number of genera 18 38 44
Number of families 13 22 28
Major weed species (ranked by density)
Monochoria vaginalis Echinochloa crus‐galli Echinochloa crus‐galli
Ludwigia hyssopifolia Echinochloa colona Leptochloa chinensis
Fimbristylis miliacea Leptochloa chinensis Fimbristylis miliacea
Cyperus diVormis Scirpus grossus Marsilea crenata
Limnocharis flava Fimbristylis miliacea Monochoria vaginalis
Source: Ho (1991) for the MUDA area in Malaysia.
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 183

and sedge species in transplanted rice to competitive grassy weeds in DSR
has been related by several authors to the continuous use of herbicides in
weed‐control operations since all rice farmers who practiced direct‐seeding
adopted chemical weed control (Azmi and Baki, 1995; Ho, 1998). Azmi and
Baki (1995) observed that Echinochloa crus‐galli was dominant in the plots
Sagittaria guayanensis
Fimbristylis miliacea
Scirpus grossus
Najas graminea
100.00
Echinochloa colona
1989
1991
0.01
Echinochloa crus-galli var. crus-galli
Rank order
0.01
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52
0.10
10.00 Fimbristylis miliacea
2001
Ischaemum rugosum
Leersia hexandra
1993
Weedy rice
0.10
Echinochloa crus-galli var. formosensis
Fimbristylis miliacea
Proportion abundance (log scale)
Monochoria vaginalis
Limnocharis flava
0.10
1.00
10.00
Echinochloa crus-galli
0.10
1.00
10.00
Echinochloa oryzicola
Leptochloa chinensis
1.00
Weedy rice
Ischaemum
rugosum
Panicum repens
Leptochloa chinensis
100.00 Oryza rufipogon
Panicum repens
1.00
10.00 Echinochloa spp.
Sagittaria guayanensis
Echinochloa spp.
Rank order
Figure 4 Changes in weed species composition in response to direct‐seeding. Species are
ranked in order of proportional abundance based on area coverage per m
2
. Data from Azmi and
Mashor (1995), Mortimer and Hill (1999).
184 A. N. RAO ET AL.

repeatedly sprayed with 2,4‐D, whereas M. vaginalis became dominant
with the repeated use of molinate/propanil, thiobenacarb/propanil, pretila-
chlor, quinclorac, propanil, and fenoxaprop‐ethyl. Molinate use suppressed
Echinochloa crus‐galli but caused an increased infestation of Leptochloa
chinensis and I. rugosum (Azmi and Mashor, 1995). In wet‐seeded rice,
when serial applications of bensulfuron and 2,4‐D were applied, Scirpus
grossus and Echinochloa crus‐galli, respectively, predominated (Azmi and
Mortimer, 2002). In Malaysia, Azmi and Baki (2003) reported that Limno-
phila erecta Benth. and Bacopa rotundifolia Wettst. had become dominant
weeds in areas where sulfonylurea herbicides were used continuously.
Changes in rankings of dominant weeds were observed by Singh et al.
(2005a) in response to both dry‐and wet‐seeding of rice in the Indo‐Gangetic
plains (Fig. 5). After four seasons of rice cropping, at 56 days after planting,
Ischaemum rugosum and F. miliacea were the dominant species of unweeded
wet‐seeded plots and Echinochloa colona and Commelina diVusa of dry‐
seeded plots in comparison with Cyperus iria and Echinochloa colona in
transplanted rice. Manual weeding 30 days after sowing led to increased
importance of Cyperus rotundus in wet‐seeded rice and Ischaemum rugosum
in dry‐seeded rice.
Yaduraju and Mishra (2005) reported that sedges such as Cyperus iria
and Cyperus diVormis were common under both wet‐and dry‐seeded con-
ditions, whereas C. rotundus and Fimbristylis dichotoma (L.) Vahl were
dominant in rainfed uplands where dry‐seeding was practiced.
In Jiangsu and Zhejiang provinces in eastern China, the long‐term use of
butachlor and molinate in rice has led to the elimination of Echinochloa crus‐
galli, with an associated increase in other species, including Leptochloa
chinensis, Sagittaria montevidensis, Alternanthera philoxeroides (Mart.) Gri-
seb., Juncellus serotinus, and Scirpus planiculmis (Zhang, 2003). Similarly, in
Sri Lanka, usage of alternate herbicides for controlling propanil‐resistant
Echinochloa crus‐galli has resulted in a shift in dominance to Ischaemum
rugosum with continuous use of quinclorac and to Leptochloa chinensis with
continuous use of bispyribac‐sodium (Marambe, 2002).
2. Weed Population Dynamics
Understanding the processes governing the persistence of a weed species
or the underlying reasons for an increase with a change in management
requires a reductionist approach examining the comparative population
dynamics of individual species under alternative management regimes.
Figure 6A shows the flux in population size of Echinochloa crus‐galli in
wet‐seeded rice and emphasizes the species’ ability to compensate for losses
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 185

during the seedling recruitment phase due to early flooding and in the fallow
periods between cropping seasons through both adult plant fecundity and
the existence of a persistent seed bank in the soil.
Seed dormancy ensures a persistent seed bank (Fig. 6B), which at a 15‐cm
soil depth may exhibit a half‐life of more than 3 years, from which seedling
recruitment may occur as a result of land preparation. An isolated plant of
1
10
100
0.1
B
0.1
1
10
Echinochloa colona
Echinochloa colona
Cyperus iria
Caesulia axillaris
0.1
1
10
100
1000
Echinochloa
colona
Dry biomass (g m−2) Dry biomass (g m−2)Dry biomass (g m−2)
Fimbristylis miliacea
Fimbristylis miliacea
Echinochloa crus-galli
Echinochloa crus-galli
Commelina diffusa
Commelina diffusa
Cyperus rotundus
Cyperus rotundus
Cyperus rotundus
Cyperus rotundus
A
Ischaemum rugosum
Ischaemum rugosum
Ischaemum
rugosum Ischaemum rugosum
Cyperus iria
Cyperus iria
Caesulia axillaris
Caesulia axillaris
Caesulia
axillaris
Caesulia axillaris
Leptochloa chinensis
Leptochloa chinensis
Leptochloa chinensi
s
C
Echinochloa
colona
Echinochloa colona
Cyperus difformis
Cyperus difformis
Fimbristylis
miliacea
Ischaemum
rugosum
Cyperus iria
Figure 5 Abundance (dry biomass) of weeds at 56 days after establishment in:
(A) transplanted rice, (B) wet‐seeded rice, and (C) dry‐seeded rice. All plots were maintained
under saturated soil conditions 30 days after planting. The upper curve in each pair is from
unweeded plots, the lower from plots manually weeded, 30 days after establishment [Source:
Singh et al. (2005a)].
186 A. N. RAO ET AL.

Echinochloa crus‐galli may produce more than 6000 caryopses. The likeli-
hood of successful plant establishment into a developing canopy is governed
in part by the speed of attainment of autotrophy after germination. This in
turn is dependent on the water profile (depth and duration of flooding
events) and light regimes experienced in early seedling growth, particularly
in wet‐seeded rice. In Echinochloa crus‐galli, carbohydrate mobilization and
subsequent partitioning to developing biomass may result in the onset of
autotrophy before leaf emergence above the water surface in an illuminated
water column (Fig. 6C). In contrast, in total darkness (as a model of deep,
turbid water), there is a maximum shoot height that can be achieved
before the exhaustion of reserves. Mortimer et al. (2005) reported that this
height was 80 mm for one accession of Echinochloa crus‐galli, with evidence
of enhanced height extension rate when seedlings were under water. Once the
plant is committed by germination to growth, the ability to photosynthesize
under water at lowered light intensities and to mobilize resources to develop
photosynthetic structures out of water, together with adaptations to hypoxia,
are key traits influencing plant survivorship. Figure 7 contrasts the dynamics
of weed seedling recruitment in wet‐seeded and transplanted rice and high-
lights that the net recruitment of weed seedlings was confined to the first
14 DAT in flooded transplanted rice and only extended beyond that date in
graminaceous species in plots that were initially wet‐seeded and subsequently
flooded to a shallow (<50 mm) depth. Late‐emerging cohorts of grass species
predominantly comprised the large‐seeded species Ischaemum rugosum, Echi-
nochloa crus‐galli,andEchinochloa glabrescens. In dry‐seeded rice, similar
patterns of early recruitment of weeds have been observed (J. D. Janiya,
personal communication).
The preceding discussion identifies key processes in the regulation of
weed seedling recruitment in relation to direct‐seeding in which seed size is
an important attribute and in some species is ecologically correlated with
seed dormancy traits. Seed size in rice weeds varies on a logarithmic range
from about 10 mg(Sphenoclea zeylanica and Cyperus diVormis) to greater
than 10
4
mg(Rottboellia cochinchinensis). In small‐seeded species, the
absence of oxygen (M. vaginalis) or lowered oxygen concentration (Zinzania
aquatica), fluctuating diurnal temperatures (Najas graminea), and diVeren-
tial responses to spectral light ratios (Sphenoclea zeylanica) have all been
shown to be cues involved in response mechanisms that govern seed
germination rates under water. Phytochrome‐mediated switches in response
to red/far‐red light ratios provide another probable mechanism governing
germination, although there has been little detailed work on the features of
photocontrol of germination of many small‐seeded rice weeds (Sanders,
1994). Gap‐detection mechanisms are moreover likely to confer fitness advan-
tages in a rice–weed community that will experience largely uniform canopy
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 187

0 112 5
9
6
3
0
200
150
100
50
00100 200 300
19 109 DAS
DAS
1.0
15 cm depth
5 cm depth
BC
1,000,000
100,000
10,000
1000
10
1
100
Wet seasonFallow
Seedlings
Dry season
Panicles Panicles
14
Seedlings
Seeds Seeds
Biomass (mg)Shoot height (mm)
Root and shoot
biomass (illuminated)
Structural seed
biomass
Under full illumination
In dark
Root and shoot
biomass (dark)
Thermal units (⬚C day)
Population size (m−2)
0.8
0.6
0.4
0.2
0.0
0100 200 300 400
Days from burial
Proportion
A
Crop Crop
Seedlings
Figure 6 (A) Flux in population size (log scale) of Echinochloa crus‐galli from an initial
infestation of 1000 seeds over two cropping seasons of wet‐seeded rice in the Philippines. Early
flooding (4 DAS, open circles) as opposed to late flooding (12 DAS, closed circles) causes death
in 50% of seedlings (dry season) and prohibits recruitment of successive seedling cohorts (wet
season). The population suVers 97% loss (by seed removal in harvesting and in fallow germina-
tion), accounting for the reduction in seedling population size at the start of the wet season, but
surviving plants compensate by increased seed production to return the Echinochloa crus‐galli
seed population to high densities again. From Mortimer (1998). (B) Seed bank dynamics
over two seasons. Seed populations were buried in open mesh bags at depths of 5 or 15 cm,
exhumed at regular intervals and tested for viability. From Mortimer (1998). (C) Seedling
growth responses (biomass and shoot height) in response to flooding. Germinated seeds were
placed in water columns containing nutrient solution to a depth of 50 mm and either illuminated
(light/dark) on a 12 h cycle (PAR 450 mEm
2
s
1
) or maintained in total darkness at a
temperature of 30/20 C day/night. From Mortimer et al. (2005). (Source: International Rice
Research Institute.)
188 A. N. RAO ET AL.

closure, often within 30–40 days. Equally evident is the fact that small‐seeded
species such as Cyperus diVormis exhibit polymorphisms in germination
response to flooding, providing ‘‘bet‐hedging’’ tactics against unpredictable
flooding events. Conversely, some species (F. miliacea, Echinochloa colona)
exhibit little or no innate or induced dormancy and germinate rapidly on
the surface of saturated soils but not under water (Kim and Moody, 1989).
As with temperate terrestrial weed species (Cousens and Mortimer, 1995),
there also appears to be no statistical relationship between seed fecundity
and seed weight when considering a range of weed species across several
taxa, although seed size and number vary over similar orders of magnitude
(Fig. 8).
Plants m−2 (log scale)
10
100
1000
10
100
1000
0 20 40 60 80 100 120
20 40 60 80 1000 120
Transplanted rice
Days after transplanting
Wet-seeded rice
Days after seeding
Broadleaf - shallow
Broadleaf - deep
Grass - shallow
Grass - deep
Sedges - shallow
Sedges - deep
Figure 7 Changes in size of weed communities (grouped into grasses, sedges and dicotyle-
donous weeds during a cropping season in transplanted or direct‐seeded rice). Plots were either
flooded to a depth of at least 10 cm (deep) or less than 5 cm (shallow) from 7 DAS/DAT for the
duration of a direct‐seeded or transplanted crop. No weeding was done. Data from Hill et al.
(2001). (Source: International Rice Research Institute.) (See Color Insert.)
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 189

III. INTEGRATING WEED MANAGEMENT PRACTICES
IN DIRECT‐SEEDED RICE
The traditional practice of puddling soil to kill existing weeds and aid
water retention, transplanting rice seedlings into standing water to achieve
an optimum stand density, and maintaining standing water to suppress
weeds, followed by one or several periods of manual weeding, is a well‐
established example of integrated weed management (IWM). It is integrated
in the sense that it involves preventive actions followed by precise interven-
tions as a response to the consequences of suites of preventive measures.
There is, to our knowledge, no definitive rigorous experimental disassem-
bling of this entire process because the experimentation involved would be
considerable. However, inference and prevailing farmer practice are suYcient
012345
Seed biomass, µg (log scale)
1
2
3
4
5
6
Fecundity seed per plant (log scale)
Ludwigia hyssopifolia
Ludwigia octovalvis
Echinochloa crus-galli
Monochoria vaginalis
Rotala indica
Leptochloa chinensis
Fimbristylis miliacea
Cyperus iria
Cyperus rotundus
Dactyloctenium aegyptium
Leersia hexandra
Rottboelia cochinchinensis
Eleusine indica
Echinochloa
colona
Cyanotis axillaris
Eclipta alba
Portulaca oleracea
Panicum maximum
Tridax procumbens
Scirpus grossus
Celosia argentea
Ischaemum rugosum
Commelina benghalensi
s
Cyperus difformis
Figure 8 Seed size fecundity relationships in selected seed‐producing weed species [Sources:
Kim and Moody (1989); Sanders (1994)].
190 A. N. RAO ET AL.

to groundtruth much of the assertion but not to quantify the magnitude
of the individual components. Tillage and puddling soil constitute a preven-
tive process which provides the multiple functions of vegetation clearance,
soil mixing, and reduction of water loss by downward percolation. In the
absence of tillage, a young transplanted crop is placed in a resource‐poor
habitat characterized by early intense weed competition. A second preven-
tive component is standing water above which a crop canopy matures,
resulting in a systematic light reduction, with canopy development confer-
ring resource‐advantage to the crop. Poor choice of initial stand density and
variable density of planting together with variability in water depth combine
to selectively alter niche dimensions for weed germination and establish-
ment, as discussed above. Manual weeding constitutes an intervention as a
consequence of the selective failure of preventive measures.
Classifying weed control measures as either preventive or interventionist
provides a way of evaluating control procedures in the context of both
their contribution to IWM and to the biology and ecology of target species.
Most definitions of IWM have two features in common: (1) the use of
multiple control tactics and (2) the integration of knowledge of weed biology
into the management systems (Davis and Ngouajio, 2005). Liebman and
Gallandt (1997) have described IWM as the choice and application of weed
management practices as ‘‘many little hammers,’’ which in combination pro-
vide crop protection from weed competition and suppress weed commu-
nities. Long‐term management of weed communities without excessive
reliance on a single method is a key feature of the strategy of IWM.
Rice crop establishment, whether by wet‐or dry‐seeding, represents a
major change in the habitat template that interspecifically governs the
recruitment of weed species. It is this phase which removes the preventive
eVects of standing water on weed recruitment, dramatically reduces the
seedling age diVerence between the crop and weeds, and places a premium
on early interventionist selective control that is achieved with chemical
control. This is not, however, to argue that preventive tactics do not play a
role in weed management in DSR. An integrated approach involving cultural
practices, crop rotation, stale seedbed practices, selection of suitable
competitive varieties, and the use of herbicide mixtures is essential in
responding to changes in weed community structure in DSR (Sharma,
1997; Yaduraju and Mishra, 2004). As discussed by many authors (Hassan
and Rao, 1996; Radosevich et al., 1997; Zimdahl, 1999), cultural methods
of weed control are preventive in nature since they function to enhance
crop growth by precision agronomy and in so doing maximize crop
competitiveness against weeds. In this chapter, we consider land prepara-
tion, water management, and choice of cultivar in the context of preventive
weed control practices before considering the use of herbicides and manual
weeding in the context of interventions.
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 191

A. PREVENTIVE METHODS OF WEED CONTROL
1. Land Preparation
Land preparation through tillage operations such as plowing, disking,
harrowing, soil puddling, and land leveling contributes to reduced weed
growth by providing weed‐free conditions at planting in addition to the
suppression of growth of certain perennial weeds (De Datta and Baltazar,
1996). These practices in a cumulative way cause cryptic seed and seedling
mortality at depths within the soil profile and influence the dormancy status of
buried seed populations through exposure to altered temperature (Forcella
et al., 1993), gaseous (Corbineau and Co
ˆme, 1995; Pons and Schroder, 1986)
and water regimes (Dekker, 1999). As a consequence, the frequency, depth,
and timing of cultivation operations are important in governing the emerging
weed flora both in the seedbed and in designing tillage operations to maximize
the value of stale seedbeds.
In Asia, conventional farm tillage practice for double‐cropped DSR often
involves a sequence of land cultivation between rice crops. Typically, this is
dry tillage after harvest followed by cycles of wet tillage, and subsequent
land leveling, before wet‐seeding of rice (Azmi and Mortimer, 1999). Earlier
work (Ho, 1996; Ho and Itoh, 1990) showed that populations of the peren-
nial grass Echinochloa stagnina may be considerably reduced by flooding and
puddling the field twice in contrast to a single round of dry rotovation prior
to seeding. In this case, the improved control was due to suppression of
regrowth from stolons in standing water.
Hach et al. (2000) confirmed that wet tillage in contrast to dry tillage
reduced the density of weed seedling populations in wet‐seeded rice to the
greatest extent but noted increased infestations of Echinochloa crus‐galli and
Paspalum distichum in the absence of tillage. Azmi and Mortimer (1999)
noted that M. vaginalis achieved greater abundance in the developing crop
after wet tillage prior to establishment, in contrast to sedges, in particular
F. miliacea, under dry tillage. They hypothesized that two cycles of wet
tillage before crop establishment were suYcient to break seed dormancy in
M. vaginalis, which requires exposure to anaerobic conditions to promote
germination (Yamasue and Ueki, 1983). Dry tillage enforced the dormancy
of F. miliacea through lack of soil moisture, prohibiting seed losses that
would normally occur under wet tillage.
As with wet‐seeded rice, it has been shown that increasing the number of
tillage operations before dry‐sowing of rainfed rice reduces weed infestation
and contributes to rice yield gain (Sharma, 1997). Bhagat et al. (1999)
reported that two plowings at the time of land preparation significantly
reduced broadleaf weed density compared with one plowing but further
increasing tillage frequency did not aVect groups of weed species significantly.
192 A. N. RAO ET AL.

Dry land tillage is the most common method of land preparation for
much dry‐seeded rice in the United States, southern Australia, most of Latin
America and West Africa, parts of tropical Asia, and most of Europe
(De Datta and Baltazar, 1996). It usually involves one to two plowings or
disking 10‐to 20‐cm deep followed by 2–3 harrowings with a spike‐tooth
or disk harrow and sometimes a roller‐packer to break up big clods and then
land leveling (Anon, 1990). Comparison of the change in weed flora with the
adoption of zero or reduced tillage techniques provides an indication of
the preventive control measures arising from conventional tillage practices,
given similar water regimes subsequently. Ipomoea wrightii Gray is one
species that was suppressed by conventional tillage but increased with zero
tillage (Gealy, 1998). Weed species shifts have been reported to occur with
the adoption of dry‐seeding and zero tillage, with annual grass weeds in
particular increasing in density (Tuong et al., 2005). Singh et al. (2005)
examined the weed flora of drill‐seeded rice after conventional and zero
tillage following shallow (1–3 cm) flooding up to 30 DAS. Under zero tillage,
Commelina diVusa and Cyperus rotundus became dominant weed species,
whereas Ischaemum rugosum, Leptochloa chinensis, and Eragrostis japonica
predominated after conventional tillage while the abundance of Paspalum
spp. and Cyperus rotundus decreased. Smith et al. (1993) reported reduced
infestation of aquatic weeds (e.g., H. limosa) under zero or minimum tillage.
In a comparison of a range of tillage regimes, Piggin et al. (2001) reported
that the abundance of F. miliacea and Leptochloa chinensis was not
influenced by tillage practices, whereas Echinochloa colona and Ludwigia
octovalvis increased in abundance on zero‐till and dry‐seeded plots. Tillage
method was also found not to aVect the incidence of R. cochinchinensis
(Janiya et al., 2001).
Stale seedbeds, also named the false seeding technique, refer to a cultural
method of weed control commonly applied in rice monoculture (Ferrero,
2003). After seedbed preparation, the land is left unsown to allow weed
emergence. The rice is then sown after weed removal by either mechanical
(harrows) or chemical (nonselective herbicides) means. The technique
reduces both the size of the soil seedbank and the emergent weed infestation.
The success of this technology depends as much on the eYcacy of practices
promoting weed germination as it does on nonselective mortality of emerged
weed seedlings. Kartaatmadja et al. (2004) observed that paraquat applied to
minimum tillage in irrigated lowland rice (1) eVectively controlled broad-
leaves and annual grasses such as Leptochloa chinensis and Echinochloa crus‐
galli, (2) had an eYcacy of 80% against sedges such as F. littoralis and
Cyperus diVormis, and (3) had only 20% eYcacy against perennial weeds
such as P. distichum. Minimum tillage results in a higher percentage of
germination of the weed seeds that are present in the upper soil layer,
compared with moldboard plowing (Ferrero and Vidotto, 1999), and in
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 193

Rio Grande do Sul, Brazil, about 250,000 ha are cropped using this tech-
nique (Noldin and Cobucci, 1999). Renu et al. (2000) observed that the use
of paraquat in a stale seedbed was more eVective than mechanical weeding in
dry‐seeded rice. Application of glyphosate before planting rice can reduce
labor input for weeding by 30–60% (Roder et al., 2001).
In Louisiana (USA), typically, water‐seeded rice fields are mechanically
tilled after flooding to destroy existing weed vegetation and create a uniform
seedbed. This cultural practice is generally referred to as ‘‘mudding in’’
(Bollich and Feagley, 1995). Public perceptions and increasing legislation
concerning water quality have resulted in a shift of some of the water‐seeded
rice area to varying levels of conservation tillage. Since 1998, no‐till and stale
seedbed conservation tillage area has fluctuated between 7% and 15% of the
water‐seeded rice (Anon, 1998, 2004). Leon (2005) examined imazethapyr
use in a water‐seeded system receiving no tillage or tilled in the water before
seeding and observed that (1) imazethapyr provides producers the option to
use conservation tillage in a water‐seeded rice production system and (2) the
benefits of reducing soil erosion and surface water contamination from
muddy‐water discharge at seedling establishment can be obtained in a stale
seedbed system without experiencing a decrease in weed control or rice yield.
2. Water Management
Water management is arguably the most important cultural practice in
DSR (Caton et al., 2002). Floodwater management aVects the density, vigor,
and uniformity of rice stands; the severity of weed competition; and the
eVectiveness of herbicides (Kim et al., 2001). The main reason that prompted
rice farmers in California to convert entirely from dry‐seeding to water‐
seeding in the early part of the twentieth century was primarily to manage
Echinochloa crus‐galli (Hill et al., 2001).
Water depth alone exerts a dominant eVect on the structure of weed
communities and the fate of weeds recruited into the growing crop (Janiya
et al., 1999). Early flooding, 4–5 DAS in contrast to 12 DAS, to a 5‐cm depth
reduced the density of Echinochloa crus‐galli (Dizon et al., 1999) as well as
other species (Chin et al., 2002; Hach et al., 1998). Bhagat et al. (1999)
concluded that, irrespective of herbicide application, continuous shallow
water ponding throughout the life of a wet‐seeded rice crop, as well as
up to panicle initiation, was eVective in reducing weed diversity, number,
density, and biomass compared with saturated soil maintained throughout.
Balasubramanian and Krishnarajan (2001) similarly recorded lower weed
growth with continuous submergence in wet‐seeded rice.
Flooding depth, however, has a diVerential eVect on the survival and
growth of weed species. Kent and Johnson (2001) reported that, compared
194 A. N. RAO ET AL.

to saturated soil conditions, flooding to 2–8 cm increased the density of
Sphenoclea zeylanica and Heteranthera callifolia Rchb. ex. Kunth. and
decreased that of Echinochloa colona and E. crus‐pavonis,withnoeVect on
F. miliacea. Where flooding of 4‐or 8‐cm depth was maintained for 2 or
4 days out of 7 days, there were greater densities of Ammannia prieuriana
Guill. & Perr. than in soil maintained in a saturated condition at the same
intervals. Increased flood duration from either 2 or 4 days out of 7 days to
continuous flooding increased the density of H. callifolia, had no eVect on
Sphenoclea zeylanica, but decreased plant numbers in Spilanthes filicaulis,
Echinochloa colona, and E. crus‐pavonis (Kent and Johnson, 2001). Gealy
(1998) observed that cultural practices such as deep‐flooding in both dry‐
seeded and water‐seeded rice may reduce morning‐glory (Ipomoea lacunosa
L. and I. wrightii Gray) infestations. This cultural practice suppresses the
germination and establishment of Echinochloa crus‐galli but promotes
aquatic weeds such as Cyperus diVormis, Damasonium minus, Sagittaria
montevidensis, and Alisma plantago‐aquatica (Seal et al., 2004).
Smith et al. (1977) observed that seed of H. limosa (Sw.) Willd. germi-
nated only in saturated or flooded soils due to low oxygen levels required
for germination (Marler, 1969). Baskin et al. (2003) have suggested that it
may be possible to reduce H. limosa infestation by exploiting the tempera-
ture response of the species by (1) flooding fields during the winter to
decrease dormancy break and (2) sowing rice as early as possible before
daytime temperatures reach 30–35C as very few seeds of H. limosa, flooded
during winter, gain the ability to germinate even under flooded conditions at
25/15C.
Cohort recruitment of weeds in dry‐seeded rice may extend over a longer
period than in wet‐seeded rice, depending on the time of initiation and depth
of flooding. In dry‐seeded rice of temperate regions, it is recommended that
the permanent flood be applied as soon as possible in order to suppress weed
seed germination. Typically, the permanent flood is applied 3–6 weeks after
planting (Rainbolt and Bennett, 2005).
3. Rice Cultivars
a. Weed Competitiveness.Interest in crop competitiveness as a weed
management tool in DSR has stemmed in part from the evolution of
weed resistance to herbicides and the lack of alternative control options
(Gibson et al., 2003), and also because many rice farmers have limited cash
and labor resources and are unable to invest in herbicides (Johnson et al., 1998).
Sanint et al. (1998) estimated that enhancing crop competitiveness against
weeds could reduce weed control costs by 30%.
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 195

The significance of crop interference with weed growth is historically well
established and variation in competitive ability has been established in many
crops (Berkowitz, 1998; Callaway, 1992). Caton et al. (2001) argued that,
even with this observation, breeding for crop competitiveness was rare
because (1) there was a lack of understanding about which traits conferred
competitiveness; (2) competitive processes are intrinsically dynamic and
changes in crop management, environment, and season markedly influence
outcomes; and (3) alternative weed management technologies oVered both
greater and more cost‐eVective weed control. Crop competitiveness may be
judged by either crop tolerance (the ability to maintain shoot or grain
biomass in the presence of weeds) or weed suppression (the ability to reduce
weed biomass or reproductive propagules) or both (Jannick et al., 2000). The
literature reflects arguments for and against (Callaway, 1992; Jordan, 1993)
the use of these measures which stem in part from the hypothesis that a
trade‐oVexists between the two (Jennings and Aquino, 1968; Kawano et al.,
1974). Subsequent analysis (KropVet al., 1993) and studies (Fischer et al.,
1997; Gealy et al., 2000; Johnson et al., 1998) have led to the conclusion that
trade‐oVs are not inherent and the extent to which the abilities of tolerance
or weed suppression are linked is as yet unresolved (Pester et al., 1999).
Harnessing competitiveness as a preventive weed control measure for DSR
may be achieved by focusing on both early vigor as well as traits influencing
competitiveness throughout the growth cycle. DiVerences between rice culti-
vars in their competitiveness with weeds have been reported from Asia (Caton
et al., 2003; Garrity et al., 1992; Zhao et al., 2006), Latin America (Fischer
et al., 1997; Kawano et al., 1974), the United States (Gealy et al., 2005b), and
Africa (Fofana and Rauber, 2000), and between O. sativa and Oryza
glaberrima rice (Johnson et al., 1998).
Zhao et al. (2006) reported that vegetative vigor scored at 2 weeks after
seeding and weed‐free yield accounted for 87% of the variation in yield
between cultivars in competition for weeds and that these two traits could
be eYcient means of indirect selection for improving rice yield in competi-
tion with weeds. Competitive ability in rice is often associated with traits
related to light interception and is correlated with height and leaf area
index (Garrity et al., 1992), droopy leaves, tiller production (Estorninos
et al., 2002, 2005; Fischer et al., 1997; Fofana and Rauber, 2000), higher
specific leaf area, earlier tiller production (Dingkuhn et al., 1999; Johnson
et al., 1998), root length density (Fofana and Rauber, 2000), and biomass of
roots and stems (Gealy et al., 2005b). Most studies have related to rice
grown under upland conditions, though Haefele et al. (2004) found that
height, tiller density, specific leaf area, leaf area index, and growth duration
were negatively related to yield loss under lowland conditions.
After studying the competitive relationships in a number of crops,
including rice, KropVet al. (1993) concluded that it was the morphological
196 A. N. RAO ET AL.

traits that contributed to early ground cover and height that were the most
important traits for competitiveness. Further, they suggested that ecophysio-
logical models may help to understand the integration of component
traits. After studying a range of O. sativa and O. glaberrima rice cultivars,
Dingkuhn et al. (1999) suggested that specific leaf area and tillering ability,
as major components of vegetative vigor, were predictive of competitiveness.
In addition, Asch et al. (1999) reported that the superior competitiveness of
O. glaberrima is partly due to the early onset of autotrophic growth, higher
partitioning coeYcients to laminae, and high specific leaf area. After study-
ing the growth of rice cultivars with and without competition from weeds,
Caton et al. (2003) reported that early vigor was highly repeatable and that it
could be used to discriminate between more and less competitive cultivars
even in monoculture.
The use of more competitive cultivars has been proposed as a tool to
improve weed control in water‐seeded rice (Dingkuhn et al., 1999; Fischer
et al., 1997; Gibson et al., 2001). It is well known (from early work,
Kira et al., 1953) that the asymptotic response of yield to sowing density is
resource dependent and that the competitiveness of a crop is inherently
related to stand density (Rainbolt and Bennett, 2005). Tropical rice systems
under precision resource management, high tillering rate, and consequent
rapid canopy closure (Peng et al., 1994) can allow wet‐seeded rice farmers to
use low seeding rates (50–80 kg ha
1
), especially where seed costs are high
(Luat et al., 1998). However, in the Philippines and Vietnam, high seed rates
(150 and 250 kg ha
1
) are used in wet‐seeded rice to suppress weeds in early
growth (Balasubramanian and Hill, 2002). While this may contribute to
weed management of some species, Gibson et al. (2001) found no significant
eVect of rice‐seeding rate in reducing Echinochloa growth in water‐seeded
rice. Some authors (e.g., Labrada, 2002) have argued that the use of high
seed density for weed management should be reconsidered in DSR, within
the context of integrated crop management. However, in resource‐poor
environments such as dryland rice, this may not prove to be a viable option
because it often leads to low and patchy stand establishment.
b. Submergence Tolerance.There has been renewed interest in the
potential for selecting rice germplasm that is tolerant of very early flooding
(48 h after germination) for use in direct‐seeding (Yamauchi et al., 1993),
particularly to control problematic grasses and sedges in wet‐and water‐
seeding (Hill et al., 2001).
Rice can germinate and develop a coleoptile in the absence of oxygen, and
there are genotypic diVerences in the rates of coleoptile, root, and leaf
development at low oxygen concentrations (Turner et al., 1981). In anaero-
bic conditions, rapid coleoptile elongation hastens the shoot emergence
from soil or water, leading to rapid oxygen transport to the apical meristem
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 197

(Yamauchi et al., 1995). Tolerance of early flooding, where rapid coleoptile
growth is considered important, therefore contrasts with tolerance of flood-
ing at the later stages of growth, when submergence injury is exacerbated
by rapid leaf extension and consumption of assimilates (Das et al., 2005;
Jackson and Ram, 2003).
Cultivars with a superior ability to germinate under anaerobic conditions
have been identified, and these have developed coleoptiles that are longer
and elongate faster than others (Yamauchi and Biswas, 1997). Anaerobic
germinability and flooding tolerance are complex traits, and it seems that
they are regulated by several genes (Fukao et al., 2003). Combining the
ability to germinate in anaerobic conditions with other key agronomic traits
will enable greater use of floodwater in IWM.
B. INTERVENTION METHODS OF WEED CONTROL
It can be argued that both chemical and manual means of weed control
are preventive in that they aim to prohibit weed increase in the long term.
However, the decision to intervene in the management of a crop and commit
expenditure to the application of a herbicide or to labor for mechanical or
hand weeding is a proximal one as, not least, it is seasonally dependent.
Although indirect (preventive) methods aim mainly to reduce the number of
weeds emerging in a crop, direct (intervention) methods also aim to increase
crop competitive ability against weeds (Barbery, 2003).
1. Manual and Mechanical Methods of Weed Control
In spite of the increasing herbicide use in weed control in DSR, hand
weeding is either partially or extensively practiced in countries of Asia, Latin
America, and Africa (Ahmed et al., 2001; Chin, 2001; Chin and Mortimer,
2002; de Dios et al., 2005; Fischer and Antigua, 1996; Islam et al., 2004;
Jashim et al., 2004; Makara et al., 2001; Noldin et al., 2004; Oteng and
Sant’Anna, 1999; Son and Rutto, 2002; Yaduraju and Mishra, 2004; Zhang,
2003).
In dry‐seeded rice in Africa, farmers normally rely on family labor for
weeding, which usually starts at 15–30 DAS and continues over many days
(Oteng and Sant’Anna, 1999). Similarly, in Cambodia manual weeding is
practiced at least twice and commonly three times (Makara et al., 2001) as is
done in flood‐prone areas of Bangladesh (Jashim et al., 2004). Weed control
in slash‐and‐burn rice production in northern Laos requires about
140–190 days ha
1
or 40–50% of the total labor input (Roder et al., 1997)
and probably represents one example of the maximum time allocated to a
198 A. N. RAO ET AL.

practice of crop protection. In these situations, manual weeding of rice is
done by hand or, if weeds are small, with simple tools. In wet‐seeded rice in
Vietnam, Chin et al. (2000a) considered that hand weeding twice was the
most eVective treatment in terms of both controlling weeds and crop safety
but noted that the labor cost was high and often prohibitive.
Manual weeding can be implemented only when weeds have reached a
suYcient size to be pulled, and it has an inherent opportunity cost. Manual
weeding is therefore often practiced late as evidenced by yield loss compar-
isons of the eVects of manual weeding at 21–30 DAS with those from the use
of early postemergence herbicides (Singh et al., 2005a). Labor scarcity, high
labor cost, poor weather conditions, and the presence of perennial weeds
that fragment on pulling may all lead to lowered eYcacy in weeding.
Mechanical weed control with the use of simple implements remains a
practical and economic method for many small and marginal farmers of
Asia and Africa. Mechanical weeding is almost universally practiced on row‐
seeded rice since interrow cultivation with either hand tools or animal
traction equipment reduces time in weeding and minimizes crop damage.
An exception to this is the practice of beushening in which dry‐seeded
broadcast rice is flattened by ‘‘planking’’ (drawing of a heavy flat wooden
object over the crop) after crop tillering has commenced. The process kills
weed species with single main stems, whereas rice is able to retiller from basal
nodes on stems pressed to the ground in addition to those at the plant base.
Comparisons of traditional and mechanically modernized weeding equip-
ment suggest the value of these technologies in resource‐limited farming
communities. Sarma and Gogoi (1996) reported that in rainfed upland rice
in India a manually operated peg‐type dryland weeder and a twin wheel hoe
were eVective in weed control when used twice at 20 and 30 days after
emergence. Another dryland weeder (with a straight‐line peg arrangement)
has also shown excellent performance across a range of soil types with
varying soil moisture levels and weed intensity providing a labor saving of
57% compared with hand weeding (127 person‐days ha
1
) (Subudhi,
2004).
Similar observations have been made elsewhere in identifying the practi-
cal eYcacy of mechanical weeders and economies in labor use. EVective
weed control has been demonstrated in Gambia, with a donkey‐drawn
implement (the Super‐Eco seeder), cultivating twice (21 and 42 DAS) with
the animal‐drawn hoe followed by selective hand pulling (Remington and
Posner, 2000). In Bangladesh, farmers’ principal weed management practices
in transplanted rice remain hand weeding and the use of a push weeder
(Ahmed et al., 2001). A rake‐type weeder developed in Bangladesh has been
shown to work in light and heavy soils and has a capacity of 0.04 ha h
1
(Islam et al., 2004). Power‐operated rotary weeders have improved weeding
eYciency (Victor and Verma, 2003). In northeast Thailand, a mechanized dry
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 199

DSR management system has been advocated, which includes weeding with a
soil cultivator involving tillage between rows of rice twice at 2 and 4 weeks
after seeding (Kabaki et al., 2003). In wet (row)‐seeded rice, an improved and
modified IRRI conoweeder (Parida, 2002) gave a weeding eYciency of 80%
during the first weeding, with a field capacity of 0.02 ha h
1
.
With labor scarcity, mechanical weed control methods may be more
eYcient and suitable in DSR as they help save labor. Hence, eVorts to improve
the eVectiveness of existing weeding tools and implements are desirable,
especially for developing countries.
2. Chemical Method of Weed Control
Labor unavailability, increasing labor costs, and the pressing need to raise
yields and maintain profit on a progressively limited land base have been
major drivers for farmers to seek alternatives to manual weeding. Herbicides
are one such alternative. EVective weed management practices are an impor-
tant prerequisite in DSR culture, with herbicide application seemingly indis-
pensable (Azmi et al., 2005). The trend for an increase in herbicide use has
been reinforced by the spread of DSR (Naylor, 1994). In Asia, herbicide use
grew dramatically from 1980 to 1995, with more than a threefold increase in
herbicide sales amounting to more than US$900 million per annum (Naylor,
1996). In the Americas, Australia, Europe, and East Asia, over 90% of DSR
areas are treated with herbicides (De Datta and Baltazar, 1996). Parts of
South and Southeast Asia and Latin America have seen moderate but fast
expansion in herbicide use. In Asia, more herbicide is used in irrigated DSR
than in rainfed DSR, with much less use in upland rice in tropical Asia and
Africa. From 1996 to 2003, however, there was a decline in the value of sales
at annual growth rates of –5.8% in North America, –3.6% in Latin America,
and –2.4% in Asia (Cropnosis, personal communication). This has been partly
due to the changes in the pricing structure of the herbicide market and also the
influx of generic products from China and India. Declines in sales of herbicide
volumes may, however, be a misleading indicator of herbicide use as there has
been a shift to low‐volume, sulfonylurea‐based products in many countries in
Asia, excluding Japan.
Herbicide options for weed control in DSR diVer according to method of
crop establishment because the performance of herbicides varies in relation
to water regimes. Extensive research has been conducted over the years by
many researchers to find out the optimum rate, time, type, and method of
herbicide application. This information is summarized in Table V.
a. Dry‐Seeded Rice.In irrigated dry‐seeded systems, 4–6 weeks may
elapse between planting and permanent flood establishment and controlling
200 A. N. RAO ET AL.

weeds during this period is critical to optimize grain yield. Thus, for dry‐
seeded rice, in general, two herbicide applications are recommended: one at
the dry period either just before or after rice emergence and the other at the
flood period (Kim and Ha, 2005). Pendimethalin, quinclorac, and thioben-
carb have residual activity and control annual grasses and some broadleaf
weeds (Jordan et al., 1998b; Smith and Hill, 1990). Pendimethalin and
thiobencarb can be applied after rice has imbibed water for germination
but before rice and weeds emerge (Jordan et al., 1998b). Quinclorac can be
applied preemergence, delayed preemergence, or postemergence. Although
these herbicides can provide season‐long barnyardgrass control in silt loam
soils (Helms et al., 1995), they generally do not provide control for an entire
season on alluvial clay soils (Jordan, 1997). The capacity of these soils to
change in volume by swelling and shrinkage allows exposure of nontreated
soil to conditions that promote weed germination and emergence from soil
below the zone treated with herbicide. Subsequent weed flushes are generally
controlled with postemergence herbicides (Jordan et al., 1998b) or manual
weeding. Failure to apply postemergence herbicide treatment may reduce
irrigated dry‐seeded rice yield by 9–60% (McCauley et al., 2005). Examples
of postemergence herbicides are acifluorfen, bensulfuron, bentazon, bispyr-
ibac, carfentrazone, clomazone, cyhalofop, 2,4‐D, fenoxaprop, halosulfuron,
molinate, propanil, quinclorac, and triclopyr. Bentazon, acifluorfen, bensul-
furon, 2,4‐D, and triclopyr target broadleaf species. DiYculties in achieving
grass and sedge weed control have led to a continued call for new graminicides.
Clomazone and halosulfuron are relatively recent introductions. Clomazone
provided the most consistent residual barnyardgrass control over a range of
environmental conditions and water management practices (Jordan and Kendig,
1998). Halosulfuron provides protection against Cyperus spp., including Cyperus
rotundus, with activity on broadleaf weeds also (Table V). The diverse weed flora
(terrestrial and aquatic) in dry‐seeded rice fields usually necessitated the use of
two or more herbicides for wide‐spectrum weed control (Gianessi et al., 2002)
and determining compatability of herbicides is important in developing manage-
ment strategies (Jordan, 1995). In Arkansas, propanil, molinate, clomazone,
quinclorac, glyphosate, triclopyr, pendimethalin, 2,4‐D, acifluorfen, and holo-
sulfuron are used in order of decreasing percentage area applied for dry‐seeded
rice (Gianessi et al., 2002) and, in Mississippi, clomazone, propanil, quinclorac,
2,4‐D, glyphosate, molinate, halosulfuron, and acifluorfen are used.
Chemical control in dry‐seeded rice has been inconsistent from site‐to‐site and
from year‐to‐year at the same site because of diverse interacting factors such
as varying weed species, weed populations, and soil and climatic conditions
(Ho, 1996). Therefore, flexibility in herbicide usage to suit conditions is critical
to the acceptance of herbicides (Moody, 1981; Sankaran and De Datta, 1985).
Chemical weed control is widely used in rice agriculture in Korea. Recom-
mendations for use (Kim and Ha, 2005; Kim et al., 2001) include (1) early
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 201

Table V
Herbicides and Their Combinations Reported to be EVective in Controlling Weeds in Direct‐Seeded Rice
Herbicide
Mode of action,
chemical group
a
Type of rice direct‐seeding Class of
weeds
controlled
b
(i) Application time
c
References
e
Dry‐seeded Wet‐seeded Water‐seeded (ii) Water regime
d
Cyhalofop‐butyl Inhibition of acetyl CoA
carboxylase (ACCase),
aryloxyphenoxy‐propionates
(FOPs)
þþ þG, B, CS (i) EPOE, POST Bocchi et al. (2005); Esqueda and
Tosquy (2004); Ferrero et al.
(2002); Ntanos et al. (2000);
Saini and Angiras (2002);
Subbaiah and Sreedevi (2000);
UOA (2005)
(ii) WDRA (EPOE),
ESMC (POST)
Fenoxaprop‐p‐
ethyl
As above þþ þG, CB, C (i) EPOE, POST Azmi and Mortimer (2002);
Karim et al. (2004); Saini and
Angiras (2002); UOA (2005);
Yang et al. (2004)
(ii) WDRA (EPOE),
ESMC (POST)
Quizalofop‐p‐
ethyl
As above þG (i) BSR Eleftherohorinos and Dhima
(2002)
Clefoxydim Inhibition of acetyl CoA
carboxylase (ACCase),
cyclohexanediones, lipid
biosynthesis inhibitor
(DIMs)
þþG (i) BSR, POST Anon (2003); Tabacchi and
Romani (2002)
Clethodim As above þG BSR Ferrero et al. (2002)
ImazethapyrInhibition of acetolactate
synthase (ALS)
(acetohydroxyacid synthase,
AHAS), imidazolinones
þþG, CB, CS (i) PPI fb POST
(in IT rice)
Pellerin and Webster (2004);
UOA (2005)
(ii) FARF (PPI)
Bispyribac‐
sodium
Inhibition of acetolactate
synthase (ALS),
(acetohydroxyacid synthase,
AHAS), pyrimidinylthio‐
benzoates
þþ G, B (i) EPOE Anon (2003); Chin et al. (2000a);
Fischer et al. (2004); Rainbolt and
Bennett (2005); Risi et al. (2004);
UOA (2005)
Pyribenzoxium As above þG, CB, CS (i) POST Baron (2005); Dasanayaka (2003)
202 A. N. RAO ET AL.

Azimsulfuron Inhibition of acetolactate
synthase (ALS)
(acetohydroxy‐acid synthase,
AHAS), sulfonylureas
þþ þ G, S (i) EPOE, POST Anon (2003); Bocchi et al. (2005);
Ferrero et al. (2002); Kim et al.
(2001); Tabacchi and Romani
(2002)
Bensulfuron‐
methyl
As above þþ þ B, S (i) EPOE, POST Azmi and Mortimer (2002);
Clampett and Stevens (2002);
Peterson et al. (1990); Pratley
et al. (2004)
(ii) HWAP
Chlorimuronþ
metsulfuron
As above þþ G, B (i) EPOE, POST Singh et al. (2006); Yaduraju and
Mishra (2004)
Cinosulfuron As above þþ þ S, B (i) EPOE, POST Azmi and Supad (1990); Bocchi
et al. (2005); Dasanayaka (2003);
Ferrero et al. (2002); Son and
Rutto (2002)
Ethoxysulfuron As above þþ þ B, S (i) EPOE, POST Bocchi et al. (2005); Ferrero et al.
(2002); Kolhe (1999); Moorthy and
Saha (2002); Saini and Angiras
(2002a); Singh et al. (2006)
Halosulfuron‐
methyl
As above þþB, S (i) POST Rainbolt and Bennett (2005);
Suarez et al. (2004); UOA (2005)(ii) DAFL
Imazosulfuron As above þþ (i) PPI, EPOE,
POST
Kim et al. (2001); Ottis et al. (2004).
Metsulfuron As above þG, B (i) POST Anon (2003); Bocchi et al. (2005);
Peterson et al. (1990)
Pyrazosulfuron‐
ethyl
As above þþ G, S, B (i) PRE, POST Anon (2003); Kim et al. (2001);
Moorthy (2002); Ooi (1988);
Yaduraju and Mishra (2004)
Metosulam Inhibition of acetolactate
synthase (ALS),
(acetohydroxyacid synthase
(AHAS), triazolopyrimidines
þB, S (i) POST Bocchi et al. (2005)
Penoxsulam As above þG, S, B (i) EPOE Lam et al. (2005)
Propanil Inhibition of photosynthesis at
photosystem II‐amides
þþ þ G, CB, S POST Ampong‐Nyarko (1996);
Bocchi et al. (2005); Fischer and
Antigua (1996); Hill and Fischer
(1999); Lo and Cheong (1995);
Navarez et al. (1979)
(continued )
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 203

Bentazon Inhibition of photosynthesis at
photosystem
II‐benzothiadiazinones
þþ þB, S (i) POST Anon (2003); Bocchi et al. (2005);
Ferrero et al. (2002); Fischer and
Antigua (1996); Rainbolt and
Bennett (2005); Taylor (2004)
(ii) PALW
Paraquat Photosystem‐I‐electron
diversion: bipyridiliums
þG, B, S (i) BSR Eleftherohorinos and Dhima (2002);
Lacy and Stevens (2005)
Acifluorfen Protoporphyrinogen oxidase
(PPO) inhibitors, diphenylethers
(chlorophyll synthesis inhibitor)
þþB (i) POST Smith and Hill (1990); UOA (2005)
Oxyfluorfen As above þþ B, G (i) PRE Anon (2003); Biswas et al. (1991);
Fischer and Antigua (1996);
Vongsaroj (1995)
Oxadiargyl Inhibition of protoporphyrinogen
oxidase (PPO): oxadiazoles
þG (i) PRE Chin et al. (2000a); Gnanasambandan
and Murthy (2002)
Oxadiazon As above þþ þG, B (i) BSR, PRE Adigun et al. (2003); Ampong‐Nyarko
(1996); Antigua (1993); Bhagat
et al. (1977); Bocchi et al. (2005);
Fischer and Antigua (1996);
Hassan and Rao (1996); Lim (1988)
(ii) MWAT
Carfentrazone‐ethyl Inhibition of protoporphyrinogen
oxidase (PPO), triazolinone
þþ þB, S (i) PRE
FLOOD/POST
Dasanayaka (2003); Rainbolt and
Bennett (2005); UOA (2005)
(ii). FFEW
Benzofenap Bleaching: inhibition of
4‐hydroxyphenyl‐pyruvate‐
dioxynase (4‐HPPD), pyrazoles
(chlorophyll synthesis inhibitor)
þB, S (i) BSR to EPOE Clampett and Stevens (2002);
Pratley et al. (2004); Skinner and
Taylor (2002)
(ii) AWMF
Clomazone Bleaching: inhibition of carotenoid
biosynthesis (unknown target),
isoxazolidinones
þþG, B (i) BSR, EPOE Anon (2003); Jordan et al. (1998);
Lacy and Stevens (2005);
Pegg et al. (2002); UOA (2005)
(ii) WMBA
Glyphosate Inhibition of EPSB synthase,
glycines (aromatic amino acid
biosynthesis inhibitor)
þþG, B (i) BSR Anon (2003); Eleftherohorinos and
Dhima (2002); Ferrero et al. (2002)
Table V (continued )
Herbicide
Mode of action,
chemical group
a
Type of rice direct‐seeding Class of
weeds
controlled
b
(i) Application time
c
References
e
Dry‐seeded Wet‐seeded Water‐seeded (ii) Water regime
d
204 A. N. RAO ET AL.

GlufosinateInhibition of glutamine synthetase,
phosphinic acids
þG, B (i) BSR Ferrero et al. (2002);
Lanclos et al. (2003)
Dinitramine Microtubule assembly inhibition,
dinitroaniline (cell division
inhibitor)
þG, B, S PRE Bhagat et al. (1977)
Pendimethalin As above þþ þG, CB (i) BSR, EPOE Ampong‐Nyarko (1996); Anon (2003);
Rainbolt and Bennett (2005);
Valverde et al. (2001); Vongsaroj
(1995); Yaduraju and Mishra (2004)
(ii) AFSF
Dithiopyr Microtubule assembly inhibition,
pyridines (cell division inhibitor)
þG, B (i) PRE Hassan and Rao (1996);
Malik et al. (2002)
Acetochlor Inhibition of cell division
(inhibition of very long fatty
acids), chloroacetamides
þ(i) BSR Eleftherohorinos and Dhima (2002)
Alachlor As above þG (i) BSR Eleftherohorinos and Dhima (2002)
Butachlor As above þþ G, CB (i) PRE Ampong‐Nyarko (1996); De Datta and
Bernasor (1973); Yaduraju and
Mishra (2004)
(ii) ESWA
Butachlor
þsafener
As above þþ (i) PRE Moorthy and Saha (2002);
Piggin et al. (2001)
Metolachlor As above þG (i) BSR Eleftherohorinos and Dhima (2002)
Pretilachlor As above þþ þG, S, CB (i) BSR, POST Angiras and Rana (1998); Bocchi et al.
(2005); Karim et al. (2004)
Pretilachlor
þsafener
As above þþ þG, CB (i) PRE Kyau and Win (2000); Moorthy and
Saha (1999); Ooi and Chong (1988);
Yaduraju and Mishra (2004)
Anilofos Inhibition of cell division
(inhibition of very long chain
fatty acids), others
þþ G, CB (i) BSR, PRE,
EPOE
Tamilselvan and Budhar (2001);
Yaduraju and Mishra (2004)
(II) ESWA
Dalapon Inhibition of lipid synthesis,
not ACCase inhibition:
chloro‐carbonic‐acids
þG (i) BSR Ferrero et al. (2002)
Dimerpiperate Inhibition of lipid synthesis:
not ACCase inhibition‐
thiocarbamates
þG (i) EPOE Bocchi et al. (2005);
Ferrero et al. (2002)
(continued )
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 205

Molinate As above þþ þG, B (i) PPI, POST Bocchi et al. (2005); Ferrero et al.
(2002); Fischer and Antigua
(1996); Hill and Fischer (1999);
Ho and Zuki (1988); Lo and
Cheong (1995)
Thiobencarb As above þþ þG, CB (i) BSR, PRE,
EPOE, POST
Antigua (1993); Clampett and
Stevens (2002); De Datta and
Bernasor (1973); Fischer and
Antigua (1996); Hassan and
Rao (1996); Hill and Fischer
(1999); Okafor (1986);
Singh (2005)
(ii) ESMC, FARF
2,4‐D Synthetic auxins (action like
indole acetic acid),
phenoxy‐carboxylic acids
þþ þB, S (i) POE Anon (2003); Antigua (1993);
Azmi and Mortimer (2002);
Fischer and Antigua (1996);
Kim et al. (2001); Singh (2005);
UOA (2005)
MCPA
sodium salt
As above þþB, S (i) POST Ampong‐Nyarko (1996);
Clampett and Stevens (2002);
Pratley et al. (2004)
(ii) PALW
Triclopyr Synthetic auxins (action
like indole acetic acid),
pyridine carboxylic acids
þB, S (i) POST Ferrero et al. (2002);
Jordan et al. (1998a); Lo and
Cheong (1995); Singh et al.
(2006); UOA (2005);
Valverde et al. (2001)
(ii) FDAA
Quinclorac Synthetic auxins (action like
indole acetic acid),
quinoline carboxylic acids
þþ þG, CB (i) PRE, EPOE,
POST
Abeysekera and Wickrama
(2005); Anon (2003); Bocchi et al.
(2005); Chang (1988); Fischer and
Antigua (1996); Valverde
et al. (2001)
(ii) DWPF
a
Herbicide classification by mode of action, 2003 Weed Science.org and Tomlin (1997).
Table V (continued )
Herbicide
Mode of action,
chemical group
a
Type of rice direct‐seeding Class of
weeds
controlled
b
(i) Application time
c
References
e
Dry‐seeded Wet‐seeded Water‐seeded (ii) Water regime
d
206 A. N. RAO ET AL.

application (0–10 days after flooding) of soil‐applied herbicides for grass weeds
such as butachlor, thiobencarb, dithiopyr, anilofos, molinate, esprocarb, pyra-
zolynate, pentoxazone, and several mixtures of these with bensulfuron‐methyl,
pyrazosulfuron‐methyl, and imazosulfuron; (2) an intermediate application
(15–25 days after flooding) of sulfonylurea mixtures for controlling annual
grasses and perennial sedges; and (3) late application (30–40 days after flood-
ing) of primarily foliar‐applied herbicide mixtures using bispyribac‐sodium,
cyhalofop‐butyl, fenoxaprop‐ethyl, pyribenzoxim with propanil, bentazon,
azimsulfuron, and ethoxysulfuron.
Drill dry‐seeding of rice using resource conservation technologies (RCTs)
such as the furrow‐irrigated raised‐bed planting system (FIRBS) is more
eYcient in irrigation water use than transplanted rice on puddled soil
(Balasubramanian et al., 2003). Singh et al. (2006) reported that, in the
FIRBS (1) ethoxysulfuron at 18 g a.i. ha
1
applied at 21 DAS was eVective
for controlling broadleaf weeds and (2) fenoxaprop‐p‐ethyl þethoxysul-
furon at 50 þ18 g a.i. ha
1
, applied at 21 DAS, and pendimethalin followed
by chlorimuron þmetsulfuron at 1000 fb 4 g a.i. ha
1
applied at 3 DAS
followed by 21 DAS gave broad‐spectrum weed control.
Remington and Posner (2000) observed eVective control of within‐row
weeds by broadcasting oxadiazon at 0.75 kg a.i. ha
1
at 1 DAS or by banding
b
G, grasses; CG, certain grasses; B, broadleaf weeds; CB, certain broadleaf weeds; S, sedges; CS,
certain sedges.
c
PPI, preplant incorporation; BSR, before seeding rice; PRE, preemergence (0–3 DAS); EPOE,
early postemergence (4–20 DAS); POE, postemergence (20 DAS and later).
d
AFGC, apply to flooded field and maintain flood until grass is controlled; AFSF, apply after
the first flushing and ensure that the soil surface is sealed by flushing or rainfall before applica-
tion. Apply a second flush or permanent water after 2 days but not later than 5 days after
application; AWMF, apply within 10 days of commencement of flooding. Water movement to
and within bays should cease 12 hours before application and for 5 days after application, but
maintaining permanent flood; DAFL, do not apply into flood; DWPF, in dry‐seeded rice, if
weeds emerge after preemergence application, rainfall or flushing may be required for activation
and reactivation; ESMC, excellent soil moisture is critical for good activity; ESWA, ensure
suYcient moisture at the time of application; FARF, flush for activation if rainfall does not
occur within a few days of planting. Repeat flushing as needed to keep soil‐applied treatment
active; FDAA, flood should be delayed 3 days after application; FFEW, postflood/POST to
exposed weeds: apply to rice and weeds after permanent flood and when 80% of the foliage of
the weeds is exposed; FLEW, the flood water must be lowered to expose small weeds for foliar
absorption of the herbicide; HWAP, as it is highly water soluble, avoid pumping water for
7 days after treatment; MWAT, maintain water after treatment for 14 days; PALW, prior to
application, lower water levels to expose more than two‐third of the weed growth to direct
contact with the spray; WDRA, works by direct contact with weeds. Re‐flood after 2 hours and
fill as soon as possible to limit germination of new weeds; WMBA, water movement must cease
before application and for 3 days after to ensure suYcient water to maintain permanent flood.
e
References are given here as examples.
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 207

thiobencarb and propanil at 0.72 and 1.30 kg a.i. ha
1
over the row at
21 DAS. Seed treatment with cinosulfuron at 0.2–0.6 g l
1
and optimal
N management have been reported to delay Striga emergence (Adagba
et al., 2002a,b).
The following treatments were eVective in controlling weeds in dry‐seeded
rice: pendimethalin or pretilachlor as preemergence application followed by
hand weeding in India (Singh, 2005); oxadiazon followed by one hand
weeding in Bangladesh (Mazid et al., 2005); oxadiazon, butachlor/2,4‐D,
and thiobencarb/propanil in Indonesia (Pane and Mortimer, 2002); 2,4‐D
and propanil in Laos (Roder et al., 2001); oxadiazon, butachlor, thioben-
carb, propanil, butachlor, bentazone, and oxadiazon or their combinations;
quinclorac; pretilachlor followed by bensulfuron‐methyl þthiobencarb;
pretilachlor followed by bensulfuron þquinclorac; pretilachlor þfenclorim;
cyhalofop; fenoxaprop‐p‐ethyl; oxaziclomefone; fentrazamide; thiobencarb
þbensulfuron; pretilachlor þbensulfuron; pendimethalin þbensulfuron;
and bispyribac‐sodium in China (Guan et al., 2004; He et al., 2000; Wang
et al., 2000). Bispyribac‐sodium when applied with thiobencarb or fenox-
aprop‐p‐ethyl was eVective against L. chinensis (Wang et al., 2000). Pretila-
chlor þbensulfuron‐methyl applied at 0 or 4 DAS was very eVective against
Echinochloa crus‐galli and L. chinensis (He et al., 2000). In Mexico,
clomozone controlled Echinochloa colona and partially controlled Scleria
setuloso‐ciliata but had no eVect on Cyperus iria and Cyperus rotundus
(Esqueda, 2000). A mixture of clomazone þ2,4‐Dþpropanil controlled
all weeds.
b. Wet‐Seeded Rice.Herbicides are considered indispensable for cost‐
eYcient weed control in wet‐seeded rice (De Datta et al., 1989). In general,
the choice of herbicide for wet‐seeding, where soil may be saturated or there
is standing water, is relatively narrow compared with dry‐seeding because
modes of action cannot necessarily rely on adsorption to soil particles or
uptake in an aqueous environment. For early applications (10–20 DAS),
molinate, dimepiperate, dymron, fenclorim, pyrazolate, mefenacet, cyhalofop‐
butyl, and pyriminobac‐methyl, applied singly or in a mixture with sulfonyl-
urea herbicide, are recommended. Subsequent application of herbicides at
30–40 DAS depends on the composition of surving weed flora (Kim and Ha,
2005). For late application, foliar‐applied herbicide mixtures are recommended
using bispyribac‐sodium, cyhalofop‐butyl, fenoxaprop‐ethyl, and pyribenzoxim
with propanil, bentazon, azimsulfuron, or ethoxysulfuron (Kim and Ha, 2005;
Kim et al., 2001). Mixtures combining graminicide with a herbicide for
sedges and broadleaf weed control such as quinclorac þbensulfuron, molinate
þbensulfuron, molinate þ2,4‐D, and thiobencarb þpyrazosulfuron are com-
monly recommended (Karim et al., 2004). In India, cyhalofop‐butyl followed
208 A. N. RAO ET AL.

by 2,4‐D was found eVective against mixed weed populations (Angiras and
Attri, 2002).
The success of weed control with herbicide is closely linked to water
management because of the precise chemical requirements for achieving
specificity in weed control and minimizing the risk of phytotoxicity to rice
(Hill et al., 2001). Foliar‐active herbicides (e.g., bentazon, 2,4‐D, triclopyr)
require spray contact with the leaf and require draining the field to
completely expose weeds to herbicide. Molinate needs to be applied into
water as application into drained paddies would result in its loss by volatility.
Sulfonylureas work best when applied into flooding water, with floodwater
acting as a carrier for their even distribution. Hach et al. (1997) concluded
that increased flooding depth enhanced the eYcacy of early postemergence
pyrazosulfuron‐ethyl (20 g a.i. ha
1
), a synergy not exhibited by butachlor
and thiobencarb. Metsulfuron methyl is being used by farmers to control
Marsilea minuta (Chin and Mortimer, 2002). The combination of presowing
treatment of pretilachlor at 0.6 kg a.i. ha
1
in flooded conditions, drainage
before sowing, and seed treatment with fenclorim (as safener) is eVective
in controlling weedy rice (Azmi et al., 2003), which is becoming a
major weed of wet‐seeded rice. Maneechote et al. (2004) reported that
quizalofop‐p‐tefuryl at 50 g a.i. ha
1
induced sterility of wild rice when
applied at either flowering or booting stage, thereby increasing rice yield.
In the Philippines, butachlor þsafener at 0.75 kg a.i. ha
1
and pretilachlor
þsafener at 0.3 kg a.i. ha
1
applied at 3 DAS eVectively controlled weeds
in the farmers’ practice (Tuong et al., 2000) and with controlled irrigation
(de Dios et al., 2005). The rate of pretilachlor þsafener can be reduced
from the recommended rate and still achieve adequate control of weeds by
flooding the field continuously with 2 cm of water (Janiya and Johnson,
2005).
Chemical weed control is practiced by 90% of the rice farmers in Sri Lanka
(Abeysekera, 1999) and 82% of the farmers in South Vietnam (Chin et al.,
2000a). Propanil, quinclorac, bispyribac‐sodium, and fenoxaprop‐ethyl are
most commonly used by farmers as per a survey in Sri Lanka (Sangakkara
et al., 2004). The use of a wider spectrum of chemical modes of action
may also help delay the development of herbicide resistance (De Datta and
Baltazar, 1996).
Mohankumar et al. (1996) reported that in wet‐seeded rice EC formula-
tions of preemergence herbicides could be eVectively applied by mixing with
sand instead of spray. Splash application is a new method of herbicide
treatment that uses a relatively small amount of water, 5–10 liters ha
1
(Lojo et al., 2001). The splash technique was reported to be simple, easy to
use, and fast, and it requires no spray equipment and improves farmers’
eYciency and applicator safety.
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 209

c. Water‐Seeded Rice.Water‐seeding rice into continuously flooded
fields was developed as a cultural control for severe infestation of barnyard-
grass and herbicides have become an integrated component of these rice‐
cropping systems (Hill et al., 1994). Herbicides are necessary to achieve
levels of weed control suYcient to maintain economic viability (Hill, 2000).
EVective control of weeds is dependent on herbicide rate, environmental
conditions, and growth stage at the time of application (Scherder et al.,
2004). Matching water management to the translocation characteristics of
the herbicide used is extremely important to the success of the application
(Hill and Fischer, 1999). For example, for triclopyr, a translocated herbicide,
only 70% of the foliage need to be exposed. Most of the postemergence
herbicides need to be applied to partially or fully drained fields to ensure full
coverage of weeds with herbicides. Fischer et al. (2004) observed synergism
on late watergrass (Echinochloa phyllopogon) in the field when 10 g a.i. ha
1
bispyribac‐sodium was mixed with 1120–2240 g a.i. ha
1
thiobencarb. This
synergism can have multiple benefits. The eYciency of control can be
enhanced using lower herbicide rates. Propanil plus molinate and benta-
zone were more compatible with fenoxaprop at 0.075 kg ha
1
for control
of barnyardgrass, whereas bensulfuron, carfentrazone, halosulfuron, and
triclopyr can antagonize fenoxaprop activity on barnyardgrass (Zhang et al.,
2005).
In water‐seeded systems of Arkansas, Louisiana, and Mississippi in the
United States, thiobencarb or molinate is applied before seeding to suppress
aquatic weeds and grasses. After water‐seeding, propanil, propanil þmoli-
nate and quinclorac are used to control grasses and broadleaf weeds. These
herbicides are often supplemented with broadleaf weed‐controlling herbi-
cides (Shipp, 2005). In California, where 95% of the rice area is water‐seeded,
propanil, molinate, thiobencarb, triclopyr, bensulfuron, fenoxaprop, 2,4‐D,
MCPA, and pendimethalin are used in order of decreasing percentage of
area applied (Gianessi et al., 2002). In water‐seeded rice in California,
Williams et al. (1990) evaluated the interaction of water depth from 5 to
20 cm, and the control of Echinochloa spp. improved from 16% to 77%.
Cyperus diVormis was also suppressed by deeper water. Although all flood-
ing depth treatments were improved by the application of herbicides,
shallow water treatments were much more dependent on the herbicides to
achieve high yields (Hill et al., 2001).
Common postemergence herbicides used in Europe are propanil, pre-
tilachor, thiobencarb, molinate, cyhalofop, fenoxaprop, azimsulfuron,
quinclorac, bentazon, bensulfuron, cinosulfuron, propanil, and 2,4‐D
(Bocchi et al., 2005; Gianessi et al., 2003; Su
¨rek, 2000). Pendimethalin,
thiobencarb, and molinate are used as preemergence herbicides. In fields
where propanil‐resistant barnyardgrass exists, an alternative strategy is to
apply molinate as preemergence followed by a postemergence application of
210 A. N. RAO ET AL.

quinclorac þazimsulfuron or cyhalofop (Gianessi et al., 2003). The farmers
in Turkey sometimes apply herbicides late or in high dosage. Some of them
try to control broadleaf or sedge weeds with propanil and inappropriate
herbicide application makes them apply herbicides two or more times
(Su
¨rek, 2000).
In Australia, thiobencarb for controlling barnyardgrass and benzofenap
for controlling Cyperus diVormis, Damasonium minus, Sagittaria montevi-
densis, A. lanceolatum,andAlisma plantago‐aquatica are recommended
(Clampett and Stevens, 2002). Bensulfuron‐methyl has been applied to
more than 90% of the New South Wales rice crop, most often in combi-
nation with molinate (Skinner and Taylor, 2002). The herbicides used in
Australian rice are bensulfuron, benzofenap, clomazone, molinate, thio-
bencarb, dicamba, MCPA sodium, glyphosate, paraquat, pendimethalin,
and propanil (Taylor, 2004).
C. DEVELOPING WEED MANAGEMENT FOR DIRECT‐SEEDED RICE
In spite of eVorts in developing and disseminating eVective management
strategies for crop protection from competition by weeds, weeds remain of
major importance in DSR. In this section, we discuss a range of issues
that are important in developing weed management for DSR both in terms
of the response of the flora and in the development of new and existing
technologies.
1. Grass Weed Control
Section II outlined the responses in the weed flora that arose with the
change to direct‐seeding in Asia, and many authors have highlighted the
need to focus on grass weed control, particularly for Echinochloa spp. and
weedy rice.
a. Echinochloa spp.The genus Echinochloa consists of 50 species that
exhibit polyploidy and ecotypic race diVerentiation and comprise both annual
and perennial species (Kim, 1994; Michael, 1983; Yabuno, 1983). It has long
been recognized as a contributor of weeds in both tropical and temperate
rice. Echinochloa crus‐galli (and closely related species) is more competitive in
direct‐seeded than in transplanted rice (Rao and Moody, 1987, 1992) in Asia
and is well known as a serious weed in a range of crops elsewhere (Barrett,
1983; Norris, 1992). Despite possessing a C
4
photosynthetic pathway, Echino-
chloa crus‐galli has evolved cold temperature adaptations and is native to
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 211

Europe, North America (Potvin and Simon, 1989; Robert et al., 1983), and
China (Zhang, 2003). Range expansion to West Africa has been reported
(Danquah et al., 2002). Despite its worldwide distribution and economic
importance, little is known about the population genetic structure of members
of the genus (Lopez‐Martinez et al., 1999). The closely related species Echino-
chloa phyllopogon and Echinochloa oryzoides constitute the main grass weed
species in California and Echinochloa phyllopogon has developed resistance to
most herbicides currently used by farmers (Fischer et al., 2000a,b), indicative
of genetic diversity. Resistance to butachlor has been reported in China
(Huang and Gressel, 1997) and resistance to propanil in Sri Lanka (Marambe
et al., 1997). Ecological amplitude (Mortimer, 2001), however, is characteris-
tic of members of this genus (Gibson et al., 2004), and several authors have
argued that there is a need to recognize the genetic structure and likely
phenotypic variability of target populations in developing appropriate man-
agement strategies (Danquah et al., 2002). Gibson et al. (2002, 2003) and
Gibson and Fischer (2001, 2004) have pointed to the importance of natural
resource management that favors resource acquisition by the crop at the
expense of the weed in this respect.
Analysis of the reasons why an individual species continues to renew
populations under weed management and persists in the face of control
measures can only come through understanding the population dynamics
of the species as argued earlier. Understanding the factors that govern
seedling recruitment from the soil seed bank and cohort establishment
within the rice crop in relation in particular to the water regime prior to
germination and during early establishment is an important research issue
in developing Echinochloa management in wet‐seeded rice. Equally, such
understanding may explain the diVering abundance of Echinochloa spp.
Echinochloa colona co‐occurs with Echinochloa crus‐galli in rice agroecosys-
tems but tends to dominate in rainfed agriculture. It is a plausible hypothesis
that the water profile (depth, duration, and frequency of flooding) immedi-
ately after land preparation and sowing interspecifically selects species. Pons
(1982) commented that seed burial in saturated soil conditions prohibited
Echinochloa colona germination, and Sahid and Hossain (1995) showed that
early flooding increased seedling mortality. A similar mechanistic explana-
tion can be advanced to explain the spatial distribution of Leptochloa
chinensis in rice fields, where dense infestations are more often observed on
land that has remained saturated but has not been flooded early (21–30 DAS)
after crop sowing. Pane and Mansoor (1994) have experimentally indicated
the subtlety of the interactions of plant size at time of flooding and depth in
determining seedling survivorship.
b. Weedy and Red Rice.Rice (O. sativa or other species) that is grown
unintentionally in and around cultivated rice‐growing areas is regarded as a
212 A. N. RAO ET AL.

weed (Vaughan and Morishima, 2003). Generally, the term ‘‘weedy rice’’
refers to populations of Oryza spp. that diminish farmers’ income both
quantitatively through yield reduction and qualitatively through lowered
commodity value at harvest (Baki et al., 2000).
Weedy rice poses perhaps the greatest threat to DSR because of the
close similarity between weedy forms and the cultivated crop. Watanabe
et al. (1997) recorded a yield loss of 60–74% in DSR, with an 35% weedy
rice infestation. Weedy rice phenotypes occur in more than 50 countries
in Africa, Asia, and Latin America (Valverde, 2005). Weedy rice infesta-
tions are reported for 40–75% of the rice area in European countries
(Ferrero, 2003), 55% in Senegal (Diallo, 1999), 80% in Cuba (Garcia and
Rivero, 1999), 60% in Costa Rica (Fletes, 1999), and 0.5–35.2% in southern
Korea (Kim and Ha, 2005). In Latin America (Noldin, 2000), weedy rice is
a much older problem than in Asia where it has been recorded at damag-
ing infestation levels in Vietnam (Mai et al., 2000), Malaysia (Azmi et al.,
2003), the Philippines (Fajardo and Moody, 1995; Moody, 1994; Rao and
Moody, 1994), Sri Lanka (DA, 1997), Korea (Choi et al., 1995; Kim and
Ha, 2005), and Thailand (Azmi et al., 2005). In Thailand, wild rice (Oryza
rufipogon GriV.), a close relative of cultivated rice, is a noxious weed in
fields of the central region (Maneechote et al., 2004). Invasion in some fields
has been so severe that the crop has had to be abandoned occasionally.
Similarly, a weedy form of rice (O. sativa Luolijing) has been reported to
seriously interfere with cultivated rice in Liaoning Province of China (Yu
et al., 2005).
Historically, perhaps the most well‐known form of weedy rice is red rice,
O. sativa possessing a caryopsis with a pigmented pericarp, which was
identified as a weed as early as 1846 (Craigmiles, 1978). The name red rice
is derived from the red color of pericarp, which, during milling, causes
contamination of commercial rice grain.
Red rice has been reported to contribute to lower grain quality in Greece
(Eleftherohorinos and Dhima, 2002; Eleftherohorinos et al., 2002), Turkey,
(Su
¨rek, 2000), and Portugal (da Silva and Rodrigues, 2000). It is a common
weed in most irrigated rice production areas in the Americas: Bolivia, Brazil,
Chile, Colombia, Guyana, Italy, the United States, and Venezuela (Ferrero
and Vidotto, 2002; Noldin, 2000). The status of red rice and its management
in the United States were reviewed by Noldin (2000) and Sadohara et al.
(2000). Rice growers in Louisiana (USA) have considered red rice of special
importance since no selective herbicides were available prior to the advent of
herbicide‐resistant rice (Williams et al., 2001). Gealy et al. (2000) reviewed
the scale of red rice infestations in the southern rice belt of the United States
and estimated that severe economic infestations occurred in 65% of the
rice area in Louisiana, 25% in Arkansas, Texas, and Missouri, and 15% in
Mississippi. The only areas where red rice is not considered a major problem
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 213

are California and Uruguay. In California, water‐seeding and the use of
certified seed have prohibited the ingress of weedy rice (Hill et al., 1994).
Until recently, red rice was considered to be taxonomically identical to
commercial rice. Genetic studies have shown that this classification is inade-
quate and that there are at least three distinct types of red rice (Vaughan
et al., 2001). Red rice accessions have varying degrees of relatedness with
cultivated rice (O. sativa subsp. indica and O. sativa subsp. japonica), and
wild rice spp. Oryza nivara and O. rufipogon (Vaughan et al., 2001). O. sativa
has a tendency to become weedy in areas where wild and cultivated rice grow
sympatrically. In such regions hybrids that compete with cultivated rice and
reduce yield may occur (Oka, 1988). However, weedy rice had also arisen in
areas without native wild rice populations (Vaughan and Morishima, 2003).
Weedy rice is postulated to be derived from (1) hybridization between
diVerent cultivars and between cultivars, and wild species, (2) through the
selection of weedy traits present in cultivars, and (3) by segregation from
land races. Microsatellite marker analysis (Gealy et al., 2002) comparing the
genetic relationships between populations of cultivars and weedy rice has
provided insights into their origin in DSR‐growing areas.
A global workshop on red and weedy rice control (FAO, 1999) recom-
mended integrated approaches that combine preventive, cultural, and chem-
ical methods. A key control measure is the use of clean and certified seeds.
In the United States, the commercial availability of imidazolinone‐tolerant
(IT) rice cultivars oVered the opportunity for selective chemical control of
red rice as well as other weed species (Ottis et al., 2004). However, rapid gene
introgression into rice (Burgos, 2005, personal communication) has now
constrained the utility of this germplasm.
Weedy rice has infested several rice‐growing areas in Malaysia, in particu-
lar in the MUDA region for the last decade, and rice farmers have resorted to
practicing manual weeding for control (Azmi et al., 2003). Water‐seeding and
control strategies combining preventive and cultural measures have also been
shown to be eVective (Azmi et al., 2003; Chin et al., 2000b). However,
designing and implementing strategies, particularly integrated weed control
tactics, are a real challenge as the biological characteristics of weedy rice are
similar to those of cultivated rice (Valverde, 2005). Vaughan et al. (2001) have
emphasized that, since red rice is much more diverse than previously assumed,
this diversity must be considered when developing management strategies.
2. Weed Management Technologies
a. Improving Herbicide Use.Direct‐seeding systems are less robust
than transplanting as the control of elements of soil moisture, irrigation,
drainage, and weed control are more critical in DSR for successful crop
214 A. N. RAO ET AL.

establishment and growth. With management playing a more decisive role in
crop establishment and weed control, direct‐seeding can be described as a
‘‘knowledge‐intensive’’ practice (Johnson and Mortimer, 2005). The transi-
tion from traditional transplanting to direct‐seeding renders ineVective the
experience of traditional rice production systems with their reliance on
indigenous knowledge and manual inputs. Instead, substantial information
is required to enable farmers to judge the best technological options, espe-
cially during the transition from transplanting to direct‐seeding. In DSR, the
timing and rate of application of herbicides need to be more precise than in
the transplanted system. Improving farmers’ knowledge can substantially
raise productivity, as was shown in the Sahel, where farmers were able to
improve yields by 1.0 ton ha
1
by combining appropriate timing and dose of
herbicide application (Haefele et al., 2000). Such changes had a benefit‐to‐
cost ratio of more than 4, but they did require the acquisition and under-
standing of appropriate knowledge of both weed and crop management.
The challenge for researchers is to analyze the existing knowledge gap,
synthesize available knowledge, develop location‐specific practical solutions,
and make them available to users.
Rice varieties display diVerential sensitivities to a number of herbicides in
relation to the soil environment and water regimes postapplication (Jordan
et al., 1998a). For example, clomazone has been identified for weed control
in dry‐seeded rice in the southern United States (Zhang et al., 2004) and is
currently used in both dry‐seeded and water‐seeded rice (Scherder et al.,
2004). Although rice has shown acceptable tolerance of clomazone (Mitchell
and Gage, 1999), substantial injury can occur (Bollich et al., 2000; Jordan
et al., 1998) especially on soils low in clay content.
While not causing mortality, high doses of oxadiargyl (150 g a.i. ha
1
)
can cause crop injury and presowing application of oxadiargyl with sub-
sequent flooding may reduce selectivity in wet‐seeding (Gitsopoulos and
Froud‐Williams, 2004). Butachlor at 1.5 kg ha
1
without a safener may
damage wet‐seeded rice seedlings when applied at 2 or 7 DAS, and toxicity
is significant at the later application time (Angiras and Rana, 1998). Similar-
ly, anilofos þ2,4‐D ethyl ester at 0.40 þ0.53 kg ha
1
applied at 5 DAS
may damage wet‐seeded rice (Behera and Jena, 1997; Choudhary and
Thakuria, 1998). In wet‐seeded rice, phytotoxicity has also been observed
with oxyfluorfen at 0.17 kg a.i. ha
1
(Natarajan and Kuppuswamy, 1997),
butachlor at 1, 1.5, and 2 kg a.i. ha
1
(Angiras and Rana, 1998; Mathew
and Jagadeeshkumar, 1999), and anilofos at 0.3–0.4 kg a.i. ha
1
applied at
3 DAS (Mathew and Jagadeeshkumar, 1999; Sreedevi et al., 2001) and
8 DAS (Madhavi and Reddy, 2002). Informing farmers of the phytotoxic
eVects of herbicides and the potential for rice recovery is of special relevance
as direct‐seeding is adopted.
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 215

b. Herbicide Resistance in Weeds and Its Management in Direct‐Seeded
Rice.The evolution of herbicide resistance is now a common and undesirable
feature of most cropping systems—313 herbicide‐resistant biotypes across 183
weed species have been reported at the time of writing (www.weedscience.org/
in.asp). Weed species resistant to herbicides have been reported in countries
with high herbicide adoption rates, including the Philippines (Migo et al.,
1986), Malaysia (Azmi and Baki, 2003; Kone et al., 2001), Japan (Itoh et al.,
1999), Sri Lanka (Marambe et al., 1997; Sangakkara et al., 2004), Thailand
(Maneechote et al., 2005), Korea (Kim and Ha, 2005; Kim et al., 2001),
Colombia and Costa Rica (Rubin, 1997), Italy, Portugal, Spain, France, and
Greece (Bocchi et al., 2005; Ferrero and Nguyen, 2004; Ntanos, 2001), North
and Central America (Fischer et al., 2000b; Heap, 1997), and Australia
(Graham et al., 1996; Pratley et al., 2001, 2004). Weeds of DSR that are
resistant to herbicides in diVerent countries are summarized in Table VI.
While herbicides remain inexpensive and eVective options, sustainable use
requires a prudent mix of agronomic practices and the rotation of herbicides
(Kone et al., 2001), and the use of herbicide mixtures with diVerent modes of
action (Schmidt, 1997). The theory behind the evolution of herbicide resis-
tance is well understood (Cousens and Mortimer, 1995; Gressel and Segel,
1990). However, resistance prevention ultimately depends on knowledge
dissemination by a diversity of routes, and not least the product supply
chain. In turn mitigating measures, when herbicide resistance has occurred,
are essential. For example, in managing propanil‐resistant Echinochloa
colona, pendimethalin has been considered an excellent partner for propanil
to prevent propanil resistance evolution and as an alternative product when
propanil resistance has already evolved (Riches et al., 1997). Table VII
illustrates options for controlling herbicide‐resistant weeds.
c. Herbicide‐Resistant Transgenic Rice and Weed Management in Direct‐
Seeded Rice.Rice containing transgenes that impart resistance to postemergence,
nonselective herbicides such as glyphosate and glufosinate allow farmers use
of reduced‐or no‐tillage cultural practices, and may potentially reduce the in-
tensity of herbicide application while controlling nearly the entire spectrum of
weed species (Duke, 1999).
Three herbicide‐tolerance systems are being developed in rice: Clearfield
(nontransgenic technology, providing tolerance of imidazolinones; Stidham
and Singh, 1991), Liberty Link (transgenic technology with resistance to
glufosinate) and, to a lesser extent, Roundup Ready rice (transgenic tech-
nology with resistance to glyphosate) (Williams et al., 2002). However, no
transgenic rice cultivar has yet been approved for commercial cultivation
anywhere in the world. Clearfield rice was registered in 2001 and fully
216 A. N. RAO ET AL.

Table VI
Herbicide Resistance Among Major Weeds of Rice in DiVerent Countries
Weed species Resistance to the herbicide Country (year of report)
Alisma plantago‐
aquatica
Bensulfuron‐methyl and
cinosulfuron
Italy (1994)
Alisma plantago‐
aquatica
Bensulfuron‐methyl Portugal (1995), Spain
(2000)
Ammannia coccinea Bensulfuron‐methyl USA (2000)
Bacopa rotundifolia Bensulfuron‐methyl Malaysia (2000)
Bacopa rotundifolia Bensulfuron‐methyl,
metsulfuron‐methyl, and
pyrazosulfuron‐ethyl
Malaysia (2001)
Bidens pilosa Metsulfuron‐methyl China (1999)
Cyperus diVormis Bensulfuron‐methyl USA (1993), Australia
(1994), Spain (2000)
Cyperus diVormis Cyclosulfamuron and
pyrazosulfuron‐ethyl
Brazil (2000)
Cyperus diVormis Azimsulfuron, bensulfuron‐
methyl, and cinosulfuron
Italy (1999)
Cyperus diVormis Azimsulfuron, bensulfuron‐
methyl, cinosulfuron,
cyclosulfamuron,
ethoxysulfuron,
halosulfuron‐methyl,
imazosulfuron, and
pyrazosulfuron‐ethyl
South Korea (2002)
Damasonium minus Bensulfuron‐methyl Australia (1994)
Echinochloa colona Propanil Colombia (1988), Costa
Rica (1987), El Salvador
(1999),
Guatemala (1999),
Honduras (1999),
Panama (1999),
Venezuela (2000)
Echinochloa colona Quinclorac Colombia (2000)
Echinochloa colona Azimsulfuron, fenoxaprop‐
p‐ethyl and propanil
Costa Rica (1998)
Echinochloa colona Fenoxaprop‐p‐ethyl Nicaragua (2000)
Echinochloa crus‐galli Quinclorac Brazil (1999), USA (1998)
Echinochloa crus‐galli Butachlor China (1993)
Echinochloa crus‐galli Thiobencarb China (1993)
Echinochloa crus‐galli Propanil Greece (1986), Italy (2000),
Sri Lanka (1997), USA
(1990)
Echinochloa crus‐galli Butachlor and propanil Thailand (1998)
Echinochloa crus‐galli Cyhalofop‐butyl,
fenoxaprop‐p‐ethyl and
quizalofop‐p‐tefuryl
Thailand (2001)
Echinochloa crus‐galli Propanil and quinclorac USA (1999)
(continued)
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 217

Table VI (continued)
Weed species Resistance to the herbicide Country (year of report)
Echinochloa crus‐galli Cyhalofop‐butyl,
fenoxaprop‐p‐ethyl,
molinate, and
thiobencarb
USA (2000)
Echinochloa phyllopogon Thiobencarb USA (1998)
Echinochloa phyllopogon Fenoxaprop‐p‐ethyl USA (1998)
Echinochloa phyllopogon Bispyribac‐sodium USA (2000)
Echinochloa oryzicola Cyhalofop‐butyl,
fenoxaprop‐p‐ethyl,
molinate, and thiobencarb
USA (2000)
Fimbristylis miliacea Pyrazosulfuron‐ethyl Brazil (2001)
Fimbristylis miliacea 2,4‐D Malaysia (1989)
Ischaemum rugosum Fenoxaprop‐p‐ethyl Colombia (2000)
Leptochloa chinensis Clefoxydim, fenoxaprop‐p‐
ethyl, and quizalofop‐p‐
ethyl
Thailand (2002)
Limnocharis flava 2,4‐D Indonesia (1995)
Limnocharis flava 2,4‐D and bensulfuron‐
methyl
Malaysia (1998)
Limnophila erecta 2,4‐D, bensulfuron‐methyl,
cinosulfuron
Malaysia 2003)
Lindernia dubia Bensulfuron‐methyl and
pyrazosulfuron‐ethyl
Japan (1996)
Lindernia dubia Azimsulfuron, bensulfuron‐
methyl, cinosulfuron,
cyclosulfamuron,
ethoxysulfuron,
halosulfuron‐methyl,
imazosulfuron, and
pyrazosulfuron‐ethyl
South Korea (2000)
Lindernia procumbens Bensulfuron‐methyl, and
pyrazosulfuron‐ethyl
Japan (1997)
Monochoria vaginalis Bensulfuron‐methyl South Korea (1999)
Rotala indica Bensulfuron‐methyl Japan (1998)
Rotala indica Imazosulfuron South Korea (2002)
Sagittaria guayanensis Bensulfuron‐methyl Malaysia (2000)
Sagittaria montevidensis Bensulfuron‐methyl Australia (1994), USA
(1993)
Sagittaria montevidensis Bispyribac‐Na,
cyclosulfamuron,
ethoxysulfuron,
metsulfuron‐methyl, and
pyrazosulfuron‐ethyl
Brazil (1999)
Sagittaria pygmaea Sulfonyl urea herbicides South Korea (2005)
Scirpus juncoides Bensulfuron‐methyl Japan (1998)
Scirpus juncoides Azimsulfuron, bensulfuron‐
methyl, and
pyrazosulfuron‐ethyl
South Korea (2001)
(continued)
218 A. N. RAO ET AL.

released by BASF in 2002. So far, Newpath (imazethapyr) is the only
imadazolinone herbicide registered in the Clearfield system.
i. Imidazolinone‐tolerant rice. Rice tolerance of imidazolinone herbi-
cides was developed from a single plant that survived a chemically induced
mutation trial in 1993 (Sanders et al., 1998). This rice line is considered
nontransgenic because it was developed through seed mutagenesis and not
through gene transfer. Imazethapyr is a broad‐spectrum herbicide that con-
trols many annual and perennial grass and broadleaf weeds preemergence or
postemergence in drill‐and water‐seeded rice (Kent et al., 1991; Williams
et al., 2002).
In both systems, imazethapyr eVectively controlled Echinochloa crus‐galli
(91–98% mortality) with 70 or 87 g ha
1
soil applied followed by 70 or53 g ha
1
early postemergence or late postemergence application (Pellerin and Webster,
2004). Pellerin et al. (2004) concluded that, in drill‐seeded rice, a preemer-
gence application of imazethapyr followed by a postemergence herbicide
mixture of imazethapyr plus bentazon plus acifluorfen, carfentrazone, halo-
sulfuron, or propanil plus molinate would provide total weed control for
grass (including red rice) and broadleaf weeds. Because of the possibility of
outcrossing between IT rice and red rice, Ottis et al. (2004) argued the need to
apply the maximum allowable imazethapyr rate of 70 g a.i. ha
1
in two
separate applications to ensure complete control of red rice, even on coarse‐
textured soils. As mentioned earlier, widespread outcrossing of imazethapyr
resistance has now been detected in red rice (Shivrain et al., 2007).
ii. Glufosinate‐resistant rice. The rice varieties Gulfmont and Koshihi-
kari possess the bialaphos resistance (BAR) gene for glufosinate resistance
through genetic engineering (Agracetus, Inc., 1991), and if glufosinate‐
resistant rice is released, it is anticipated that lines will be developed from
medium‐grain variety BNGL‐62 (Lanclos et al., 2003; Webster et al., 2003).
Glufosinate is a nonselective herbicide that controls many grass and broadleaf
Table VI (continued)
Weed species Resistance to the herbicide Country (year of report)
Scirpus mucronatus Azimsulfuron, bensulfuron‐
methyl, cinosulfuron, and
ethoxysulfuron
Italy (1994)
Scirpus mucronatus Bensulfuron‐methyl USA (1997)
Scirpus planiculmis Sulfonyl urea herbicides South Korea (2004)
Solanum
phoreinocarpum
Metsulfuron‐methyl China (1999)
Sphenoclea zeylanica 2,4‐D Malaysia (1995), Philippines
(1983), Thailand (2000)
Source: References cited in the review and www.weedscience.org/in.asp
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 219

weeds (Ahrens, 1994). It has been evaluated extensively for weed control in
rice (Braverman and Linscombe, 1994; Hatzois, 1998; Lanclos et al., 2002).
Postemergence application of glufosinate provides eVective control of
most weeds, including red rice, with little injury to glufosinate‐resistant rice
(Lanclos et al., 2003).
In glufosinate‐resistant rice, glufosinate at 0.6 kg ha
1
controlled red rice
(Sankula et al., 1997) and at 0.42 kg ha
1
controlled barnyardgrass and
broadleaf signalgrass [Brachiaria platyphylla (Griseb.)] Nash (Lanclos et al.,
2002). Control increased with the addition of propanil (Lanclos et al., 2002;
Table VII
Alternative Herbicides Identified for Managing Herbicide‐Resistant Weeds
Herbicides Weeds controlled Country References
Oxadiazon, thiobencarb,
fentrazamide, oxadiargyl,
butachlor; simetryn,
carfentrazon‐ethyl,
pyriminobac‐methyl,
bezobicyclon
Sulfonylurea‐
resistant weeds
South Korea Kim and Ha (2005)
Bispyribac‐sodium Thiobencarb,
molinate and
fenoxaprop‐ethyl‐
resistant
Echinochloa crus‐
galli and
Echinochloa
phyllopogon
USA De Witt et al. (2002)
Bentazon/MCPA 2,4‐D and
sulfonylurea‐
resistant weeds
Malaysia Azmi (2003)
Quinclorac, bispyribac‐
sodium
Propanil‐resistant
Echinochloa crus‐
galli
Sri Lanka Marambe and
Amarasinghe
(2002)
Imazethapyr Propanil‐resistant
Echinochloa crus‐
galli and Urochloa
platyphylla
USA Scherder et al. (2001)
Quinclorac þpropanil Propanil‐resistant
Echinochloa colona
Costa Rica Valverde et al. (2001)
Pendimethalin Propanil‐resistant
Echinochloa colona
Costa Rica Riches et al. (1997);
Valverde et al.
(2001)
Benzofenap Bensulfuron‐
resistant weeds
Australia Pratley et al. (2001,
2004)
Piperophos and anilofos Propanil‐resistant
Echinochloa colona
Costa Rica Valverde et al.
(1997, 1999)
220 A. N. RAO ET AL.

Sankula et al., 1997), acifluorfen at 0.6 kg ha
1
(Sankula et al., 1997) or
propanil plus molinate (Lanclos et al., 2002). Antagonism was observed for
control of barnyardgrass with mixtures of glufosinate at 0.42 kg ha
1
with
bensulfuron, halosulfuron, and quinclorac (Lanclos et al., 2002).
d. Gene Flow and Its Implications in Weed Management.The use of
herbicide‐resistant GM crops poses the risk of gene flow via pollen and seed,
resulting in (1) contamination of nearby non‐GM crops with the transgene,
(2) establishment of herbicide‐resistant volunteer weeds in the crop field and
nearby non‐cropland, and (3) unintended and unanticipated eVects on close-
ly related species (Cohen et al., 1999; Kwon et al., 2001).
Although rice is primarily a self‐pollinated crop, cultivars in farmers’
fields are rarely isogenic at all loci and cross‐pollination occurs at a suYcient
frequency to cause hybrids. Moreover, wild rice occurs sympatrically with
cultivated rice and each is sexually compatible (Song et al., 2003). Weedy rice
in particular often grows within cultivated rice fields (Messeguer et al., 2004).
Experimentally, Song et al. (2003) demonstrated that gene flow from
cultivated rice to the wild species O. rufipogon occurred at a considerable
rate (around 3%) and at a distance up to 43 m. In China, Lu et al. (2002) and
Chen et al. (2004) confirmed that cultivated rice and its wild relative
O. rufipogon had a sympatric distribution and overlapping flowering time,
which met the spatial and temporal conditions necessary for transgene
escape from cultivated rice to wild relatives, and that most of the AA
genome wild Oryza spp. had relatively close biosystemic relationships and
could cross with cultivated rice, particularly O. rufipogon, O. nivara,and
O. spontanea (weedy rice). However, other studies have reported a lower rate
of gene flow from transgenic rice to red rice (Zhang et al., 2003) and
conventional rice (Messeguer et al., 2004).
Transgene escape may become a much more serious problem where
weedy rice is abundant because its life cycle within the cropping system is
closely aligned with the crop. Because weedy rice is a global constraint to rice
production, the merit of transferring herbicide‐resistance genes into modern
rice varieties needs careful evaluation. Recognizing this threat, Valverde and
Gressel (2005) have called for genetic mitigation constructs in developing
new lines of herbicide‐resistant rice.
Certainly, one of the possible and immediate risks is herbicide‐resistant
GM crops becoming volunteer weeds (Kwon and Kim, 2001). Dry direct‐
seeded rice is one of the most likely crops in this respect. In the event of
commercialization of glufosinate‐resistant rice, IWM programs would
therefore need to be adopted to control gene transfer (Sankula et al., 1998).
Control strategies such as tillage, competitive crops in rotation and the use of
herbicides with diVerent modes of action represent mainstays in this regard
and emphasize the ‘‘knowledge‐intensive’’ nature of this technology.
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 221

3. Potential Weed Control Methods
a. Allelopathy.Perusal of the literature on weed management in rice
indicates that there has been a long interest in the potential role of allelopa-
thy in weed management. Rice cultivars with allelopathic ability, that is,
producing root exudates that target competing species, have long been
hypothesized to occur. Extensive evaluation of the allelopathic potential of
rice germplasm in drill‐seeded systems began in the early 1980s, and rice
cultivars were reported to suppress H. limosa and partially suppress Echino-
chloa crus‐galli in the United States (Dilday et al., 1998, 2001; Gealy et al.,
2003, 2005a) and Asia (Olofsdotter et al., 2002). Allelopathy has been a
contentious area of ecological research since its inception, and Harper (1977)
made the point that allelopathy in the field has proved extraordinarily
diYcult to illustrate, and stated that ‘‘it is logically impossible to prove
that it does not happen and perhaps nearly impossible to prove absolutely
that it does.’’ He went on to point out that almost any plant species could, by
appropriate digestion, extraction, and concentration, be persuaded to yield a
product that was toxic to one species or another. Common laboratory
screening of rice cultivars for allelopathy uses a ‘‘relay seeding technique’’
the results of which are correlated with field studies of rice competitiveness
against selected weed species (Olofsdotter, 2001). Numerous studies on
allelopathy have continued, and rice accessions with putative allelopathic
potential have been identified (Ahn et al., 2005; Dilday et al., 1994; Gealy
et al., 2003, 2005a; Olofsdotter et al., 2002). Much of this work ignores the
findings of Stowe (1979), who concluded that the results from bioassay
methods were highly variable and that incidence of autotoxicity was as
high as allotoxicity. As a consequence, it is likely that allelopathy as
measured in bioassays does not occur in the field. Further, he concluded
that ‘‘bioassays never adequately simulate natural conditions, and the inves-
tigator can neither demonstrate allelopathy if the results are positive, nor
discount them if they are negative.’’ Further complications arise from the
diYculties of separating the individual eVects of plant interactions. It can be
hypothesized that an allelopathic trait may have evolved in response to
resource (light, water, nutrients, space) competition. Is competition required,
however, for allelopathic ability to be expressed and, if so, can researchers
distinguish between the outcome of resource competition and allelochemical
interactions? Williamson (1990) argued that the application of Koch’s postu-
lates was required to establish proof of allelopathy. In this test, the putative
chemicals would be added on a rate release basis and, to remove all alternative
interactions, the allelopathic plant would be absent. Until such critical experi-
ments are undertaken, the significance of allelopathy in the field can only be
inferred and remains conjecture. This, however, has not halted discussion of
the potential value of allelopathic rice cultivars as a component of IWM
222 A. N. RAO ET AL.

(Duke et al., 2000, 2002; Jensen et al., 2001; Kim and Shin, 2005; Ni and
Zhang, 2005; Olofsdotter, 2001).
b. Bioherbicides.Biological control can be divided into the ‘‘classical’’
and ‘‘bioherbicide’’ approaches (Hallett, 2005). The classical approach
involving the use of exotic predators or pathogens has not been implemented
in rice. However, there has been greater research interest in the development
of bioherbicides. This technology commonly seeks to exploit the use of a
naturally occurring coevolved plant pathogen which is applied at inundative
levels in a simple formulation.
The status of bioherbicides has been reviewed by a number of authors
(Charudattan, 2001; Hallett, 2005; Li et al., 2003), and some eVective biocon-
trol agents are listed in Table VIII. Although a substantial number of patho-
gens for target species have been identified, relatively few have been
commercialized. Colletotrichum gloeosporiodes (Penz.) Penz. & Sacc. aeschy-
nomene (Collegot) was registered in 1982 for the control of northern join-
tvetch [Aeschynomene virginica (L.) B.S.P.] in rice in the United States.
Puccinia canaliculata (Biosedget) was shown to control Cyperus esculentus
L. and limit new tuber formation by 66% (Boyetchko, 1997), though it has yet
to be commercialized (Li et al., 2003). Damasonium minus (R. Br.) Buch is an
important native plant species considered to be the most important weed in
rice‐growing areas of Australia. Rhynchosporium alismatis (Oudem.) J. J.
Davis is a coevolved pathogen of both these species and others in the Alisma-
taceae (Jahromi et al., 2004), and leaf necrosis occurs on leaves of Damaso-
nium minus, Sagittaria guyanensis H.B.K., Alisma lanceolatum L., and Alisma
plantago‐aquatica L. (Cother et al., 2002). Farzad et al. (2001) concluded that
R. alismatis had characteristics to be a successful mycoherbicide as it can be
readily multiplied in artificial culture. It has also been reported to have a
synergistic eVect with bensulfuron‐methyl (Cother et al., 2002).
Good control of Sphenoclea zeylanica with Alternanthera alternate (Fr.)
Keissler f. sp. sphenocleae has been reported (Mabbayad and Watson, 1995;
Masangkay et al., 1999) for specific humidity and temperature regimes.
Similarly, foliar application of conidial suspensions of Curvularia tuberculata
Jain and Cyperus oryzae Bugnicourt killed seedlings of Cyperus diVormis,
C. iria,andF. miliacea (Luna et al., 2002a,b). Exserohilum monoceras and
Cochliobolus lunatus have been reported to kill Echinochloa sp. after 14 days
leaving rice unaVected (Chin, 2001). Spraying of the fungus Setosphaeria
rostrata on Leptochloa chinensis resulted in almost complete leaf death
(Thi et al., 1999). Synergism has also been reported between Setosphaeria
rostrata and pyrazosulfuron ethyl for L. chinensis control (Chin et al., 2002).
Drechslera monoceras hasbeentestedonEchinochloa crus‐galli in rice; better
control being observed under low (25/15C, day/night) temperatures (Hirase
et al., 2004).
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 223

Table VIII
Potential Biocontrol Agents Reported to be EVective in Controlling Weed Species
Associated with Rice
Weed species
Potential biocontrol
agent Country References
Alismataceae weeds Rhynchosporium
alismatis
Australia Cother et al. (2002)
Alternanthera
philoxeroides
Fusarium sp. China Tan et al. (2002).
Cyperus diVormis Curvularia tuberculata Philippines Luna et al. (2002a,b)
Cyperus diVormis Curvularia oryzae Philippines Luna et al. (2002a,b)
Cyperus esculentus Dactylaria higginsii USA Kadir and Charudattan
(2000)
Cyperus globulosus Dactylaria higginsii USA Kadir and Charudattan
(2000)
Cyperus iria Dactylaria higginsii USA Kadir and Charudattan
(2000)
Cyperus iria Curvularia tuberculata Philippines Luna et al. (2002a,b)
Cyperus iria Curvularia oryzae Philippines Luna et al. (2002a,b)
Cyperus rotundus Dactylaria higginsii USA Kadir and Charudattan
(2000)
Damasonium minus Rhynchosporium
alismatis
Australia Cother et al. (2002);
Jahromi et al. (2001,
2004)
Echinochloa crus‐galli Exserohilum monoceras China Huang et al. (2001)
Echinochloa crus‐galli Drechslera monoceras China Huang et al. (2001)
Echinochloa crus‐galli Exserohilum monoceras Vietnam Chin (2001)
Echinochloa crus‐galli Cochliobolus lunatus Vietnam Chin (2001)
Echinochloa crus‐galli Exserohilum monoceras Philippines Zhang and Watson (1997)
Echinochloa
glabrescens
Exserohilum monoceras Philippines Zhang and Watson (1997)
Echinochloa spp. Drechslera monoceras Japan Hirase et al. (2004, 2004a)
Eichhornia crassipes Fusarium pallidoroseum India Praveena and Naseema
(2003)
Eichhornia crassipes Myrothecium advena India Praveena and Naseema
(2003)
Fimbristylis miliacea Curvularia tuberculata Philippines Luna et al. (2002a,b)
Fimbristylis miliacea Curvularia oryzae Philippines Luna et al. (2002a,b)
Hydrilla verticillata Plectosporium tabacinum USA Smither‐Kopperl et al.
(1998)
Kyllinga brevifolia Dactylaria higginsii USA Kadir and Charudattan
(2000)
Leptochloa chinensis Setosphaeria rostrata Vietnam Chin et al. (2003); Thi et al.
(1999)
Sagittaria trifolia Plectosporium tabacinum Korea Chung et al. (1998)
Sphenoclea zeylanica Colletotrichum
gleosporiodes
Philippines Bayot et al. (1994)
Sphenoclea zeylanica Alternanthera alternata Philippines Mabbayad and Watson
(1995); Masangkay et al.
(1999)
224 A. N. RAO ET AL.

Despite the demonstrated eYcacy of some bioherbicides under laboratory
test conditions, adoption has been rare. An intrinsic constraint of inundative
bioherbicides is that they most commonly target a single weed species in a
weed community and moreover that species itself will have coevolved with
the pathogen. In consequence genetic variability in resistance to the bioher-
bicide is to be expected and evolution of resistance may be an expected
response. Hallett (2005) concluded that the current status of bioherbicide
applications does not promise to make significant advances unless the per-
formance of bioherbicides themselves can be reliably enhanced. With more
eVort devoted to developing techniques for the cultural and genetic enhance-
ment of bioherbicidal organisms, they may have a role in future weed
management systems.
IV. FUTURE RESEARCH NEEDS
Weed infestation is a major threat to yield and further expansion of DSR
throughout the world. In most developed nations, direct‐seeding is the sole
method of rice establishment and is reliant on mechanization and close
attention to weed management. While shortage of labor in Asian agricul-
ture is encouraging the adoption of direct‐seeding, so too is the need to im-
prove water productivity (Tuong et al., 2005). ‘‘Aerobic’’ rice (Bouman,
2003) may reduce water consumption by up to 50% (Yang Xiaoguang
et al., 2002). Sustainable rice production by direct‐seeding therefore will
critically depend on ensuring that on‐farm weed management technologies
are implemented in the absence of a historical free public good—standing
water for weed suppression. This represents a paradigm shift in the applica-
tion of weed control measures for rice farmers that have solely transplanted
in the past. At the research and extension level, it places a premium on the
design of integrated management measures to respond to weed species shifts
arising from changes in crop establishment and/or the evolution of herbicide
resistance. Some future research needs focusing on this issue are highlighted
below, in addition to those discussed earlier.
1. Characterizing scales of yield loss: Knowledge of the yield losses at farm
level in developing countries due to imperfect weed control remains
incomplete, as too, do the underlying reasons for them. Adoption of
DSR is likely to increase the variance in these yield losses since it is likely
that knowledge transfer for successful cropping may be incomplete.
Information on farmers’ resource base and knowledge will be needed to
successfully implement suitable weed management practices. In the first
instance, studies may best be conducted in selected agroecosystems
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 225

in order to provide broad‐based understanding on which location‐specific
options can be based.
2. Understanding the role of water management on weed population dynamics:
Water management plays an important role in weed control in direct‐
seeded rice, yet greater understanding is still required to be able to
maximize the role of water in weed management in itself and in combina-
tion with herbicides. For example, alternate wetting and drying by con-
trolled irrigation has water‐saving potential (B. A. M. Bouman, personal
communication), yet at the same time, this procedure will have consider-
able impact on the germination and establishment of weeds. Relevant
research in this area includes (1) understanding the relationship between
water depth/drainage and weed recruitment and (2) identifying rice germ-
plasm and practices to improve rice‐seedling survival and emergence
under flooding (Jackson and Ram, 2003). The corollary at the extension
level includes perfecting decision tools to target particular weeds species
and improving infrastructure at the field level to allow for precision
flooding, uniform water depths, and drainage.
3. Developing cultivars with competitive ability: Improved competitive ability
of rice cultivars, as a component of IWM, is yet to be exploited fully.
Vigorous, early vegetative growth has been identified as the key charac-
teristic (Asch et al., 1999), but there remains much to be learnt about
the characteristics that impart competitiveness, the component traits of
vegetative vigor, possible trade‐oVs, and the relative importance of these
in diVerent cropping environments and management systems (Caton,
2002). Unraveling these relationships requires mechanistic process‐based
modeling coupled with detailed experimentation as emphasized by many
authors (Bastiaans et al., 1997; Caton et al., 1999a,b; Weigelt and JolliVe,
2003). Several studies (Assemat et al., 1981; Gibson et al., 1999; Perera
et al., 1992) have suggested that root competition plays a major role
in the interaction between rice and Echinochloa spp. Gibson et al.
(1999) found that the primary mechanism by which water‐seeded
rice reduced watergrass growth was through competition for nitrogen.
Gibson and Fischer (2001) opined that for substantial Echinochloa ory-
zoides control, early nutrient deprivation by rice roots may be as relevant
as improving rice ability to intercept light. Rice cultivars can diVer in root
growth and morphology (Slaton et al., 1990) and nutrient‐uptake rates
(Teo et al., 1995). The relationship between rice root growth, competitive
ability of rice for resources against weeds, and rice yields deserves more
attention.
4. Identifying crop rotations and management practices for reducing weed
infestations: Rotation of crops with diVerent planting dates and growth
periods, contrasting competitive characteristics, and dissimilar man-
agement practices are well known to arrest the development of weed
226 A. N. RAO ET AL.

communities. Knowledge‐intensive RCTs including zero tillage, the
FIRBS, and rotary tillage have been developed for the rice–wheat system
in the Indo‐Gangetic plains (Singh et al., 2006). Singh et al. (2005a) have
shown that diVering tillage practices in wheat selectively alter the relative
abundance of grass and sedge weeds in the succeeding rice crop. The extent
to which rotation of DSR methods (dry and wet) has an impact on rice
weed communities remain unclear as does the influence of dry season
nonrice‐cropping practices. Further understanding will arise from long‐
term studies of weed communities coupled with autecological examination
of selected target species.
5. Evolving eVective herbicides and IWM: The use of herbicides will increase
with the expansion of DSR and the decreasing availability of irrigation
water, and with the development and adoption of reduced‐tillage systems.
There is an urgent need to optimize their use not only as a response to
regulations and the public concern about pesticide residues in food and
water, and to minimize possible adverse eVects on the environment, but
also to ensure that herbicides will remain an eVective and valuable tool to
farmers of DSR in the future. EVective, safe herbicides and mixtures are
required to enable farmers to use them as components of IWM. Contin-
uous monitoring to identify the emergence of new and diYcult‐to‐control
weed problems such as weedy rice and the evolution of herbicide‐resistant
weed biotypes is necessary if farmers are to be provided with timely alter-
natives. The challenge for weed management research in DSR is to develop
control strategies that sustain and enhance farm profits while safeguarding
the environment and human health. The key to the success of DSR is the
availability of eYcient weed control techniques to use as components of
IWM. Due attention is needed to identify, streamline, and adopt the ways
and means to increase farmers’ participation in developing ecologically and
economically viable IWM systems for DSR.
6. Molecular biology: Molecular methods are vital to providing a better
understanding of the potential for gene flow that might result from the
introduction of GM crops, such as herbicide‐tolerant rice varieties, in addi-
tion to elucidating the genesis of weedy rice as discussed earlier. Taxonomic
uncertainty among certain weed species (e.g., the Echinochloa spp. com-
plex) remains an additional constraint in understanding the reasons for
resilience to control and improved understanding of the genetic structure
of grass populations and their evolutionary responses to control will
underpin the development of improved weed management practices.
7. Improving decision making: Direct‐seeded rice is by nature knowledge
intensive, and ensuring that relevant, beneficial, and cost‐eVective deci-
sions are made at the farm level is prerequisite for both farm profitability
and sustainable weed management. Decision making at strategic (long
term), tactical (seasonal), and operational levels in the field (short term)
WEED MANAGEMENT IN DIRECT‐SEEDED RICE 227

ensures appropriate responses to a variable weed flora, safe use of chemi-
cal weed control in areas where water has a multiplictity of uses (domestic
and agricultural) and prohibition of herbicide resistance evolution.
In many instances, understanding of farmer decision making in these
contexts is scant, particularly in developing countries, and there is con-
siderable scope for research into eVective transfer and utilization of
knowledge (Orr, 2003).
8. Understanding the impact of global climate change: Rapid and simulta-
neous changes in temperature, precipitation, and the atmospheric con-
centration of CO
2
are predicted to occur over the next century (IPCC,
2001). As discussed in an earlier section, competitive grass weeds with a
C
4
pathway (e.g., Echinochloa spp., Paspalum spp.) are expected to
increase with the adoption of DSR. Competitiveness could be enhanced
in a C
3
crop (rice) relative to a C
4
weed (Echinochloa glabrescens) with
elevated CO
2
alone, but that simultaneous increases in CO
2
and temper-
ature could still favor a C
4
species (Alberto et al., 1996). It is generally
thought that global warming will favor C
4
species relative to C
3
species,
and indeed the distribution of many tropical and subtropical C
3
and C
4
weeds appears to be limited by low temperature although some C
4
weeds are highly successful in northern latitudes (e.g., Amaranthus spp.,
Echinochloa crus‐galli). Temperature sensitivity to growth may limit pole-
ward range expansion of some rice weeds, for example, R. cochinchinensis
(Patterson, 1993), but increased CO
2
may in itself allow such extension.
Elevated CO
2
has been found to increase tolerance to low temperatures in
several weed species (Potvin and Strain, 1985). Relatively little is known
about patterns of phenological development, flowering time, and seed
production in rice weed species in relation to changes in temperature and
CO
2
. Flowering can be faster, slower, or unchanged at elevated CO
2
depending on species (Patterson, 1995), and more rapid emergence of
weed seedlings at elevated CO
2
has been documented under field condi-
tions (Ziska and Bunce, 1993), tending to occur in small‐seeded species.
Weed scientists have as much a duty to future generations to assess the
likelihood of range expansion and altered crop competitive pressure in
weed species as do crop scientists in evaluating crop performance in
monoculture in response to climate change.
ACKNOWLEDGMENTS
The authors are grateful to Cropnosis Limited for providing data on herbi-
cide sales; Drs. Y. Singh, M. M. Kyu, H. Pane, and A. Abeysekera for their
advice on the distribution of weed species; Professor Robert E. L. Naylor,
228 A. N. RAO ET AL.

University of Aberdeen, UK; Drs. N. T. Yaduraju and H. Pathak, Indian
Agricultural Research Institute, India for providing critical and constructive
comments on the chapter; and Dr. Bill Hardy, Senior editor, IRRI for editing
the chapter.
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