Weed species differ in their ability to emerge in no-till systems that include cover crops

Article (PDF Available)inAnnals of Applied Biology 166(3) · May 2015with 159 Reads
DOI: 10.1111/aab.12195
Abstract
No-till cropping systems that include cover crops could lead to important changes in weed communities by decreasing some annual weed populations. In this study, we predicted that seed burial depth and the presence of cover crop would affect the emergence and initial growth success of annual weed species. We tested two factors on 14 weed species in a greenhouse: the seed burial depth of weeds (buried versus soil surface) and the presence/absence of a cover crop (ryegrass). We counted the emerged seedlings and measured the height of weeds and cover crops (Hweed, Hcover), the dry matter content of weeds and cover crops (DMCweed, DMCcover) and the number of leaves of weeds (NLweed) on 1433 weed and 390 ryegrass individuals. Emergence of five weed species (AMBEL, ANGAR, BROST, CENCY and EPHHE) was affected by the seed location (−10.3% on average for unburied seeds), five other weed species (ALOMY, CAPBP, SONAS, VERPE and VLPMY) were affected by cover (on average −9.5% for seeds emerged in the presence of cover crop), and four weed species (GERDI, LAMPU, POAAN and VIOAR) were not affected by either. Weed growth of all weed species also decreased with the presence of a cover crop (on average Hweed: −49.9%, DMCweed: −87.2% and NLweed: −55.4%) and for unburied seeds (on average Hweed: −33.7%, DMCweed: −70.6% and NLweed: −43.3%), with various responses according to species. This study indicates that annual weeds could be disadvantaged by no-till systems using cover crops.
Annals of Applied Biology ISSN 0003-4746
RESEARCH ARTICLE
Weed species differ in their ability to emerge in no-till systems
that include cover crops
S. Cordeau1,2, J.-P. Guillemin3,C.Reibel
3& B. Chauvel1
1 UMR1347 Agroécologie, INRA, Dijon, France
2 UR LEVA, Groupe Ecole Supérieure d’Agriculture, LUNAM Université, Angers, France
3 UMR1347 Agroécologie, AgroSup Dijon, Dijon, France
Keywords
Agroecology; conservation agriculture; cover
cropping; direct-drilling; emergence;
germination; Seed trait.
Correspondence
S. Cordeau, UMR1347 Agroécologie, INRA, BP
86510, F-21000 Dijon, France. Email:
stephane.cordeau@dijon.inra.fr
Received: 10 April 2014; revised version
accepted: 5 December 2014.
doi:10.1111/aab.12195
Abstract
No-till cropping systems that include cover crops could lead to important
changes in weed communities by decreasing some annual weed populations.
In this study, we predicted that seed burial depth and the presence of cover
crop would affect the emergence and initial growth success of annual weed
species. We tested two factors on 14 weed species in a greenhouse: the seed
burial depth of weeds (buried versus soil surface) and the presence/absence
of a cover crop (ryegrass). We counted the emerged seedlings and measured
the height of weeds and cover crops (Hweed,Hcover), the dry matter content
of weeds and cover crops (DMCweed,DMCcover) and the number of leaves of
weeds (NLweed) on 1433 weed and 390 ryegrass individuals. Emergence of five
weed species (AMBEL, ANGAR, BROST, CENCY and EPHHE) was affected by
the seed location (10.3% on average for unburied seeds), five other weed
species (ALOMY, CAPBP, SONAS, VERPE and VLPMY) were affected by cover
(on average 9.5% for seeds emerged in the presence of cover crop), and four
weed species (GERDI, LAMPU, POAAN and VIOAR) were not affected by either.
Weed growth of all weed species also decreased with the presence of a cover
crop (on average Hweed:49.9%, DMCweed:87.2% and NLweed:55.4%)
and for unburied seeds (on average Hweed:33.7%, DMCweed:70.6% and
NLweed:43.3%), with various responses according to species. This study
indicates that annual weeds could be disadvantaged by no-till systems using
cover crops.
Introduction
For more than 60 years, agricultural practices in Europe
have increased productivity at the cost of reduced soil
fertility (Manlay et al., 2007), increased risk of soil ero-
sion (Montanarella et al., 2003) and drastic loss of flora
(Fried et al., 2009a) and fauna (Robinson et al., 2001)
diversity in fields. Indeed, during the first half of the 20th
century, agricultural intensification was driven mainly
by labour-saving inventions (e.g. motorisation). Ero-
sion – typically the most spectacular, immediate and irre-
versible symptom of inadequate agricultural practices
(Montanarella et al., 2003) – was the first indication of
the drawbacks of intensified practices. Erosion left large
areas of soil deprived of protective plant cover.
Negative impacts of tillage practices (soil erosion,
soil structural degradation, faster organic matter
decomposition) have motivated farmers to change
their cropping systems (Holland, 2004). In the 1970s,
no-till acreage expanded rapidly in different parts of the
world. Now, approximately 25% of the total cropland
in the USA (Kassam & Walton, 2010) and 110 million
ha worldwide is planted using no-tillage (Derpsch et al.,
2010). In some of these cropping systems, cover crops
are established during the summer to winter period to
compete with weeds (Yenish et al., 1996), to limit soil
erosion (Dabney et al., 2001) or to catch crop-decreasing
nitrate leaching (Constantin et al., 2010). Cover crops are
sown as pure or mixed stands, just after or few weeks
Ann Appl Biol (2015) 1
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Weed emergence in no-till with cover crops S. Cordeau et al.
after the harvest of the preceding crops without tillage
(e.g. seedbed preparation).
In continuous no-till cropping systems that use
cover crops, farmers stopped tillage. There are fewer
weed control possibilities because pre-sowing tillage,
pre-emergence herbicide spraying and in-crop mechan-
ical weeding are not possible. This could change weed
flora, with fewer annual species (Carter & Ivany, 2006)
because the seedbank plays a smaller role. In many arable
fields, the weed flora is dominated by annual species
(Fried et al., 2009b), and weed population dynamics are
strongly influenced by the prevailing cropping systems,
i.e. crop succession and cultivation practices (Torresen
et al., 2003). However, field surveys, simulations and
trait-based analysis on no-plough and no-till cropping
systems (Légère & Samson, 2004) have highlighted that
perennial species such as Convolvulus arvensis (Buhler
et al., 1994), Cirsium arvense or Elytigia repens (Torresen
et al., 2003) are better adapted than annual species to
survive in various crop rotation (i.e. monoculture of
wheat or maize, and diversified rotations) and growing
conditions in Europe (Torresen et al., 2003) and North
America (Miller & Nalewaja, 1985; Légère & Samson,
2004). Few field surveys report higher diversity and
abundance of annual than perennial weeds in no-till
systems (Norsworthy, 2008). Annual weed species in
continuous no-till systems are mainly summer annual
grasses (Norsworthy, 2008; Trichard et al., 2013) that are
able to produce seeds during the intercropping period.
Emergence and initial growth are crucial stages for
annual weeds (Forcella et al., 2000). The relationship
between the burial depth of seeds and their emergence
success has been well studied. Research shows that the
probability of emergence usually decreases with increas-
ing burial depth (Benvenuti et al., 2001b; Gardarin et al.,
2010), except for a few species such as Stellaria media or
Capsella bursa-pastoris. Some studies have quantified ger-
mination rates with seeds on the soil surface (Jensen,
2009; Gardarin et al., 2010). However, only a few stud-
ies have explored the emergence processes with seeds on
the soil surface and the effect of a cover crop on weed
emergence and growth.
Here our objective was to investigate the ability of
different annual weeds to overcome these new selection
pressures (no-tillage, cover crop). We hypothesise here
that the decrease in annual weed populations in no-till
systems that use cover crops could be explained by two
factors: lower emergence when seeds remain on the soil
surface (Gardarin et al., 2010), and reduced growth when
seedlings emerge in a cover crop previously established
(Baumann et al., 2001). Moreover, we expected various
responses according to weed species. We experimentally
tested the emergence and growth of 14 annual weeds
(grass and forbs) at two seed burial depths (buried seeds
and on the soil surface) and two growing conditions (with
or without grass cover crop).
Material and methods
We tested 14 annual weed species (forbs and grasses) in
a greenhouse experiment, named with their EPPO code.
All these species are monocarpic annual weed species,
commonly recorded in field surveys (Jauzein, 1995). We
collected annual weed seeds in 2007 and 2009 around
Dijon (Eastern France; Table 1). We air-dried seeds for
a month then stored them in dry conditions at 3C. We
purchased the remaining seeds from Herbiseed (Twyford,
Berkshire, UK) in 2011 (Table 1), and also stored these
under the same conditions. All seeds were dry before the
experiment.
We conducted the experiments in a greenhouse with-
out additional lighting in 2009 and 2010 in Dijon (East-
ern France) and in 2011 and 2012 in Angers (Western
France). Growth conditions were similar between sites
and years. The air temperature was recorded and the sums
of degree-days (C day) computed with base temperature
equal 0C were similar for each year of the experiment
(795 in 2009, 823 in 2010, 734 in 2011, 787 in 2012). The
maximum difference between years was 89C day (base
temperature: 0C), i.e. approximately 1.5 phyllochron
for grasses and forbs (Frank & Bauer, 1995; Gramig &
Stoltenberg, 2007). The temperature (C) and humidity
(% Volumetric Water Content) of the soil, and the Photo-
synthetically Active Radiation (PAR, mV) were recorded
with buried and unburied sensors (see Appendix S1, Sup-
porting Information).
Experimental design
The containers (0.12 m2) were filled with 8 L of clay
soil/sand (1:1, v:v) mixture. No nutrient was added dur-
ing the experiments. We tested two factors, each with two
levels: the presence of a cover crop: C(no cover) and C+
[ryegrass cover (Lolium multiflorum) cultivar Starter]; the
seed location: S (on the soil surface) and B (buried). The
burial depth (0.5 cm) was optimal for weed germination
and emergence (Gardarin et al., 2010) because seeds are
in the darkness (see Appendix S1). These four levels were
combined and repeated three times for each weed species.
Ryegrass was sown at a density of 600 seeds m2for about
500C day (i.e. around 25 days) before the weed species
were sown. The weed seeds were sown when ryegrass had
reached the ‘two- to three-leaf’ stage. In each container,
with or without cover, we placed 30 weed seeds on the
soil surface and buried 30 seeds of the same species (one
2Ann Appl Biol (2015)
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S. Cordeau et al. Weed emergence in no-till with cover crops
Table 1 Studied weed species by year and seed harvest location, and their uses in the experiment
Seed collection Experiment
Weed species Code EPPOaOriginbYear Site Year
Alopecurus myosuroides Huds. ALOMY F-Dijon 2009 Dijon 2009– 10
Ambrosia artemisiifolia L. AMBEL Herbiseed 2011 Angers 2011– 12
Lysimachia arvensis (L.) U.Manns & Anderb. ANGAR Herbiseed 2011 Angers 2011
Bromus sterilis L. BROST F-Dijon 2009 Dijon 2009– 10
Capsella bursa-pastoris (L.) Medik. CAPBP F-Dijon 2007 Dijon 2009– 10
Cyanus segetum Hill CENCY F-Dijon 2009 Dijon, Angers 2009– 12
Euphorbia helioscopia L. EPHHE Herbiseed 2011 Angers 2012
Geranium dissectum L. GERDI Herbiseed 2011 Angers 2011– 12
Lamium purpureum L. LAMPU Herbiseed 2011 Angers 2012
Poa annua L. POAAN F-Dijon 2009 Dijon 2009– 10
Sonchus asper (L.) Hill SONAS F-Dijon 2009 Dijon 2009– 10
Veronica persica Poir. VERPE F-Dijon 2009 Dijon 2009– 10
Viola arvensis Murray VIOAR Herbiseed 2011 Angers 2012
Vulpia myuros L. VLPMY F-Dijon 2009 Dijon 2009 10
ahttp://eppt.eppo.org/search.php.
bF-Dijon: Dijon in France (4720Nand5
03W); Herbiseed: weed specialist grower (Twyford, Berkshire, UK).
species per container). Seeds were placed on the soil sur-
face or in the soil with a tong that did not disturb the soil
surface. The seed density and the spatial pattern of seeds
(see Appendix S2) were the same in all containers in order
to limit and uniform the intra-specific competition. We
moistened the soil daily to avoid water stress, and were
careful when adding water to avoid seed movement at
the soil surface.
Measurements
We recorded the number of emerged seedlings at the
end of the experiment. We considered seedlings to have
emerged when the two cotyledons of forbs or the first leaf
of grasses were visible. The complete data set included
312 percent-emergence calculations; each was calcu-
lated as the number of emerged seedlings out of the 30
seeds sown.
At the end of the experiment (around 1300Cday
after sowing ryegrass), we measured the number of leaves
(NLweed), the height (Hweed) and the dry matter content
of shoots (DMCweed) on five weed individuals per repli-
cate, which were randomly selected from the emerged
seedlings. If fewer than five seedlings emerged per repli-
cate, all the seedlings were collected. Consequently, we
measured a total of 1433 seedlings, including 690 col-
lected in containers with cover. We calculated the height
of the cover (Hcover) as the average height of five indi-
viduals, randomly selected from each container. We cal-
culated the dry matter content of shoots of the cover
(DMCcover) as the total dry matter content of the container
divided by the number of grass individuals per container
(varying from 259 to 590 plants m2).
Statistical analyses
Emergence of weed species
We used R software (R Development Core Team, 2011)
for all analyses. Analysis of variance (ANOVA) tables
(Likelihood-ratio chi-square and F-tests) were calculated
with the ANOVA function of the Car library (Fox &
Weisberg, 2011). All models included the block (i.e.
container), year and site as covariates. The number of
emerged seedlings (over 30 expected) was analysed using
a generalised linear model (GLM) with a logistic distri-
bution (binomial), useful for binary outcomes. Explana-
tory variables were the weed species (14 levels), the seed
location (2 levels) and the cover (2 levels). We first fit-
ted the complete data set (n=312) to the most com-
plicated model, and then simplified by removing the
non-significant terms one-by-one (P>0.05). Finally, with
sub-datasets by weed species, we also analysed the effects
of seed location and cover on the number of emerged
seedlings using the GLM. The percent emergence (Pe)
was computed as the number of emerged seedlings of 30
sown seeds. All graphs were plotted with ggplot2 library
(Wickham, 2009).
Development and growth of weed species
To study the effects of seed location and cover on
weed growth (Hweed,DMCweed and NLweed), we anal-
ysed the complete dataset (n=1433) and then the
weed sub-datasets with linear models (LM). For each
weed growth variable, a model fitted on the complete
data set was Weed growth variable =Species +Seed loca-
tion +Cover +I(Seed location: Cover), i.e. the main factors
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Weed emergence in no-till with cover crops S. Cordeau et al.
Table 2 Effect of seed location and grass cover on emergence and development of weed speciesa
PebHweedcDMCweedcNLweedc
Term 𝜒2Df Pd𝜒2Df Pd𝜒2Df Pd𝜒2Df Pd
Complete data set
Species 571.0 13 *** ––––––
Seed location 28.3 1 *** 59.8 1 *** 74.6 1 *** 54.7 1 ***
Cover 25.7 1 *** 370.2 1 *** 971.8 1 *** 367.3 1 ***
I (Species:Seed location) 155.7 13 *** ––––––
I (Species:Cover) 20.9 13 0.07 – – – – – –
I (Seed location:Cover) 2.9 1 0.09 1.6 1 0.2 2.1 1 0.14 0.7 1 0.39
I (Species:Seed location:Cover) 8.6 13 0.80 – – – – – –
Pe: percent emergence of weeds; Hweed: Height of a weed; DMCweed: above ground dry matter content of a weed; NLweed: number of leaves of a weed
a indicates covariate introduced in the LM model but considered here as confounding variable.
bANOVA for GLM with binomial distribution, with Pe as response variable, 𝜒2: likelihood-ratio chi square..
cANOVA for LM with Gaussian distribution, with ln-transformed response variables.
d*P<0.05, **P<0.01, ***P<0.001.
(Seed location,Cover), the interaction between both, and
Species as covariate. The weed species was included in
the LM as a covariate, but the results were not presented
because the 14 weed species are inherently different in
terms of height, dry matter content and number of leaves
at similar stages. To match the distributional assumption
of LM, the weed growth variables were ln-transformed.
We again used the ANOVA (Car package) function (Fox &
Weisberg, 2011) to calculate the ANOVA tables. Then, we
performed analyses with Hweed,NLweed and DMCweed
on the weed data subsets (data subsets per species).
Results are expressed as a percentage of variation from
the control treatment, i.e. the degree of increase or
decrease in the growth weed variables in either the C
or B treatment.
Finally, the cover growth variable (Hcover,DMCcover)
varied by container, and therefore the competition
against weeds also varied. Consequently, to explore
the impact of the seed location associated with a
gradually competing cover on the weed growth vari-
ables, we fitted LM for each weed species on data
recorded in the containers with ryegrass cover (n=690)
as: Hweed =Hcover +Seed location +I(Hcover: Seed loca-
tion), DMCweed =DMCcover +Seed location +I(DMCcover:
Seed location)andNLweed =DMCcover +Seed loca-
tion +I(DMCcover:Seed location). To match the distri-
butional assumption of LM, the weed and cover growth
variables were ln-transformed. ANOVA tables were also
calculated. The multiple R-squared and F-test were
computed for each model.
Results
Emergence of weed species
The percent emergence (Pe) averaged 57.9 ±31%, and
varied from 0 to 100%. An ANOVA on the complete data
set (Table 2) showed that Pe varied first with the weed
species (Fig 1C), the interaction I(Species: Seed location), the
seed location (Fig. 1A) and then with the cover (Fig. 1B).
An ANOVA on the weed data subsets (see Table S1)
indicated that, for each species, Pe was affected by only
one factor – either the seed location or the cover. The
interaction between the factors Seed location and Cover
was never significant. Moreover, there was no significant
effect for 4 of 14 weed species (i.e. GERDI, LAMPU,
POAAN and VIOAR).
Mean Pe was high for five weed species, regardless of
treatment (VIOAR, POAAN, GERDI, VLPMY and LAMPU;
Fig. 2). Pe was always low for SONAS and ALOMY. Pe was,
on average, 10.3% lower for unburied seeds. ANOVA on
weed data subsets (see Table S1) showed that not burying
seeds significantly decreased Pe for 5 of 14 weed species
(Fig. 2): AMBEL (22.5% in comparison to buried seeds),
ANGAR (25.5%), BROST (28.6%), CENCY (12.7%)
and EPHHE (92.7%). The unburied seeds sometimes
showed greater variance in their ability to emerge (e.g.
BROST, Bartlett test, P<0.001). Finally, the presence of
crop cover reduced, on average, Pe by 9.5%. ANOVA on
weed data subsets (see Table S1) showed that the presence
of cover decreased Pe for 5 of 14 weed species (Fig. 2):
ALOMY (11.9% in comparison to Pe without cover),
CAPBP (13.3%), SONAS (8.1%), VERPE (21.9%)
and VLPMY (17.5%).
Development and growth of weed species
Post-emergence mortality was rare (1% of containers)
and negligible (less than 2% of sown seeds). First, ANOVA
on the complete dataset and then on weed data sub-
sets showed that all weed growth variables were lower
with unburied seeds and those grown along with a
cover (Fig. 3, Table 2), and that there was sometimes an
interaction between both (see Table S1). Second, linear
4Ann Appl Biol (2015)
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S. Cordeau et al. Weed emergence in no-till with cover crops
Figure 1 Effect of seed location (Panel A: B is buried, S is on the soil surface) and the presence of a ryegrass cover crop (Panel B: Cis without, C+is with
cover) on the percent emergence (Pe) of 14 weed species (Panel C: weed species named with EPPO code (http://eppt.eppo.org/search.php)). ANOVA for GLM
with binomial distribution, with Pe as response variable. Weed species with the same letter are not significantly different at the 5% level. Black dots indicate
values higher than 1.5 times the box height.
regression analysis showed that the effects of competition
with the cover differed between the weed species, some-
times mainly resulting in negative linear relationships
between weed growth variables (i.e. Hweed,DMCweed,
NLweed) and cover growth variables (i.e. Hcover and DMC-
cover). Finally, there was an interaction between initial
seed location and the growth variables of the cover. Leav-
ing seeds unburied sometimes amplified the negative lin-
ear response of cover growth variables on weed growth
variables.
Height
First, Hweed (Fig. 3A, Table 2) was lower for weeds that
emerged in the ryegrass cover (49.9%) and for those
whose seeds were initially not buried (33.7%). ANOVAs
on weed data subsets (see Table S1) showed that Hweed
was always lower in the presence of cover (e.g. ALOMY:
35.1%, LAMPU: 81.5%). Second, Hweed was lower for
unburied seeds for 10 of 14 weed species. Finally, the
interaction between Cover and Seed location only decreased
Hweed for 1 of 14 weed species (EPHHE).
To further explore the process of competition, lin-
ear regressions showed the response of Hweed to Hcover,
according to the initial weed seed location (Table 3). LM
were significant for 9 of 14 weed species with a wide
range of R-squared values. Hweed was negatively corre-
lated with Hcover for ALOMY, CENCY and VERPE. The
I(Hcover:Seed location) was significant for 2 of 14 weed
species (i.e. BROST and VIOAR), and so the negative cor-
relation of Hcover on Hweed was amplified when seeds
were not buried (see Appendix S3 for guidelines for
interpreting results of LM with graphs).
Ann Appl Biol (2015) 5
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Weed emergence in no-till with cover crops S. Cordeau et al.
Figure 2 Effect of seed location (burial depth: 0.5 cm buried (B), on the soil surface (S)) and presence of a ryegrass cover crop (with cover C+, without C)onthe
percent emergence (% over 30 sown seeds) of 14 weed species (see Pvalues in Table S1). Weed species named with EPPO code (http://eppt.eppo.org/search.php).
Black dots indicate values higher than 1.5 times the box height. Dashes indicate the average.
Dry matter content of shoots
First, DMCweed (Fig. 3B, Table 2) was lower if weed
individuals emerged in the ryegrass cover (87.2%) and
if their seeds were initially unburied (70.6%). ANOVAs
on weed data subsets showed that the presence of ryegrass
cover always decreased DMCweed (see Table S1) with
a large range of values (e.g. AMBEL: 66.5%, BROST:
76%, VERPE: 97.8%). Second, DMCweed was lower for
unburied seeds for 10 of 14 weed species (e.g. CENCY:
24.8%, AMBEL: 33.6%, ANGAR: 74.2%, SONAS:
76.8%). Finally, there was no significant interaction
between cover and seed location.
LM of 11 of 14 weed species were significant (Table 3)
with a wide range of R-squared values. DMCweed was
both negatively (6 of 14 weed species) and positively
(2 of 14 weed species) correlated with DMCcover.DMC-
cover most impacted DMCweed for LAMPU (see Table 3,
slope =−12.9). The seed location effect was significant for
8 of 14 weed species; unburied seeds were a disadvantage
for six weed species and an advantage for two weed
species (VERPE and ALOMY). I (DMCcover:Seed location)
was both positively (VERPE and ALOMY) and negatively
(BROST, GERDI, POAAN) correlated with DMCweed.
Number of leaves
First, NLweed (Table 2) was lower if weeds grew in cover
(55.4%) and if their seeds were unburied (43.3%).
ANOVAs on weed data subsets (see Table S1) showed
that NLweed was lower in plants grown with cover for all
weed species, with a wide range of values (e.g. VLPMY:
19.5%, ANGAR: 84%). Second, NLweed was lower if
seeds were unburied for 8 of 14 weed species (e.g. VERPE:
12.7%, ANGAR: 70.9%). Finally, interactions between
both factors were significant for 3 of 14 weed species;
6Ann Appl Biol (2015)
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S. Cordeau et al. Weed emergence in no-till with cover crops
0
2
4
Predicted Hweed
(1)
-1
0
1
2
3
-10
-5
0
Predicted DMCweed
(2)
BSBS
Predicted NLweed
(3)
C- C+
A
B
C
Figure 3 Effect of seed location (burial depth: 0.5 cm buried (B), on the soil
surface (S)) and presence of a ryegrass cover crop (with cover C+, without
C) on the height (Panel A), the dry matter content (Panel B) and the number
of leaves (Panel C) of weed individuals (n=1433). These predicted y-values
were projections of linear models [Weed growth variable =Species +Seed
location +Cover +I(Seed location:Cover)] where the weed species effect was
subtracted. Big, black, filled dots indicate values higher than 1.5 times the
box height. Small, empty circles are values of each plant individual.
the negative effect of cover was stronger when seeds
were unburied [e.g. ANGAR:93.8%, BROST: 47.9%,
VLPMY: 21.7%, between NLweed in I(Seed location-B:
Cover-C-) and I (Seed location-S: Cover-C+)].
LM combining the effects of DMCcover and seed location
were significant for 8 of 14 weed species (Table 3). NLweed
was negatively correlated with DMCcover for 5 of 14 weed
species (ALOMY, BROST, POAAN, VERPE and VLPMY),
but were unexpectedly positively correlated for CAPBP.
NLweed was significantly lower for unburied seeds for 3
of 14 weed species (BROST, CAPBP and GERDI). Signif-
icant I (DMCcover: Seed location) for 2 of 14 weed species
(ALOMY and BROST) amplified the negative response of
NLweed to DMCcover when seeds were not buried.
Discussion
Effect of leaving seeds unburied
Our results showed that weed emergence varied accord-
ing to weed species and their seed location (burial depth,
i.e. 0.5 cm buried versus on the soil surface). Our range of
emergence rates of buried seeds, which was higher than
for unburied seeds, was consistent with previous studies
(Boyd & Van Acker, 2003; Gardarin et al., 2010). Indeed,
germination is usually better when seeds are slightly
buried, because temperatures and soil humidity are more
stable in the first soil layer (0.51 cm) than on the soil
surface (see Appendix S1). The soil surface, which repre-
sents the interface between the seed bank and the envi-
ronment, is a crucial zone in weed population dynamics.
However, our results suggest that the germination process
occurring on the soil surface seems to be different than in
the first soil layer. Consequently, some species could have
a greater disadvantage than others when their seeds are
on the soil surface.
Our analysis, performed on a wide range of weed
species, showed that not burying seed decreased the
emergence for 5 of 14 weed species (AMBEL, ANGAR,
BROST, CENCY and EPHHE). Many studies have shown
that seeds emerge higher if they are previously exposed to
light when they are moistened (Bliss & Smith, 1985; For-
cella et al., 2000). Therefore, unburied seeds may show
greater emergence if they are well moistened because
they are exposed to light. In our experiment, all seeds
were dry before sowing, and then only moistened after
sowing. Consequently, the weaker emergence of these
five weed species when their seeds were on the soil sur-
face could be explained by weak soilseed contact com-
bined with a weaker humidity than in buried conditions
(see Appendix S1). Soilseed contact could vary with
weed traits, such as seed weight shape (Grime et al., 1981).
In our study, four of these five species (all but ANGAR)
Ann Appl Biol (2015) 7
© 2015 Association of Applied Biologists
Weed emergence in no-till with cover crops S. Cordeau et al.
Table 3 Weed growth responses (height, dry matter content and number of leaves) to the initial seed location (buried or on the soil surface) and to the ryegrass
cover growth (height and dry matter content)a,b
Term
(Intercept) Hcover Seed location S I(Hcover:Seed location S) Model
Est. ±SE PcEst. ±SE PcEst. ±SE PcEst. ±SE PcR2Pc
HweeddALOMY 28.2 ±4.9 *** 7.3 ±1.4 *** 0.39 ***
AMBEL 0.17 *
ANGAR 4.7 ±1.6 ** 0.62 ***
BROST 16.8 ±6.5 *4.8 ±1.8 * 0.43 ***
CAPBP 0.08 0.20
CENCY 3.9 ±1.1 *** 0.6 ±0.3 *3.2 ±1.56 *0.11 **
EPHHE 0.7 ±0.1 *** 0.72 ***
GERDI 0.03 0.62
LAMPU 0.26 *
POAAN 0.04 0.54
SONAS 0.32 0.20
VERPE 12.3 ±2.6 *** 3.2 ±0.7 *** 0.33 ***
VIOAR 16.6 ±7.6 *5.1 ±2.3 * 0.19 0.13
VLPMY 0.25 ***
(Intercept) DMCcover Seed location S I(DMCcover:Seed location S) R2P
DMCweeddALOMY 8.6 ±0.5 *** 1.4 ±0.3 *** 2.1 ±1.0 *1.3 ±0.5 *0.31 ***
AMBEL 6.7 ±0.8 *** 1.8 ±0.6 ** 0.30 ***
ANGAR 8.0 ±2.1 *** 0.30 *
BROST 4.8 ±0.2 *** 2.1 ±0.3 *** 0.6 ±0.2 ** 0.63 ***
CAPBP 5.6 ±0.4 *** 1.1 ±0.3 *** 1.7 ±0.6 ** 0.39 ***
CENCY 4.7 ±0.4 *** 0.09 *
EPHHE 3.1 ±0.9 ** 1.4 ±0.6 *0.28 0.09
GERDI 4.1 ±0.9 *** 3.0 ±1.3 *2.2 ±1.1 *0.11 0.07
LAMPU 25.2 ±4.9 *** 12.9 ±3.3 *** 0.44 **
POAAN 8.3 ±0.16 *** 0.5 ±0.1 *** 0.7 ±0.2 ** 0.4 ±0.1 *0.65 ***
SONAS 8.4 ±0.7 *** 0.37 0.14
VERPE 7.9 ±0.1 *** 0.5 ±0.1 *** 0.5 ±0.2 ** 0.5 ±0.1 *** 0.39 ***
VIOAR 8.4 ±2.3 ** 0.30 *
VLPMY 8.3 ±0.2 *** 0.4 ±0.1 ** 0.36 ***
(Intercept) DMCcover Seed location S I(DMCcover:Seed location S) R2P
NLweeddALOMY 0.5 ±0.1 *** 0.3 ±0.1 *0.33 ***
AMBEL 1.7 ±0.3 *** 0.14 *
ANGAR 0.27 *
BROST 0.7 ±0.1 *** 0.1 ±0.1 *0.7 ±0.2 *** 0.2 ±0.1 *0.57 ***
CAPBP 1.5 ±0.1 *** 0.2 ±0.1 *0.6 ±0.2 *0.22 **
CENCY 1.2 ±0.2 *** 0.05 0.10
EPHHE 0.15 0.27
GERDI 1.3 ±0.2 *** 0.8 ±0.4 *0.09 0.11
LAMPU 0.15 0.21
POAAN 0.8 ±0.1 *** 0.1 ±0.01 ** 0.22 **
SONAS 0.43 0.09
VERPE 0.6 ±0.1 *** 0.2 ±0.1 *** 0.42 ***
VIOAR 0.6 ±0.2 ** 0.08 0.51
VLPMY 0.9 ±0.1 *** 0.2 ±0.04 *** 0.39 ***
Hweed: Height of a weed; DMCweed: above ground dry matter content of a weed; NLweed: number of leaves on a weed; Hcover : average height of a grass
plant; DMCcover: average above ground dry matter content of a grass plant; Seed location S: not buried seeds; (Intercept): buried seeds without cover; Est.:
Parameter estimates; SE: standard errors; and Pvalue (P) for effect of terms in the LM.
aSee Appendix S3 for guidelines for plotting and interpreting results of linear models. Weed species named with EPPO code (http://eppt.eppo.org/search.php).
bEmpty cells indicate no significant effects of the terms at the 5% level.
c*P<0.05, **P<0.01, ***P<0.001.
dANOVA for LM with Gaussian distribution, on weed sub-datasets, with ln-transformed response and quantitative dependent variables.
8Ann Appl Biol (2015)
© 2015 Association of Applied Biologists
S. Cordeau et al. Weed emergence in no-till with cover crops
had bigger seeds than the remaining species, and this
could alter the soilseed contact. Soilseed contact is
influenced also by others factors, such as the type of soil
(Rogers & Dubetz, 1980), but it cannot explain the dif-
ferences in our experiment. Moreover, based on a study
of 400 species, Grime et al. (1981) observed that germina-
tion rate is associated with seed shape, and that smaller
and more elongated seeds had higher germination rates.
Our results invite further investigations to find relation-
ships between emergence success and weed seed traits, as
initiated by Gardarin et al., (2011) on the speed of emer-
gence of buried seeds. Better understanding the traits or
combinations of traits that allow for high emergence rates
could lead to better understanding of which species might
persist in undisturbed soil such as in no-till fields.
We also showed that weed growth variables (Hweed,
DMCweed and NLweed) were sometimes lower when seeds
were sown on the soil surface. This could be explained by
a slower germination and root establishment. Our results
suggested further investigations on the dynamic of emer-
gence with daily records, to show that unburied seeds
emerged later than buried seeds, and so have reduced
growth.
Effect of grass cover
Our study demonstrated that emergence of 5 of 14 weed
species (ALOMY, CAPBP, SONAS, VERPE and VLPMY)
decreased in the presence of ryegrass cover. These were
not the same species that were sensitive to having
unburied seeds. Our greenhouse experiment with a
ryegrass canopy showed results that were consistent with
the reduced germination rate of other annual species
(i.e. Amaranthus quitensis H.B.K., Carduus acanthoides L.,
Galinsoga parviflora Cav., Portulaca oleracea L. and Raphanus
sp.) sown on the soil surface in a field experiment with a
wheat canopy (Kruk et al., 2006) and in no-tilled systems
with alfalfa canopy (Huarte & Arnold, 2003). The nega-
tive effect of cover on emergence could be explained by a
decrease in the quantity and quality of light transmitted
through the cover (Benvenuti et al., 2001a; Juroszek &
Gerhards, 2004). In our experiment, the ryegrass cover
was sown with a 600 seed m2density and an average
DMCcover of 42.69 g m2(0.42 t ha1). This ryegrass cover
was not thick enough to completely reduce the penetrat-
ing light as, for example, in a sown grass margin strip
where a three-year-old grass cover limits 60100% of
the light transmittance (Cordeau, 2010). However, it was
probably enough to reduce the quantity of penetrating
light and alter its quality, which in turn decreased the
germination rate. Indeed, the reduced red:far-red ratio
decreases germination of some weed species under crop
canopies (Kruk et al., 2006). The effect of modifications in
light environment would only be magnified for unburied
seeds. Therefore, some species were negatively impacted
twice over: by the cover and by being unburied.
Firstly, cover had no effect on emergence in most of
weed species we tested (9 of 14). This result is consistent
with the literature; some weed species are not photosen-
sitive and exposure to light or modifications of light qual-
ity do not break their dormancy (Juroszek & Gerhards,
2004). Milberg et al. (1996) concluded that photocon-
trol of weeds would require management of much vari-
ability between sites, and possibly years, and acceptance
of much unpredictability because of the great variation
between populations. Many researchers have found great
differences in the light sensitivity of different populations
of the same species (Naylor & Abdalla, 1982). Finally,
Bergelson & Perry (1989) showed that seed density could
affect seedling emergence of annual species. Indeed, in
our experiment the weed seeds were sown with the same
spatial pattern (see Appendix S2) with distances between
seeds that avoid early competition. However, the ryegrass
seeds in our experiment (600 seeds m2) could compete
with the weed seeds, resulting in low emergence in con-
tainers with ryegrass.
Potential weed community evolution in no-till systems
with cover crop
This study showed a lower percent emergence in the
majority of the annual weeds when seeds were located
on the soil surface and/or under a ryegrass cover crop. We
demonstrated that the resulting height (Hweed), dry mat-
ter content (DMCweed) and number of leaves (NLweed)
were also lower when weeds grew in a cover. These same
variables were either the same or lower when seeds were
also initially not buried. Consequently, our results sug-
gested that no-tillage systems, which concentrate the seed
bank on the soil surface (Bàrberi & Lo Cascio, 2001),
and the use of cover crops, which compete with weeds
during their initial stages, could affect a wide range of
annual weed species. Unburied seeds could also lead to
delayed emergence, and could benefit weed management
in no-till systems that use cover crops. Indeed, not bury-
ing seeds could lead to increased competition between
weedsandcovercrops,favouringcover(Mirskyet al.,
2011; Uchino et al., 2012; Mobli et al., 2013).
However, many results show that annual weed species
such as grasses (Norsworthy, 2008) or forbs (Reddy,
2003) still appear frequently in no-till systems. Some
authors explain the success of annual weed species in
no-till systems by their biological traits: weed species
with earlier emergence (Norsworthy, 2008) or smaller
seed weight should have an advantage over crops or
cover crops. In addition, even if our results showed that
Ann Appl Biol (2015) 9
© 2015 Association of Applied Biologists
Weed emergence in no-till with cover crops S. Cordeau et al.
weed species’ growth decreased with ryegrass cover, weed
species should be able to produce mature seeds and
replenish the soil seed bank if the cover is chemically
or mechanically killed too late. However, buried seeds
should persist for longer than unburied seeds because of
dormancy enforced by the dark (Chauhan et al., 2006),
and possible seed predation on unburied seeds (Bohan
et al., 2011).
Our results expand what is known about emergence
and initial growth processes when seeds are located on the
soil surface and are grown in the presence of a cover crop.
An innovative next step would be to correlate species
traits to predict the behaviour of a wide range of species
(Keddy, 1992). Based on this principle, model parame-
ters that are difficult to measure could be inferred from
easily accessible seed traits using generic functions estab-
lished with a few contrasting model species that rep-
resent the diversity of the weed flora (Gardarin et al.,
2009). Then, a better understanding of weed response
to modifications in the photothermal environment below
a crop canopy (e.g. monospecific crop such as ryegrass
or plurispecific synergetic cover crop) should allow us to
improve weed management strategies by manipulating
crop canopy attributes. This could be carried out by mod-
ifying sowing date, crop density, spatial arrangement and
species of cover crops. For example, increasing the plant
density of the cover crop and choosing the right cover crop
species to minimise the light transmittance would dimin-
ish the number of weeds that emerge during the first stage
of crop establishment. This strategy would be appropriate
in situations where weed seeds are predominantly located
on the soil surface, as has been typically found by numer-
ous studies of no-till cropping systems (Swanton et al.,
2000; Torresen et al., 2003).
Conclusion
This study showed that both not burying weed seeds
and the ryegrass cover crop (which both could be con-
sequences of no-till systems using cover crops) nega-
tively influenced emergence of different studied annual
weed species. These factors lead to decreased growth of
emerged seedlings for all species. These results indicate
that changes in the weed communities observed by field
surveys in previous studies in no-till cropping systems
with cover crops could be partly explained by the trait
studied here: the ability of seeds to emerge when they are
not buried.
Acknowledgements
Authors greatly appreciate the technical assistance of
the technicians of the greenhouse platform of INRA,
and technicians from the research unit LEVA (Groupe
Ecole Supérieure d’Agriculture), Sylvain Pineau, Vincent
Oury, Solange Renaud, Christine Goyer and Maud Goi-
chon. Some experiments were also conducted by engi-
neering students from the High School of Agronomy in
Angers (Groupe ESA). This work was financially sup-
ported by INRA, the school department of Agronomy
and Ecology of the High School of Agronomy in Angers
(Groupe ESA, Guillaume PIVA, department manager)
and the network “RMT Florad”. Authors acknowledge
Sharilynn Wardrop (Science Editor and Writer, clarity-
science.com) and anonymous referees for their construc-
tive revisions.
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Supporting Information
Additional Supporting Information may be found in the
online version of this article:
Appendix S1. Measurements of the environmental
conditions of the trials.
Appendix S2. Weed seed arrangement in container
(one weed species per container) with or without grass
cover).
Tab l e S 1. Effect of seed location and grass cover on the
emergence and development of 14 weed species.
Appendix S3. Guidelines for plotting and interpreting
results of GLMs.
12 Ann Appl Biol (2015)
© 2015 Association of Applied Biologists

Supplementary resources

  • ... Les couverts d'interculture réduisent la germination des adventices ( Cordeau et al., 2015) en diminuant en quantité et en qualité la lumière transmise à travers le couvert ( Juroszek et Gerhards, 2004). En effet, la réduction du ratio rouge/rouge lointain réduit la germination de quelques adventices sous la canopée ( Kruk et al., 2006). ...
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  • ... Cover crops can influence weeds either in the form of living plants or as plant residue remaining after the cover crop is killed. A fast growing living cover crop will decrease weed emergence[30]and compete with weeds growing at the same time, decreasing their growth up to 68%[31]. Incorporation of cover crop biomass can improve subsequent maize growth[32]. ...
    ... Higher weed infestation has been observed in conservation tillage agriculture[54]and if weeds are not well managed, like Echinochloa crus-galli (L.) P.Beauv. in MM CT which reached maturity at high densities (seeTable S3for list of species observed with mature or immature seeds at maize maturity in the four cropping systems), they can replenish the weed seedbank and create challenges for weed management over the long term. However, it is not always observed[12]because weed emergence is reduced when seeds remain on the soil surface as in zero-tillage systems[30]. Indeed, under these conditions seed-soil contact is poor and seeds are exposed to light[59]. ...
    ... & Schult.), which completely dominated after the first two years of the experiment, as reported in previous studies[4]. Grasses are adapted to conservation agriculture because no-till maintains weed seeds on the soil surface, where annual grass seeds are able to germinate[30]. Convolvulus arvensis L., a geophyte perennial broadleaf, most likely benefited from this system due to minimal soil disturbance[11]. ...
    Article
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    Conventional Maize Monoculture (MM), a dominant Cropping System in South-Western France, is now questioned for environmental reasons (nitrate leaching, pesticide use and excessive irrigation). Three low-input Cropping Systems (CS) using diverse weeding strategies (MMLI, a Low-Input MM implementing ploughing, a combination of on-row spraying and in-between row cultivation and cover crops; MMCT, Conservation Tillage MM implementing chemical control and cover crops; Maize-MSW, maize managed similar to MMLI but rotated with soybean & wheat) were compared to a reference system (MMConv, a conventional MM with tillage and a high quantity of inputs). Potential of Infestation of weeds (PI), weed biomass and crop production of these CS were compared during the first five years after their establishment. Yields were also assessed in weed-free zones hand-weeded weekly in 2014 and 2015. Weed communities did not drastically differ among CS. PI and weed biomass were higher in MMCT, especially for Echinochloa crus-galli (L.) P.Beauv. and were comparable between MMConv, MMLI and Maize-MSW. Analysis of covariance between CS and weed biomass did not reveal a significant interaction, suggesting that weed biomass affected yield similarly among the CS. Comparison between weedy and weed-free zones suggested that weeds present at maize maturity negatively affected yields to the same extent for all four CS, despite having different weed biomasses. Grain yields in MMConv (11.3 ± 1.1 t ha−1) and MMLI (10.6 ± 2.3 t ha−1) were similar and higher than in MMCT (8.2 ± 1.9 t ha−1). Similar yields, weed biomasses and PI suggest that MMLI and Maize-MSW are interesting alternatives to conventional MM in terms of weed control and maize productivity and should be transferred to farmers to test their feasibility under wider, farm-scale conditions.
  • ... The depth of the buried weed seeds (0.5 cm) was optimal for germination and emergence ( Gardarin et al. 2009). The photosynthetically active radiation at this depth was recorded in a previous study, showing no light transmittance for the buried seeds (see Appendix S1 in Cordeau et al. 2015). The seed location was a split-plot treatment, as it was nested within containers, while the cover crop, Weed species and hydric stress all were applied to entire containers, making them the main plot treatments. ...
    ... This study confirmed that the presence of a cover crop can decrease weed emergence, as previously demonstrated with ryegrass ( Cordeau et al. 2015), wheat ( Kruk et al. 2006) and alfalfa ( Huarte & Arnold 2003). In addition, it was demonstrated here that the negative effect of a cover crop on weed emergence was magnified under hydric stress, with a greater impact on weed growth than on weed emergence. ...
    Article
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    In conservation agriculture, weed seed germination could decrease with the presence of a cover crop, surface weed seed location and temporal drought in summer just after seed shedding. This study simultaneously examined the effects of a cover crop, burial depth (seed location) and hydric stress on weed emergence and early growth. It was hypothesized that drought would reduce weed emergence and the initial growth of weed seeds and that this effect would be greater when the seeds were on the soil surface and in the presence of a cover crop. Four annual weed species were chosen that are frequently found (Anisantha sterilis, Vulpia myuros, Sonchus asper, Veronica persica) and not frequently found (Alopecurus myosuroides, Poa annua, Cyanus segetum, Capsella bursa-pastoris) in fields that implement conservation agriculture. The unburied seeds had 26% lower emergence, on average, than the buried seeds (significant for six of the eight species), hydric stress reduced emergence by 20% (for seven of the eight species) and the presence of a cover crop reduced the level of emergence by 17% (for all species). The unburied seeds with hydric stress were emerging under the “most stressful” set of factors, with a 45% decrease in emergence, compared with the seeds emerging under the “least stressful” set of factors (buried seeds without hydric stress). All the weed growth measurements (height, dry matter content and number of leaves) decreased with the presence of a cover crop. The species that are found frequently in the fields that implement conservation agriculture, compared with the species that are not frequently found in conservation agriculture fields, had higher rates of germination and a higher tolerance of hydric stress when their seeds were unburied.
  • ... In spring-sown crops, associated filters such as primary tillage, herbicide application or crop canopy closure occur early in the growing season, while in fall-sown crops, these filters operate later in the growing season or early in the next growing season. Weed species differ in their phenology, seed germination requirements, periodicity and duration of germination, and response to secondary soil disturbance (Guillemin et al., 2013;Cordeau et al., 2015). Consequently, whereas primary tillage can be a strong driver of seedbank dynamics (Bàrberi & Lo Cascio, 2001;Sosnoskie et al., 2006) and can determine the composition of the weed community emerging later in the crop (Schutte et al., 2014), the interaction between the timing of tillage and the specific traits of individual weed species with regard to their emergence periodicity and other germination behaviours suggests that the seasonal timing of tillage should also be a strong weed community assembly filter (Smith, 2006;Ryan et al., 2010). ...
    ... Certainly there is a strong positive correlation between seed size and ability of buried seed to emerge from various soil depths (Chauhan et al., 2006b). However, germination and emergence of seeds is generally low regardless of seed traits when seeds remain on the soil surface (Cordeau et al., 2015). Moreover, intraspecific variability in seed traits is generally very high (Esser & Overdieck, 2013). ...
    Article
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    Previous research has demonstrated that the season in which soil is tilled (spring versus fall) can strongly influence weed community assembly and subsequent species composition and abundance in annual cropping systems. Despite this understanding, it is unknown whether finer-scale, within-season variation in the timing of tillage has similar impacts on weed community assembly. We conducted an experiment on four research farms across the northeastern USA to test the effects of tillage timing on weed emergence periodicity. Soil was tilled at 12 different times that were 2 weeks apart from 29 April to 30 September (the entire growing season) and the composition and abundance of the weed seedlings that emerged was measured 6 weeks later. Weed species clustered into three tillage timing groups at the two New York locations and clustered into five tillage timing groups at the New Hampshire and Maine locations. Individual species associated with each window of tillage time varied by location. No single trait or combination of traits were consistently associated with species-by-tillage time groupings across locations; however, within each location several traits were associated with particular groups of species, including: (a) seed length, (b) seed weight, (c) cotyledon type, (d) life span, (e) ploidy level and (f) photosynthetic pathway. These results suggest that fine-scale variation in the timing of tillage can lead to predictable changes in the species composition and trait distribution of weed communities in annually tilled agroecosystems.
  • ... The total vegetation cover never reached 100%. This suggests that open area exist in the sown grass cover and that light can reach the ground and stimulate germination [9]. Weed species richness was not affected by the age (i.e. ...
    Article
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    Sown grass strips could be an opportunity (refuge) or a threat (source of crop infestation) for weed management in arable landscapes. Firstly, our field surveys recorded 187 weed species, among which 90% were arable species, mostly perennial and wind dispersing species, here mostly represented by the Asteraceae family. Even if sown grass strips harboured a high richness of unsown species (26 weed species/grass strip), the richness did not vary according to the time since establishment. Weed community composition drastically changed over time, with a quick shift (within the 2 first years) from annual to perennial species, and from common agricultural weeds to non-arable species. Secondly, flora surveys performed with continuous transects from the field margin to the field centre showed clear plant spatial patterns. The sown grass strips acted as an ecotone with a sharp vegetation transition. Weeds occurring within the centre of the sown grass strips radically differed from species occurring within the first 0.5m of the cultivated area (only 15% of similarity between communities occuring in both habitats). Moreover, classical field margins influenced the weed composition up to 4.5m in the field whereas sown grass strips influenced the weed composition only up to 2m. We concluded that sown grass strips could be an opportunity to maintain plant diversity at the landscape scale and to decrease weed dispersion from the field margin to the field core in the short term. However, we discussed the long term impacts, especially to maintaining high level of annual species and segetal species in these perennial semi-natural habitats.
  • ... However, Fried et al.[2]also reported that, after partitioning out the effects of timing of crop sowing and associated tillage practices, and weather conditions, only 18% of the explained variance in weed composition was due to crop type. Tillage is one of the main drivers of weed community assembly because primary tillage concomitantly buries and stimulates the germination of weed seeds[15], and secondary tillage kills the resulting seedlings, thereby decreasing seed density in the soil[16,17]. Weed control practices often drive weed community assembly, as only species that can tolerate or avoid these practices survive and persist in the weed community. ...
    Article
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    The effect of tillage timing on weed community assembly was assessed at four locations in the Northeastern United States by tilling the soil every two weeks from April to September and quantifying the emerged weed community six weeks after each tillage event. Variance partitioning analysis was used to test the relative importance of tillage timing and weather on weed community assembly (106 weed species). At a regional scale, site (75.5% of the explained inertia)—and to a lesser extent, timing—of tillage (18.3%), along with weather (18.1%), shaped weed communities. At a local scale, the timing of tillage explained approximately 50% of the weed community variability. The effect of tillage timing, after partitioning out the effect of weather variables, remained significant at all locations. Weather conditions, mainly growing degree days, but also precipitation occurring before tillage, were important factors and could improve our ability to predict the impact of tillage timing on weed community assemblages. Our findings illustrate the role of disturbance timing on weed communities, and can be used to improve the timing of weed control practices and to maximize their efficacy.
  • Conference Paper
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    Les espèces annuelles produisent des graines chaque année pour maintenir leur population. Ces graines tombent à la surface du sol. Si aucun travail du sol n'est réalisé (ex. cas du semis direct), les graines doivent germer en surface du sol, exposées aux variations environnementales, et parfois même dans un couvert. Cette étude a permis d'étudier l'effet du non-enfouissement des graines, et des conditions environnementales (humidité, lumière, présence d'un couvert) sur la germination, l'émergence et la croissance de nombreuses espèces annuelles. La germination est réduite (en moyenne, toutes espèces confondues) de 26% lorsque les graines sont laissées en surface, de 19% en situation de stress hydrique, et de 17% avec la présence d'un couvert. L'effet négatif du stress hydrique est amplifié quand les semences sont en surface. La croissance et le développement des adventices (hauteur, biomasse, nombre de feuilles) sont réduits, par ordre d'importance, par le couvert, puis le stress hydrique et enfin la position initiale de la graine. Ces réponses varient selon les espèces adventices étudiées. Cette étude confirme que la germination est une phase cruciale pour les espèces annuelles et qui peut être perturbée en cas de non-travail du sol.
  • Article
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    In the context of a shift towards pesticide reduction in arable farming, weed management remains a challenging issue. Integrated Weed Management currently recommends agronomic practices for weed control, but it does not integrate the use of biodiversity-based options, enhancing the biological regulation of weeds. Here, wereview existing knowledge related to three potentially beneficial interactions, of crop–weed competition, weed seed granivory, and weed interactions with pathogenic fungi. Our main finding are the following : (1) promoting cropped plant–weed competition by manipulating cropped cover could greatly contribute to weed reduction ; (2) weed seed granivory by invertebrates can significantly lower weed emergence, although this effect can be highly variable because seed predation is embedded within complex multitrophic interactions that are to date not fully understood ; (3) a wide range of fungi are pathogenic to various stages of weed development, but strain efficacy in field trials does not often match that in controlled conditions. We present a framework that superimposes biodiversity-based options for weed biocontrol on a classical Integrated Weed Management system. We then describe the current state of knowledge on interactions between agronomic practices and the organisms at play and between the different biological components of the system. We argue that further advances in our understanding of biodiversity-based options and their performance for weed biocontrol will require farm-scale experimental trials.
  • Article
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    The objective of this work was to evaluate the dry matter yield of cover crops and their suppressive effects on weeds. The experiment was carried out during three years in a cerrado area of the state of Goiás, Brazil, and consisted of 16 treatments with fallow and cover crops cultivated in single cropping and intercropping. Fallow allowed high weed infestation. Cover crops affected the composition of weeds, which showed greater diversity in fallow, followed by the Pennisetum glaucum 'BRS 1501' and Cajanus cajan crops. In the average of the three experimental years, the highest dry matter yield was observed for the treatments Panicum maximum (10,857 kg ha-1), Urochloa brizantha 'Piatã' (11,437 kg ha-1), U. ruziziensis (9,463 kg ha-1), and U. ruziziensis intercropped with Crotalaria spectabilis (9,167 kg ha-1), which prevented weed infestation. Pennisetum glaucum 'BRS 1501' had a low dry matter yield (<5,000 kg ha-1) and did not suppress weeds. Panicum maximum, U. brizantha 'Piatã', U. ruziziensis, and U. ruziziensis intercropped with C. spectabilis provide high dry matter yield and suppress weed infestation in the cerrado area.
  • Article
    Cyanus segetum is an iconic, colourful weed in arable fields that provides ecological and societal services. To understand better both the infestation dynamics of C. segetum as an abundant, harmful weed and maintain sustainable populations where it provides beneficial services, we compared information on seed dormancy, seed longevity and germination conditions in two populations. Persistence of seeds buried in the soil was low, with <10% viable after 3 years. Periodic dormancy cycling was observed over the 4 years in the soil, with a maximum of dormant seeds in the spring and a minimum in the autumn; however, 20% of the seeds were non-dormant all the time. Seeds of C. segetum were positive photosensitive, but light requirement varied among populations. Base water potential for germination was −1 MPa. Base temperature ranged from 1 to 2°C. Optimum temperature for germination was about 10 to 15°C, but the mean thermal time varied greatly between populations, from 80 to 134 day °C. Photoperiod and temperature combinations had no effect on germination percentage, but both reduced the germination rate. Burial deeper than 2 cm greatly reduced germination and seedling emergence strongly decreased at depths >0.5 cm. No seeds buried deeper than 8 cm emerged. Low seed longevity and a wide range of germination conditions could partly explain the rapid disappearance of C. segetum populations after herbicide application began in western Europe. However, yearly sowing in restoration areas does not seem to be essential.
  • Article
    Trials were carried out to investigate the effects of seed burial depth on seedling emergence rate of 20 weed species. Marked depth-mediated variation in emergence ability of the different species was observed, together with a general pattern of decreasing emergence with increasing soil depth. At 10 cm, only johnsongrass, velvetleaf, catchweed bedstraw, and cutleaf geranium emerged, albeit only in limited numbers. Species most severely inhibited by burial depth were buckhorn plantain, large crabgrass, common purslane, chickweed, and corn spurry, none of which emerged from beyond 6 cm. In all species, depth-mediated inhibition was found to be sigmoidal (polynomial regression). In addition, the number of seedlings and rate of seedling emergence decreased when depth of burial increased. The depth at which the number of emerged seedlings was halved varied by species and ranged from 3.6 cm for common purslane and chickweed to 7 cm for velvetleaf and catchweed bedstraw. Excessive burial depth generally induced dormancy (in roughly 85% of cases) rather than suicide germination. A close inverse relation (second-degree equation) between seed unit weight and depth-mediated inhibition was observed. The physiological involvement of depth inhibition in seed bank ecology is discussed.
  • Article
    Weed control by rye, crimson clover, subterranean clover, and hairy vetch cover crops was evaluated in no-tillage corn during 1992 and 1993 at two North Carolina locations. Weed biomass reduction was similar with rye, crimson clover, and subterranean clover treatments, ranging between 19 and 95% less biomass than a conventional tillage treatment without cover. Weed biomass reduction using hairy vetch or no cover in a notillage system was similar averaging between 0 and 49%, but less than other covers approximately 45 and 90 d after planting. Weed biomass was eliminated or nearly eliminated in all cover systems with PRE plus POST herbicide treatments. Weed species present varied greatly between years and locations, but were predominantly common lambsquarters, smooth pigweed, redroot pigweed, and broadleaf signalgrass. Corn grain yield was greatest using PRE herbicides or PRE plus POST herbicides, averaging between 16 to 100% greater than the nontreated control across all cover treatments depending on the year and location.
  • Article
    Conservation agriculture is designed to deliver more sustainable cropping systems by preserving agricultural soils with tillage abandonment. However, knowledge on the impacts of Conservation agriculture adoption on weed infestation level and potential shifts in the composition of weed communities appears low and contradictory. We used a trait-based approach to investigate whether there are shifts in values of a set of traits within weed communities following the adoption of Direct Drilling with cover-crop (DD) which is one of the Conservation Agriculture practices. Weed surveys were conducted across a range of times since conversion to DD in 52 winter wheat fields located in north-eastern France. A three-table ordination method (RLQ analysis) was performed to relate environmental data to species traits data using weed community composition data. We found a shift in the weed community toward perennial and monocotyledon species with increasing time since conversion to DD. Weeds tended to invest more in maintaining their roots system than in seed production as time since conversion increased. Thus, weeds developing in DD systems tended to be more persistent, and this poses a challenge for management with current practices.
  • Article
    The interactions between seeds in the soil are poorly understood. We performed greenhouse experiments to investigate the effects of seed density, relative frequency, and relative planting date on the emergence of seedlings in the species Senecio vulgaris, Capsella bursa-pastoris, and Poa annua. We found that for both Poa and Senecio, the probability of emergence significantly decreased with an increase in total seed density. Neither the density of conspecifics nor heterospecifics alone could explain this decline in the probability of emergence. We also found that, for all three species studied, the rate of emergence accelerated in the presence of previously planted seeds. A second experiment indicated that this acceleration involves a response to leachate from previously germinated seeds.
  • Article
    (1) Using a standardized procedure, a laboratory study was made of the germination characteristics of seeds collected from a wide range of habitats in the Sheffield region. Measurements were conducted on freshly-collected seeds and on samples subjected to dry storage, chilling and scarification. Responses to temperature and light flux were also examined. (2) The data have been used to compare the germination biology of groups of species classified with respect to various criteria including life-form, family, geographical distribution, ecology, and seed shape, weight and colour. (3) Marked differences were observed in the capacity of freshly-collected seeds for immediate germination. Of the 403 species examined, 158 failed to exceed 10% germination but 128 attained values greater than 80%. Germination was high in the majority of grasses and low in many annual forbs and woody species. With respect to initial germinability, major families could be arranged in the series Gramineae > Compositae > Leguminosae = Cyperaceae > Umbelliferae. Many small-seeded species were able to germinate immediately after collection and seeds of these species were often elongated or conical and had antrorse hairs or teeth on the dispersule. High initial germinability was conspicuous among the species of greatest abundance in the Sheffield flora. (4) In the majority of species, germination percentage increased during dry storage; this effect was most marked in small-seeded species. Among the seventy-five species which responded to chilling, some germinated at low temperature in darkness whilst others were dependent upon subsequent exposure to light or to higher temperature or to both. Responses to chilling were characteristic of the Umbelliferae. In all of the legumes examined, rapid germination to a high percentage was brought about by scarification. (5) Under the experimental conditions, all of the annual grasses showed the potential for rapid germination. High rates were also observed in many of the annual forbs and perennial grasses. Low rates of germination occurred in the majority of sedges, shrubs and trees, and were particularly common in species of northern distribution in Britain. Rapid germination was characteristic of the species of greatest abundance in the Sheffield flora. Rate of germination showed a progressive decline with increasing seed weight, and, with some exceptions, there was a positive correlation between rate of germination and the relative growth rate of the seedling. (6) In sixteen species, germination in the light was found to be dependent upon exposure to diurnal fluctuations in temperature. Under constant temperature conditions, the majority of grasses, legumes and composites germinated over a wide range of temperature, and the same feature was evident in species of ubiquitous or southern distribution in the British Isles. A requirement for relatively high temperature was apparent in sedges, in plants of northern distribution and in a majority of the marsh plants. The range of constant temperatures conducive to germination tended to be wider in grassland plants than in woodland species. Rapid germination over a wide range of temperature occurred in many of the species which attain greatest abundance in the Sheffield flora. (7) Although germination in most species was promoted by light, some were inhibited under relatively high light flux. In 104 species a marked reduction in germination occurred if seeds were kept in the dark, and in many species this inhibitory effect could be intensified by either or both excluding temperature fluctuations and abandoning the use of a green `safety' light. The capacity for germination in darkness was observed in all of the legumes and many of the grasses. Dark germination did not occur in the Cyperaceae and was uncommon in the Compositae. The inhibitory effect of darkness was characteristic of many of the species known to form reserves of buried seeds, but it occurred also in certain species with more transient seed banks. (8) There were recurrent associations between features of seed morphology and of germination, several of which coincided with particular ecological characteristics. (9) The functional significance of some of the germination characteristics observed in this study leads us to the conclusion that certain regenerative mechanisms in the field may be predicted from the laboratory characteristics of the seed.
  • Article
    In order to evaluate the effects of some cover crops on weed populations and biomass during sunflower growth, an experiment was done in 2012 at the research field of Tabriz University, Iran. The experimental design was a randomized complete block with nine treatments in three replications. Treatments included triticale, hairy vetch, rapeseed, triticale + hairy vetch, triticale + rapeseed, hairy vetch + rapeseed, application of trifluralin herbicide, and controls (weed infested and weed free without planting cover crop). Results indicated that total weed density was reduced 44.92% in triticale + rapeseed treatment, but application of trifluralin caused 64.24% reduction in total weed density in comparison with weed infested. However, in triticale + rapeseed treatment, total weed dry biomass was reduced 72.12% compared with weed infested, so that this treatment was better than application of trifluralin. The use of cover crops as a strategy to reduce the damage of weeds and application of herbicide can be helpful.