Weed harrowing in winter cereal under semi-arid conditions

Abstract

Five field experiments on barley and wheat have been carried out in North-Eastern Spain on the same field during the cropping seasons 1999-00 to 2003-04 to compare the effect of different harrowing adjustments on weed control, weed biomass and cereal yield. The variables considered were harrowing timing (pre- or early post-emergence), one or two passes, travelling direction, harrowing depth and speed compared with an untreated control and herbicide. Excepting year 2001, with very little weed emergence, mechanical control as a whole caused a significant weed plant reduction compared to the untreated plots in all years. No influence of harrowing depth and travelling speed and of pre-emergence harrowing were found in the trials. A single harrowing treatment conducted across the sowing direction gave the same or less control compared to harrowing along the sowing direction. Two harrowing passes achieved a higher efficacy than one single pass and little differences were detected if the second pass was conducted the same day, across the sowing direction or 15 days later. Despite herbicide had generally a higher efficacy than the harrowing treatments, in three out of five years it was found a mechanical control with the same control than herbicide. The effect of the different treatments on weed biomass was lower than on weed number and no significant differences were found for grain yield. Considering that an herbicide treatment in the present conditions is three times more expensive than harrowing, a single post-emergence harrowing can be considered a valid option for low and medium-infested cereal fields.

Full text

Instituto Nacional de Investigación y Tecnología Agraria y Alimentación (INIA) Spanish Journal of Agricultural Research 2008 6(4), 661-670
Available online at www.inia.es/sjar ISSN: 1695-971-X
Weed harrowing in winter cereal under semi-arid conditions
G. Pardo1*, A. Cirujeda2, J. Aibar3, J. Cavero4and C. Zaragoza2
1EUITA Universidad de Sevilla. Carretera de Utrera, Km. 1. 41013 Sevilla, Spain.
2CITA- DGA. Apdo727. 50080 Zaragoza. Spain.
3Escuela Politécnica Superior de Huesca. Ctra de Zaragoza, Km 67. 22071 Huesca, Spain.
4EE. Aula Dei. CSIC. Apdo. 202. 50080 Zaragoza. Spain.
Abstract
Five field experiments on barley and wheat have been carried out in North-Eastern Spain on the same field during the
cropping seasons 1999-00 to 2003-04 to compare the effect of different harrowing adjustments on weed control, weed bio-
mass and cereal yield. The variables considered were harrowing timing (pre- or early post-emergence), one or two passes,
travelling direction, harrowing depth and speed compared with an untreated control and herbicide. Excepting year 2001,
with very little weed emergence, mechanical control as a whole caused a significant weed plant reduction compared to the
untreated plots in all years. No influence of harrowing depth and travelling speed and of pre-emergence harrowing were
found in the trials. A single harrowing treatment conducted across the sowing direction gave the same or less control com-
pared to harrowing along the sowing direction. Two harrowing passes achieved a higher efficacy than one single pass and
little differences were detected if the second pass was conducted the same day, across the sowing direction or 15 days later.
Despite herbicide had generally a higher efficacy than the harrowing treatments, in three out of five years it was found a
mechanical control with the same control than herbicide. The effect of the different treatments on weed biomass was lower
than on weed number and no significant differences were found for grain yield. Considering that an herbicide treatment in
the present conditions is three times more expensive than harrowing, a single post-emergence harrowing can be conside-
red a valid option for low and medium-infested cereal fields.
Additional key words: barley, durum wheat, flex-tine harrow, mechanical weed control.
Resumen
Control mecánico de malas hierbas en cereal de invierno en condiciones semiáridas
En este trabajo se muestran los resultados de cinco experimentos de campo localizados en el noreste de España desde
1999-00 hasta 2003-04. En ellos se comparó el efecto de distintas formas de control mecánico de la flora arvense con grada
de varillas flexibles sobre la densidad y biomasa de la misma y sobre la producción de cereal comparado con un control
sin tratar y el uso de herbicidas. Las variables consideradas fueron: momento y número de pases de grada, profundidad,
dirección y velocidad de los pases. Excepto en 2001, con una baja infestación, el control mecánico redujo la densidad de
arvenses en comparación con las parcelas sin tratar. La profundidad y la velocidad de la labor no tuvieron influencia clara
y realizar el pase en preemergencia del cereal resultó ineficaz probablemente debido a la falta de humedad. Dos pases
resultaron más eficaces que uno y se encontraron pocas diferencias si el segundo se hacía en el mismo día, perpendicular
o 15 días después. Aunque el herbicida tuvo generalmente mayor eficacia que los tratamientos mecánicos, en tres años se
encontró un tratamiento mecánico con eficacia similar. El efecto de los distintos tratamientos sobre la biomasa de arven-
ses fue menor que sobre el número de las mismas y no hubo diferencias significativas sobre la producción. Dado que el
tratamiento herbicida en las condiciones del ensayo fue tres veces más caro que el tratamiento mecánico, un único pase en
post-emergencia puede ser una opción válida con infestaciones medias o bajas de malas hierbas.
Palabras clave adicionales: cebada, grada de varillas flexibles, trigo duro.
Abbreviations used: a.i. (active ingredient), BBCH (Biologische Bundesanstalt Chemical), df (degrees of freedom), MCPA (2-methyl-
4-chlorophenoxyacetic acid), 2,4-D (2,4-dichlorophenoxyacetic acid).
* Corresponding author: gpardo@us.es
Received: 28-11-07. Accepted: 28-10-08.

Introduction
Herbicides have played a key role in the advancement
of agriculture in industrialised countries since they are
efficient controlling weeds, are selective, fast-acting
and relatively cheap. However, nowadays there is a need
to consider alternatives to chemical control because
weeds may develop resistance against almost any herbi-
cide (Heap, 2008) and an abusive use can contribute to
environmental contamination (Garrido et al., 1998).The
important reduction of accepted active ingredients in
the European Union leads to consider alternative weed
control methods. In addition, the general public is more
concerned about agrochemical residues in the environ-
ment and, more recently, in the agricultural products it-
selves. To reduce these problems, mechanical weeding
is an alternative tool which can be used in combination
with herbicides, with other cultural methods as rotations
or alone. Moreover, the changes in the European Com-
mon Agriculture Policies enhance farmers to adopt
more environmentally-friendly practices.
The flex-tine harrow is widely used for mechanical
weed control in organically grown cereals. It tills the
soil surface superficially, so that weed seedlings are
uprooted and covered by soil. Some authors have sug-
gested that the most important effect is covering (Jones
et al., 1995), while in other experiments uprooting was
probably the main cause (Kurstjens and Kropff, 2001;
Cirujeda et al., 2003). Uprooting and covering may not
completely kill the weeds, but the damage caused may
be enough to slow down their growth (Lambain et al.,
1993) although Rasmussen (1992) did not find that sur-
viving weeds had a lower competitive ability.
Authors agree in the large amount of factors affecting
harrowing efficacy. Some are the weed species, pheno-
logical stages of the weed and crop, soil type and mois-
ture, weather conditions and the way tillage is carried
out (Böhrnsen, 1993) including driving speed, tine
angle, etc. However, contradictory results can be found
in the literature for many of these parameters. For exam-
ple, concerning the effect of speed on weed control,
Rasmussen and Svenningsen (1995) and Kurstjens and
Perdock (2000) related efficacy with higher speed. Ras-
mussen (1990), however, did not find a clear relations-
hip between both. Also Cirujeda et al. (2003) did not
find efficacy increases when comparing 2, 5 and 8 km
h-1 and Rydberg (1993) described the biggest weed
reduction at 5 km h-1 and increasing harrowing speed up
to 9 or 13 km h-1 did not improve weed control but could
affect grain yield.
Concerning the direction of harrowing, most trials
have been conducted along the crop plant rows. Few
experiments reported results of trials harrowing across
the sowing direction. Neururer (1977) quoted by
Rydberg (1994) found more weed reduction harrowing
across but Rydberg (1994) did not find any differences.
The flex-tine harrow can be used at pre-emergence,
early post-emergence and selective harrowing can be
conducted at late tillering of the cereal (Rasmussen and
Svenningsen, 1995). Efficacy in pre-emergence is only
expected if the majority of weeds germinate earlier than
the crop (Rasmussen, 1996). Most of the trials descri-
bed in the literature have been conducted in early post-
emergence of both the weeds and the crop and it is often
recommended to harrow in early stages of the weeds,
especially when tap-rooted weeds dominate (Wilson et
al., 1993; Welsh et al., 1997; Cirujeda et al., 2003).
Selective harrowing is not usually conducted as weeds
are better controlled at early stages. Repeating post-
emergence harrowing the same day usually increases
efficacy and crop damage (Rasmussen, 1991; Wilson et
al., 1993) and no additional effect is observed in some
cases (Welsh et al., 1997).
The tine angle of the harrow should have an effect on
weed control and yield. However, Bàrberi et al. (2000)
combined four different positions which corresponded
to different aggresiveness with single and go-and-back
treatments on durum wheat and did not find significant
effects on weeds and crop.
In addition to controlling the weeds it is necessary to
do the adjustments in order to avoid crop damage, as it
happens with chemical treatments (Kurstjens and Per-
dock, 2000). The relationship between weed control and
crop damage is very important (Rasmussen, 1991),
since control is normally improved by deeper harrowing
or more passes, even though both factors may increase
crop damage (Rasmussen and Rasmussen, 1995). The
selectivity of weed control is mostly determined by the
difference between the root system and/or stiffness of
the crop versus the weed, where the weeds should be
less developed than the crop for the treatment to be
effective and selective.
With the exception of the work conducted in Italy
(Bàrberi et al., 2000) and some conducted in Spain
(Lacasta et al., 1997; Moyano et al., 1998; Cirujeda and
Taberner, 2006), most of the research on weed harro-
wing has been conducted in Northern countries (Den-
mark, Netherlands, Great Britain, Sweden, Germany,
etc.). In a semi-arid environment as found in Southern
Europe, Northern Africa, huge parts of North America
662 G. Pardo et al. / Span J Agric Res (2008) 6(4), 661-670

and other parts of the world rainfall is very irregular. In
the semi-arid conditions of North-Eastern Spain
drought periods in autumn or winter are frequent but
also wet years occur. It is thus important to conduct
field trials during several years in order to assess the
limitations given by the climatic conditions in the pre-
sent locations. Pre-emergence harrowing can be effecti-
ve if moisture enhances germination but if sowing is
conducted in dry conditions the effect is probably very
little. Dry soils in autumn are expected to be more com-
mon in the semi-arid conditions, opposite, survival of
weeds should be more difficult in the semi-arid clima-
tes after harrowing. On one hand, more continuous
moisture in Northern Europe allows recovery of weeds,
but on other hand, moisture favours crop development
and competition potentially suppressing weeds.
The cropping cycle is also shorter in the semi-arid
conditions compared to Northern Europe, as cereal is
normally sown in mid autumn (November) to benefit
from autumn and winter rain or rarely at the end of win-
ter (February) if too wet conditions did not allow earlier
sowing. Harvest is made in mid June so that the cropping
cycle comprises around 7 months, compared to the 8 or
9 months in Northern European conditions. Despite
sowing density for winter cereal is similar (150-200 kg
ha-1) a normal yield in Spanish rainfed areas is 2500 kg
ha-1, compared to the 5000 or 6000 kg ha-1 in Northern
Europe. Specially, in low-yield areas harrowing can be
an interesting weeding technique as costs are lower com-
pared to any herbicide application (Pardo et al., 2004).
In Spain, this type of weeding is poorly developed but
adopted by some organic growers. The typical treatment
tested in winter cereal in semiarid zones is one pass
during the tillering phase (Lacasta et al., 1997; Moyano
et al., 1998; Zaragoza et al., 1999; Lezáun et al., 2001).
The studies carried out by Cirujeda and Taberner (2006)
included repeated passes but focused on one weed spe-
cies, namely Papaver rhoeas. Due to the scarce informa-
tion available in semi-arid conditions it was considered
necessary to do harrowing experiments in winter cereal
crops during several years on a mixed weed flora, sear-
ching optimising the use of the flex-tine harrow by adjus-
ting variables such as the number of passes, harrowing
depth, direction and travelling speed, as well as the time
of intervention (pre- or post-emergence of the crop).
The objective of this work was to study the effect of
different adjustments of the flex-tine harrow on weed
control in winter cereals under semi-arid conditions on
a mixed weed population assessing both effects on
weeds and on crop yield.
Material and methods
Site
Trials were carried out in Zaragoza (the township of
Montañana, latitude 41º43’ N and longitude 2º52’ W;
225 m above sea level) on a soil with a loamy texture
(37.75% sand, 49.08% silt and 13.17% clay) and with
3.37% organic matter, which is high for the area and
caused by the previous crop, which was an orchard.
Monthly data on rainfall and temperatures for the period
of the experiment are shown in Figure 1.
Weed harrowing in winter cereal under semi-arid conditions 663
0
20
Oct-99
Dec-99
Feb-00
Apr-00
Jun-00
Aug-00
Oct-00
Dec-00
Feb-01
Apr-01
Jun-01
Aug-01
Oct-01
Dec-01
Feb-02
Apr-02
Jun-02
Aug-02
Oct-02
Dec-02
Feb-03
Apr-03
Jun-03
Aug-03
Oct-03
Dec-03
Feb-04
Apr-04
Jun-04
40
60
80
100
120
140
mm
0
5
10
15
20
25
30
ºC
mm ºC
Figure 1. Monthly rainfall and temperature data at the trial.

Sowing and crop management
Barley was sown in November in the first and third
year of the study (1999-00, 2001-02) and in February in
the second year (2000-01) due to heavy rainfall in
autumn and winter impeding earlier sowing. The wheat
was sown in November in 2002-03 and in January in
2003-04 due to technical problems. Sowing rate was
150 and 175 kg ha-1 in 1999, 2001 and 2002, and 200 kg
ha-1 on the second and last year in anticipation of a scar-
ce tillering (Table 1).
Fertilization consisted in 120-60-60 N-P2O5-K2O kg
ha-1. Phosphorus, potassium and a third of the nitrogen
were applied preplant. The rest was applied as side dress
at tillering (Table 1). The crop was harvested in June-
July each year.
Treatments
The experimental design was a randomized block with
three replicates and elemental plots measuring 18 x 5 m.
Ten treatments were tested in the first two years and eight
in the other three seasons considering only those with the
most interesting results (Table 2). Pre-emergence harrow -
ing was considered since 2002.Trials included every year
herbicide control plots and non-weeded plots.
The treatments varied in terms of the number of passes
(one or two, either on the same day or with an interval of
15 days), of the direction (parallel and/or across to the
sowing lines), harrowing depth [changing the angle of the
tines on the soil following Bàrberi et al. (2000) low β= -
70, middle β= -60º, high β= -50º and α= 135º] and of the
travelling speed (low 9 km h-1 or high 12 km h-1).
Most passes were carried out during the tillering phase
in BBCH stage 21-22 (BBCH Working Group, 1997)
except for the second pass in treatment 8, which was done
during the stem elongation phase (stage BBCH 30) and
treatment 0, performed in pre-emergence. The herbicides
were chosen depending on the flora composition and con-
sidering the phenological stage of the weeds: 2,4-D (2,4-
dichlorophenoxyacetic acid) + MCPA (2-methyl-4-chlo-
rophenoxyacetic acid) (0.33 + 0.33 kg a.i. ha-1, Horma,
Key) in year 2000, diflufenican + MCPA (0.05 + 0.5 kg
a.i. ha-1, Yard, Bayer) in year 2001, chlortoluron + terbu-
trine + triasulfuron (0.795 + 0.161+0.00375 kg a.i. ha-1,
Tricuran 64 WG, Syngenta) in year 2002, MCPA (0.8 kg
a.i. ha-1 Agroxone, Syngenta) in year 2003 and 2,4-D (0.6
kg a.i. ha-1 Primma Din, Agrodan) in year 2004.
Weed plants were counted 15-20 days after the last
harrowing in three 0.27 m2squares and the weed num-
ber was added to evaluate the effectiveness of the diffe-
rent treatments. Efficacy was calculated comparing
weed density (plants m-2) with the untreated plots
follow ing: [(density untreated plots - density treated
plots) (density untreated plots)-1*100]. Crop and weed
dry weight at flowering of the crop were also assessed to
evaluate selectivity. Aboveground biomass was cut in
664 G. Pardo et al. / Span J Agric Res (2008) 6(4), 661-670
Sowing Density
Year Crop Variety date (kg ha-1)Dominant weed species
1999-00 Barley Camelot 15/11/99 150 Sinapis arvensis, Capsella bursa-pastoris,
Polygonum aviculare, Anacyclus clavatus,
Senecio vulgaris, Rumex crispus.
2000-01 Barley Graphic 15/2/01 200 Convolvulus arvensis, S. arvensis, Cheno-
podium vulvaria, R. crispus, Hordeum
murinum.
2001-02 Barley Hispanic 8/11/01 150 Galium aparine, Fumaria officinalis, P. avi-
culare, Stellaria media, Veronica hederifo-
lia, S. arvensis.
2002-03 Durum wheat Mellaria 15/11/02 175 S. arvensis, Diplotaxis erucoides, C. bursa-
pastoris, R. crispus, G. aparine, A. clavatus,
Taraxacum officinale.
2003-04 Durum wheat Ionio 20/1/04 200 C. vulvaria, C. arvensis, S. arvensis, Veroni-
ca hederifolia, R. crispus, Malva spp., C.
bursa-pastoris.
Table 1. Sowing date, crop density and cereal varieties sown in the different trials and dominant weed species

two 0.27 m2squares and dried at 60ºC during 96 hours.
Grain yield was obtained at 86% dry mater with an
experimental harvester harvesting 27.9 m2per plot.
Statistical analysis
To satisfy normality and variance homogeneity, effi-
cacy was arcsin [ ] transformed and weed bio-
mass needed transformation. Block was nested with
year, which was taken into account in the ANOVA
model, assigning the correspondent error terms. A con-
trast analysis was performed to compare the different
treatments in a more detailed way using the statistic pac-
kage SAS (SAS Institute, 1991).
Results and discussion
Effect on weed number
The average weed infestation was highest in 2000
and 2004 with more than 200 plants m-2 in the non-
weeded plots, moderate in 2002 and 2003 with 42 and
105 plants m-2, respectively and very low in 2001 with
only 12 plants m-2, probably due to the delayed sowing.
Concerning the effect of harrowing on the different
weed species, no efficacy was appreciated on Sinapis
arvensis and Rumex crispus even in the repeated
harrowing treatments. In the first case the plants and
especially the rooting system developed faster than the
crop, and in the second case the perennial weed was
strongly anchored.
The interaction year x treatment was significant
for percentage weed control (Table 3). Percentage
weed control of the mechanical treatments was very
irregular ranging from 1 to 80% (Table 4). Therefore
the results were considered for each year indivi-
dually. The interaction was not significant for weed
biomass so that the mean of all five years was consi-
dered (Table 4). The contrast analysis showed that
the mechanical treatments considered as a whole
were effective for weed density reduction in relation
to the untreated control in all years excepting 2001
where the infestation was very low (Table 5, contrast
a).
Weed harrowing in winter cereal under semi-arid conditions 665
Harrowing date
Treatment
number Number of
passes
Harrowing
direction
regarding sowing
direction
Harrowing
depth Speed
(km h
-1
)Barley Wheat
2000 2001 2002 2003 2004
0- Pre-emergence 1
a
Parallel Middle 9 --
27/11 4/12 9/2
1- Single pass at low depth 1 Parallel Low 9 29/2 27/3 - - -
2- Single pass at medium depth 1 Parallel Middle 9 29/2 27/3 11/2 14/2 4/3
3- Single pass at high depth 1 Parallel High 9 29/2 27/3 - - -
4- Single pass at high speed 1 Parallel Middle 12 29/2 27/3 - - -
5- Two parallel passes the same day 2
b
Parallel Middle 9 29/2 27/3 11/2 14/2 4/3
6- Two passes: one parallel,
one across 2
b
First parallel,
second across Middle 9 29/2 27/3 11/2 14/2 4/3
7- Single pass across the sowing line 1 Across Middle 9 29/2 27/3 11/2 14/2 4/3
8- Two parallel passes.
The second 15 days later 2
c
Parallel Middle 9 29/2
14/3 27/3
10/4 11/2
27/2 14/2
28/2 4/3
22/4
9- Herbicide Only herbicide treatment 13/3 2/4 17/1 21/2 14/4
10- Untreated control Unweeded control nh
d
nh nh nh nh
Table 2. Summary of the treatments carried out in the trials
)100/(x
x
aPre-emergence. bTreatment repeated at the same day. cTreatment repeated 15 days after. dNh: not harrowed.

trolled with the second treatment as they are bigger
then. In fact, best efficacy by weed harrowing is gene-
rally achieved when weeds are small so that if it is
aimed to do two passes, following the results in semi-
arid conditions, better efficacy is generally not achieved
if the treatment is repeated 15 days after. It is probably
different in areas with constant moisture and with new
weed emergences during the cropping cycle where
harrowing at different moments can be more effective
(Rasmussen and Rasmussen, 1999).
Harrowing direction
A single harrowing treatment conducted across the
sowing direction gave the same control compared to
harrowing along the sowing direction excepting in 2000,
where harrowing across was significantly less efficient
than harrowing along the sowing direction (Table 5, con-
trast d). This tendency was also observed in 2002 and
2004, but differences were not significant. Probably the
crop was small enough in the trials to avoid that tines were
guided between the rows and impeding correct tine vibra-
tion, which does occur when harrowing after tillering as
described by Rasmussen and Svenningsen (1995).
Harrowing moment
Pre-emergence harrowing was not effective any year
in the tested conditions. In 2004, control was even sig-
nificantly lower compared to a single post-emergence
harrowing treatment (Table 5, contrast i). Weeds should
be starting germination in order to move pre-emerged
weeds so called ‘white threads’ in pre-emergence (Ras-
mussen and Svenningsen, 1995) and probably this did
not happen in the experiments, as soil moisture was low.
Another negative aspect of pre-emergence harrowing is
the risk of breaking dormancy (Grundy et al., 2003) as
the soil is refined again, but this is an issue that has not
been covered in the current study.
Comparing with the herbicide efficacy
Excepting the results of year 2001 herbicide had a sig-
nificantly higher efficacy compared to harrowing overall
(Table 5, contrast l) but in three out of five years it was
possible to find a harrowing treatment that had statisti-
cally the same efficacy than the herbicide treatment
(Table 5, contrast o) but in no case harrowing had a supe-
rior weed control than herbicide. At high-density weed
infestations and also with highly-aggressive weeds pro-
bably the use of herbicide is a more effective solution.
666 G. Pardo et al. / Span J Agric Res (2008) 6(4), 661-670
Influence of harrowing depth and speed
These parameters were analysed during the first two
years (Table 5, contrasts e, f, g, h). In year 2000 weed
control did not increase by increasing working depth or
speed and in 2001 only significant differences were
detected between middle and high depth (Table 5, con-
trast g), so that these treatments were not included in the
next field trials. Instead, a pre-emergence treatment was
tested since 2002 (Table 5, contrast i).
Number of harrowing passes
Concerning the number of passes, as a whole, two
harrowing passes achieved better efficacy than only
one, with significant differences during the years 2002
and 2004 (Table 4 and Table 5, contrast b). Little diffe-
rences were detected due to conducting the second
harrowing pass in a different direction or at a different
date. Only in 2004 harrowing the second pass across the
sowing line controlled significantly more weeds than
performing the second pass in the same direction (Table
5, contrast j) and only in 2000 doing the second pass in
the sowing direction the same day was more effective
than repeating the treatment 15 days later (Table 5, con-
trast k). This last observation is coincident with the fin-
dings of Bàrberi et al. (2000) who observed that weeds
surviving the first harrowing treatment can not be con-
d.f. F P>F
Weed control
Year 4 1.86 0.1717
Treatment 10 10.69 <0.0001
Year x Block 10 5.10 <0.0001
Year x Treatment 29 2.12 0.0048
Weed biomass
Year 4 40.30 <0.0001
Treatment 10 6.42 <0.0001
Year x Block 10 1.48 0.1620
Year x Treatment 29 0.95 0.5440
Yield
Year 4 6.92 0.0047
Treatment 10 1.84 0.0984
Year x Block 10 9.62 <0.0001
Year x Treatment 29 1.50 0.0815
Table 3. Results of the standard ANOVA on the percentage of
weed control (arcsin ( transformed), on weed biomass
in kg ha-1 ( transformed) and on yield in kg ha-1
)100/(x
x

Effect on weed biomass
Weed biomass at the end of the cropping cycle had a
similar behaviour than weed control but less significant
differences were found (Table 5). This is probably due to
the fact that a good control caused by mechanical wee-
ding generally leads to bigger surviving individual
plants due to lower competition, hiding part of the con-
trol effect as found by Rasmussen (1992). Statistically
significant differences were found only when compa-
ring mechanical control as a whole or herbicide with
untreated control (Table 5, contrasts a, p) and when cho-
osing the best mechanical control treatment concerning
weed biomass with the untreated control (Table 5, con-
trast s). These results suggest that a good harrowing is
able to reduce weed density and also weed biomass.
As commented previously, a significantly higher
weed control was found in the plots treated with herbi-
cide compared to the overall mechanical methods in
four out of five years as too little weeds were found in
2001, but herbicide did not achieve a higher weed bio-
mass reduction (Table 5, contrast l). Weed biomass was
statistically the same even when comparing herbicide
treated plots with the mechanical control treatment,
which lead to the lowest weed biomass (i.e. harrowing
twice at the same date) (Table 5, contrast p). Thus, pro-
bably the weeds found in the herbicide-treated plots
were big plants grown with little competition.
Cereal yield
Highest mean yield was obtained in 2000 and 2002
(3,511 and 3,284 kg ha-1). Lowest yield were obtained in
2001 (1,586 kg ha-1) and 2004 (1,944 kg ha-1), which is
coincident with the delayed sowing in the latter years.
However, the interaction year x treatment was not signi-
ficant for percentage weed control. Considering the mean
of all five years, despite the mechanical treatments as a
whole achieved a significantly better weed control com-
pared to the non-weeded control, this did not lead to a
yield increase for any of the tested treatments (Table 5).
No differences were found even when choosing the best
treatments considering yield (Table 5, contrasts r, s).
Weed competition is probably not the limiting factor
in cereal crops in the tested conditions, as significant
differences in yield were not found even when compa-
ring untreated plots with treated plots with the highest
Weed harrowing in winter cereal under semi-arid conditions 667
)100/(x
Weed control Weed biomass Yield
2000 2001 2002 2003 2004
Mean
2000-04
Mean
2000-04
0- Pre-emergence - - 29.2 3.3 0.5 914.0 2561.3
1- Single pass at low depth 38.1 25.0 - - - 236.9 2761.5
2- Single pass at medium depth 48.6 4.3 38.9 7.2 18.8 456.3 2839.0
3- Single pass at high depth 2
22
22
1
46.3 41.9 - - - 159.1 3060.7
4- Single pass at high speed 53.4 14.9 - - - 186.7 2346.3
5- Two parallel passes the same day 64.4 26.5 60.5 21.1 19.5 281.6 2326.9
6- Two passes: one parallel, one cross 36.0 13.8 58.1 50.0 63.1 290.0 2416.0
7- Single pass across the sowing line 20.9 30.1 35.3 18.4 11.4 506.7 2564.5
8- Two parallel passes. The second
15 days later
20.1 25.3 66.7 22.1 37.8 412.0 2558.9
9- Herbicide 75.2 19.5 79.7 83.2 84.0 149.6 2857.5
10- Untreated control 0.0 0.0 0.0 0.0 0.0 749.2 2178.3
Table 4. Mean weed control (%), weed biomass (kg ha-1) and cereal yield (kg ha-1) for the different years. Data are back-trans-
formed after analysis from arcsin ( ). No means are shown for weed control due to the significant interaction year x treat-
ment.
1Conducted in 2002-2004, only. 2Conducted in 2000 and 2001, only.

yield. However, weed control is necessary to avoid
increasing the weed seed-bank.
In the trials performed the tendency that harrowing
across the sowing line gave yielded less than harrowing
along the sowing line (Table 4) probably due to crop
plant damage. Considering that weed control was the
same or lower in one case (Table 5, contrasts c, d),
harrowing across the sowing line is probably not a good
practice in the present conditions.
Lack of significance on crop yield when comparing
the treatments (Table 3) suggests that in the present con-
ditions no important damage was caused due to repea-
ted treatments, but the tendency was lower yield for the
repeated harrowing treatment (Table 4), similar to the
results of Rasmussen (1991) who also observed just a
tendency in reduction.
The above results reveal that a single pass at middle
depth at tillering of the cereal can be considered an inte-
resting mechanical option when weed pressure is low or
medium. Moreover, although the weed control was lower
compared to herbicide use, the yield was almost the
same. Taking into account that the cost of the herbicide
668 G. Pardo et al. / Span J Agric Res (2008) 6(4), 661-670
Mean weed control Weed
biomass Yield
2000 2001 2002 2003 2004 Mean Mean
a) mechanical vs.unweeded control ** ns ** * ** ** ns
b) two passes in any form vs. one pass
at any depth, any speed and direction ns ns ** ns *ns ns
c) any parallel pass vs. one pass across ** ns ns ns ns ns ns
d) one parallel pass vs.one pass
across middle depth and 9 km h-1 both ** ns ns ns ns ns ns
e) middle vs. low depth ns ns - - - ns ns
f) high vs. low depth ns ns - - - ns ns
g) high vs. middle depth ns *-- - ns ns
h) 12 km h-1 vs. 9 km h-1 ns ns - - - ns ns
i) harrowing at tillering vs. pre-emergence - - ns ns *ns ns
j) two parallel passes vs. one parallel
and the second pass across ns ns ns ns (-)
** ns ns
k) two parallel passes vs. repeating
the second parallel pass 15 days after ** ns ns ns ns ns ns
l) herbicide vs. mechanical as a whole ** ns ** ** ** ns ns
m) herbicide vs. untreated control ** ns ** ** ** *ns
n) best mechanical concerning weed
control vs. unweeded control ** ** ** ** ** --
o) herbicide vs. best mechanical
concerning weed control ns ns ns ** --
p) best mechanical concerning weed
biomass vs. unweeded control ---- - *-
q) herbicide vs. best mechanical
concerning weed biomass ---- - ns -
r) best mechanical concerning yield
vs. unweeded control ---- - - ns
s) herbicide vs. best mechanical
concerning yield ---- - - ns
Table 5. Results from the contrast analysis
(-) means that the first term is statistically lower than the second; in the other cases, the first term is higher than the second. **: P<0.01;
*: P<0.05; ns: not significant. a) in 2000 and 2001: 1, 2, 3, 4, 5, 6, 7, 8 vs. 10; in 2002-04: 0, 2, 5, 6, 7, 8 vs. 10; b) in 2000 and 2001: 5,
6, 8 vs. 1, 2, 3, 4, 7; in 2002-2004: 5, 6, 8 vs. 2, 7. c) in 2000 and 2001: 1, 2, 3, 4 vs. 7; in 2002-2004: 2 vs. 7; d) 2 vs. 7 all years; e) in
2000 and 2001: 2 vs. 1; f) in 2000 and 2001: 3 vs. 1; g) in 2000 and 2001: 3 vs. 2. h) in 2000 and 2001: 4 vs. 2; i) in 2002-2004: 2 vs. 0;
j) 5vs. 6 all years; k) 5 vs. 8 all years l) 9 vs. 0, 1, 2, 3, 4, 5, 6, 7, 8; m) 9 vs. 10; n) 5 (2000), 3 (2001) 8 (2002), 6 (2003,2004) vs. 10; o)
9 vs 5 (2000), 3 (2001) 8 (2002), 6 (2003,2004); p) 5 vs. 10 q) 9 vs. 5; r) 2 vs. 10; s) 9 vs.2.

treatment (39.7 €ha-1 as an average of the seven years) in
the trial conditions is three times more expensive than a
single harrowing treatment (12.1 €ha-1, average of the
seven years) (Pardo et al., 2004), a single harrowing can
be a good option from an economic perspective not only
when herbicides are not permitted.
Conclusions
Harrowing efficacy was irregular but can be conside-
red a valid option for weed control in cereal fields in the
present conditions, because it reduces weed density sig-
nificantly compared to the untreated plots and because
no significant yield differences were found when com-
paring the harrowing treatments with herbicide use.
Moreover, its cost is inferior to herbicide use.
Harrowing at higher speed or higher depth did gene-
rally not lead to a higher weed control. Harrowing
across the sowing direction of the cereal did not increa-
se efficacy, either and tended to decrease cereal yield.
Concerning repeating harrowing, the best option was
to do the second pass the same day and in the sowing
direction. However, yield tended to decrease with repe-
ated harrowing so that doing the second pass will be jus-
tified with high infestations, only.
Few harrowing treatments achieved a similar efficacy
than herbicide use. Out of the tested treatments in
semiarid conditions, one single harrowing pass at a nor-
mal depth and at 9 km h-1 is probably the best option.
Finally, the convenience of any weed control method
can be discussed in the present areas as yield decreases
in the non-weeded control plots were not significant
compared to any of the mechanical methods not even
compared to the herbicide treatment, probably due to
the fact that weed competition was not the most limiting
factor in these conditions. However, reducing the weed
flora in any way is always beneficial to prevent infesta-
tions the following years.
Acknowledgements
We would like to thank Fernando Arrieta and Maria
León for their very valuable technical assistance.
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