The Weed-Suppressive Ability of Summer Cover Crops in the Northern Grains Region of Australia - MDPI
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agronomy
Article
The Weed-Suppressive Ability of Summer Cover Crops in the
Northern Grains Region of Australia
Asad Shabbir * , Lucy Hickman and Michael Walsh
School of Life and Environmental Sciences, The University of Sydney, Camden, NSW 2570, Australia;
lhic4374@uni.sydney.edu.au (L.H.); m.j.walsh@sydney.edu.au (M.W.)
* Correspondence: asad.shabbir@sydney.edu.au; Tel.: +61-409411878
Abstract: Pressure is mounting on the agricultural sector to reduce reliance on herbicides for weed
control leading to increased interest in the potential of cover crops to control weeds in summer
fallows. The weed suppression ability of three summer cover crop species, buckwheat, millet and
teff, was evaluated in field trials at two sites near Camden, NSW in 2021. Buckwheat, millet and teff
reduced weed biomass by 65%, 77% and 95%, respectively at Bringelly and by 94%, 92% and 90%,
respectively at Lansdowne. Following cover crop desiccation, teff residues reduced weed emergence
in subsequently planted wheat by 73% and 26% at Bringelly and Lansdowne, respectively. Overall
cover crops were found to be effective in suppressing weed emergence, growth, and reproductive
capacity. These studies identified teff grass as an important summer crop option for the northern
grains region.
Keywords: fallow; northern region; weed suppression; IWM; teff grass; weed emergence
1. Introduction
Citation: Shabbir, A.; Hickman, L.;
Walsh, M. The Weed-Suppressive The summer fallow period in the northern grain-based farming systems of Australia
Ability of Summer Cover Crops in is the time between winter crop harvest (December) and the following winter crop planting
the Northern Grains Region of (May). This fallow period enables beneficial outcomes of the accumulation of mineral
Australia. Agronomy 2022, 12, 1831. nutrients, increased soil water storage and improved soil health [1]. As the northern grains
https://doi.org/10.3390/ region experiences summer-dominant rainfall (Table 1), the production of winter crops such
agronomy12081831 as wheat, chickpeas and canola is dependent on effective fallow management to ensure
maximum soil water storage. Fifty percent of winter crop yield potential can be directly
Academic Editor: Ilias Travlos
attributed to summer rainfall and summer fallow management, in particular the increase
Received: 25 June 2022 in stored soil water and N [2], making it essential for farmers to efficiently manage summer
Accepted: 27 July 2022 fallow lands.
Published: 2 August 2022 Despite the potential benefits of fallow periods, there can be negative consequences
Publisher’s Note: MDPI stays neutral when the soil is left bare over this period. This includes increased evaporation, rainfall
with regard to jurisdictional claims in runoff and erosion, as well as the emergence of unwanted weeds that can persist into
published maps and institutional affil- the following cash crop. Weeds in summer fallow are a particular concern to growers in
iations. Australia’s northern grains region as they deplete the levels of available soil water and
nutrients for following crops. [3]. The lack of control of summer weeds, such as flax-leaf
fleabane (Conyza bonariensis (L.) Cronq.), awnless barnyard grass (Echinochloa colona (L.)
Link), and common sowthistle (Sonchus oleraceus L.), in fallows has been estimated to result
Copyright: © 2022 by the authors. in annual losses of up to 1.7 M tonnes of grain yield, equivalent to over AUD 428 million [4].
Licensee MDPI, Basel, Switzerland. Such losses make summer fallow weed control a major focal point for northern region
This article is an open access article grain producers.
distributed under the terms and
Typically, summer fallow weed control has been reliant on herbicides, in particular
conditions of the Creative Commons
glyphosate, which has resulted in the evolution of glyphosate resistance in 17 weed species
Attribution (CC BY) license (https://
commonly occurring in fallow and crop situations in Qld and NSW [5]. Glyphosate resis-
creativecommons.org/licenses/by/
tance is now occurring at high frequencies in populations of awnless barnyard grass (36%),
4.0/).
Agronomy 2022, 12, 1831. https://doi.org/10.3390/agronomy12081831 https://www.mdpi.com/journal/agronomyAgronomy 2022, 12, 1831 2 of 9
windmill grass (Chloris truncata R.Br.) (56%), feathertop Rhodes grass (Chloris virgata. Sw.)
(68%) and common sowthistle (14%) [6].
High frequencies of glyphosate resistance have led to an increasing interest in alterna-
tive weed control options and the adoption of integrated weed management techniques.
These include agronomic management decisions such as enhanced crop competition (e.g.,
narrow row spacing and higher plant densities) and competitive crop species and cultivars
as well as the introduction of cover crops to suppress weeds [7]. During a fallow, cover
crops can be used to effectively control weed populations while not negatively impacting
following crop yields, particularly by sustaining and improving soil water retention [8].
Cover crop is a broad term used to describe a crop that is grown for ecological benefits
other than being solely a harvestable product. Such crops are often planted in periods of
fallow, when the soil would otherwise be left bare, or with some crop residue (stubble)
cover. For cover crops to be viable in fallow, they must have a positive impact on the
farming system and/or subsequent crop [9]. There have been extensive studies on the
benefits of cover crops, including their ability to stabilise soils, fix carbon, alter nutrient
availability, increase agroecosystem diversity and complexity and supress weed emergence
and growth [10–12]. The ability of cover crops to suppress the emergence and growth
of weeds is a significant benefit for northern growers as they look towards new ways to
manage glyphosate-resistant weeds.
Common commercially available summer crop species suitable for cover crop use
in the northern region include Japanese millet (Echinochloa esculenta A. Braun), lablab
(Lablab purpureus L.) and soybean (Glycine max L. Merrill) [13], with other species such
as Sudan grass (Sorghum sudanense L.), forage rape (Brassica napus L.) and buckwheat
(Fagopyrum esculentum Moench.) also common [14]. These options all possess similar char-
acteristics of fast establishment rates, rapid growth and high levels of biomass production.
Although the species listed above are suitable as cover crops, there is always a need for
additional and potentially superior cover crop options.
Teff grass or teff (Eragrostis tef (Zuccagni) Trotter), originally from Ethiopia, is a self-
pollinating annual grass with small seeds that has been grown for human consumption
for centuries in African countries while emerging as a popular forage crop in recent
decades [15]. Teff could potentially be a successful summer cover crop in the northern
grains region as it prefers warm temperatures; has rapid establishment rates, high biomass
production and organic matter building potential; and can be incorporated with relative
ease into a cropping system [16]. While currently not widely grown, there have been
limited studies on the use of teff as a cover crop and its weed-suppressive ability [16,17]
with hope that more research into the species could help validate it as a valuable option for
northern grain production systems.
The aims of this study were to (i) determine the potential for teff as a summer cover
crop species for the northern grains region by comparing biomass production and weed
suppression with other cover crop species and (ii) determine if there is a relationship
between cover crop biomass production and weed suppression.
2. Materials and Methods
2.1. Experimental Sites
Field experiments evaluating the weed-suppressive ability of summer cover crop species
were established at two sites on 18 February 2021, at Bringelly (33.9453◦ S, 150.6821◦ E) and
Lansdowne (34.0215◦ S, 150.6647◦ E) farms of the University of Sydney. The field trials
were established at two sites to increase the validity of the results while determining
if varying factors such as soil type and background weed population impacted cover
crop establishment and growth. Both sites were prepared for crop planting through the
application of glyphosate (1 L ha−1 ). The Bringelly site has a loamy soil, in contrast with
Lansdowne, which has a finer, well-draining, sandy soil. Due to their proximity, both
Bringelly and Lansdowne experience similar climatic conditions to that of Camden, with a
predominantly summer rainfall pattern averaging 238 mm over December, January andAgronomy 2022, 12, x FOR PEER REVIEW 3
Agronomy 2022, 12, 1831 3 of 9
proximity, both Bringelly and Lansdowne experience similar climatic conditions to
of Camden, with a predominantly summer rainfall pattern averaging 238 mm over
cember, January and February. This rainfall pattern, similar to that of the northern g
February. This region,
rainfall was
pattern, similar
reflected in to that
the of the
2021 northern
climate data grains region,
; however, was
both reflected
sites experienced hi
in the 2021 climate data; however, both sites experienced
than average rainfall in March (Table 1). higher than average rainfall in
March (Table 1).
Table 1. Climate of Camden, NSW. Long-term maximum and minimum temperature data are b
Table 1. Climateonofaverages
Camden, NSW. Long-term
of 1971–2021, maximum
while rainfall andon
is based minimum temperature
data for 1943–2021. data Bureau
(Source: are of Me
based on averages of 1971–2021,
ology) [18]. while rainfall is based on data for 1943–2021. (Source: Bureau of
Meteorology) [18].
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov D
Jan Feb Mar Apr May Long-term
Jun Julclimate
Aug Sep Oct Nov Dec
Mean min temp (°C) 17 17 15 11 7 5 3 3.9 7 10 13
Long-term climate
◦
Mean min temp ( C) Mean
17 max temp
17 (°C)15 11 30 29
7 275 24 3 213.9 18 7 17 1019 22
13 24
15 26
◦
Mean max temp ( C) Total
30 rainfall
29 (mm)27 24 80 101
21 95
18 6417 5319 65 22 35 2441 38
26 62
29 74
Total rainfall (mm) 80 101 95 64 53 65 35
2021 climate * 41 38 62 74 57
Mean min temp (°C) 2021
16 climate
13 * 15 9 8 4 3 4 6 - -
Mean min temp (◦ C) 16 13 15 9 8 4 3 4 6 - - -
Mean max temp (◦ C)
Mean
30
max temp
17
(°C)25 24
30 17
20
25
17
2417
2020
17 23
17 -
20 -
23 -- -
Total rainfall (mm) Total
64 rainfall
85 (mm)327 14 64 85
82 327
45 1416 8242 45 23 16 - 42 -23 -- -
* Months averages shown in bold indicate
* Months averages shown in bold indicate experiment duration.
experiment duration.
2.2.
2.2. Experimental Experimental Design
Design
The experimentalThe design
experimental
at eachdesign at eachofsite
site consists consists
four of four
treatments treatments
(three (three cover
cover crop
species and a crop-free control-fallow), with four replicates of each treatment arranged in a arrang
species and a crop-free control-fallow), with four replicates of each treatment
a randomised
randomised complete complete
block design. block
This design.
design wasThis designat
repeated was
bothrepeated at bothareas
sites. Buffer sites. Buffer a
were left around the trials to minimise the risk of external weed contamination and spray and s
were left around the trials to minimise the risk of external weed contamination
drift. Each plotdrift. Each plot
including including
buffer zones buffer zones
and the fourand the four treatments
treatments measured measured
12 m × 4812 m.m × 48 m
three treatment cover crops used were teff grass (E. tef), buckwheat
The three treatment cover crops used were teff grass (E. tef ), buckwheat (F. esculentum) (F. esculentum)
and white Frenchwhite French
millet millet (P. milliaceum)
(P. milliaceum) (Figure 1)(Figure
with a 1) with aplot
control control
usedplot used to replicate
to replicate a a
bare fallow. fallow.
Figure 1. The three cover
Figure 1. crop species
The three grown
cover cropinspecies
field trials
grown at in
Lansdowne
field trials and Bringelly: (A)
at Lansdowne and buck-
Bringelly: (A)
wheat, (B) white wheat,
French(B)
millet and (C) teff.
white French millet and (C) teff.
Individual plots (2 × 12 m)
Individual were
plots (2planted
× 12 m) in 6 rows,
were 25 cm
planted in 6apart
rows,on
2518
cmFebruary
apart on2021
18 February
with buckwheat and millet planted using a plot seeder, model Wintersteiger Plotseed XL. Plotseed
with buckwheat and millet planted using a plot seeder, model Wintersteiger
Because of the small seed size, teff was planted by mixing the seed with potting mixture
and spreading it by hand over the designated areas. The teff plots were then run over with
the plot seeder, where the disc disturbance moved the seeds into rows. Fertiliser, MAP
supreme Zn at the rate of 50 kg ha− 1 , was applied to all plots at sowing. Trial sites wereAgronomy 2022, 12, 1831 4 of 9
irrigated with an overhead sprinkler system as required, ensuring the proper establishment
and growth of the crops. Each site had a top dressing of urea fertilizers (N) applied 6 weeks
after emergence.
2.3. Data Collection
Cover crop and weed emergence counts were conducted by counting crop and weed
plants in 4 × 0.25 m2 quadrats placed randomly in each plot three weeks after planting.
At seven weeks after planting, the crop heights of three randomly selected plants were
recorded in each plot using a metre ruler to measure height from the ground to the highest
standing point of the crop plant. At the same time, the biomass of cover crops and any
weeds present was assessed by cutting crop and weed plants at ground level in 4 × 0.25 m2
quadrats in each plot. Harvested crop and weed plant samples were placed in separate
pre-labelled brown paper bags and put in a dehydrator set at 70 ◦ C for 72 h. The dried
samples were then weighed to determine dry biomass production.
2.4. Wheat Crop Establishment and Weed Emergence in Cover Crop Residues
To investigate the effects of cover crop residues on following wheat crop establish-
ment, both cover crop trials were desiccated eight weeks after cover crop planting using
glyphosate (Roundup) at the rate of 1 L ha−1 plus saflufenacil (Sharpen) 25 g ha−1 , followed
by the slashing of the trial area in preparation for wheat planting. The entire trial site area
at both sites were planted with wheat at a rate of 175 seed m−2 in early June using the
plot seeder, as described above. Wheat crop establishment and weed emergence counts
in the trial area were collected six weeks after planting by counting plants in 4 × 0.25 m2
quadrats in each plot. Any emergent plants that were not wheat, including cover crop
residual plants, were recorded as weeds.
2.5. Data Analysis
All data were analysed using GenStat (18th Edition). The data were entered into MS
Excel sheets for the calculation of the averages, standard deviations and standard errors
of each treatment as well as the percentages of weed density and biomass suppressed by
test species compared with control at both sites. Further analysis was then carried out with
weed and crop biomass data sets transformed from both sites using log10 and square root
variates, respectively, because the Shapiro–Wilk test for normality was not initially met
(Supplementary Table S1). Analysis of variance (ANOVA) was performed on transformed
data to determine differences between treatments, with a significance probability level of
0.05. Treatment means were compared using a Tukey’s test (p = 0.05). The results presented
for crop and weed biomass are based on backtransformed data. Between-site comparisons
were few due to high levels of variations between the sites, leading to each site’s data being
analysed separately, reducing external factor variability.
3. Results
3.1. Initial Crop Establishment and Weed Emergence
Cover crop treatments, regardless of species and establishment densities, reduced
average weed emergence by more than 70% at both the Bringelly and Lansdowne sites
(p < 0.05). Buckwheat, millet and teff reduced initial weed emergence by 71%, 80% and
84%, respectively, at Bringelly compared with the control plots. Similarly, at Lansdowne,
buckwheat, millet and teff suppressed weed emergence by 92%, 76% and 85%, respectively
(Table 2). Weed counts at Lansdowne were relatively low, averaging 65 plants m−2 across
all treatments, compared with Bringelly, where there were on average 459 plants m−2 .
Bringelly had higher densities of broadleaf weeds than grass weeds, with 47% higher
counts of broadleaf weeds, while Lansdowne had similar densities of both weed types.Agronomy 2022, 12, 1831 5 of 9
Table 2. Cover crop establishment and weed densities (± standard error values) three weeks after
crop planting at the Bringelly and Lansdowne trial sites. Different letters show significant differences
(p < 0.05).
Crop Plants Weed Density (Plants m−2 )
Site Treatment
(m−2 ) Grass Weeds Broadleaf Weeds Total Weeds
Buckwheat 21 ± 3 124 199 323 ± 93 a
Millet 315 ± 23 25 150 175 ± 34 a
Bringelly
Teff 109 ± 25 68 155 223 ± 39 a
Control - 360 757 1116 ± 197 b
Buckwheat 104 ± 14 10 4 14 ± 4 a
Millet 129 ± 10 11 15 26 ± 18 a
Lansdowne
Teff 178 ± 18 32 10 42 ± 5 a
Control - 101 76 177 ± 55 b
There was poor establishment of buckwheat at Bringelly, where plant densities were
five times lower compared with the Lansdowne site. However, both millet and teff were
well established at both sites (>100 plant m−2 ). Millet establishment was higher at Bringelly
than at Lansdowne.
3.2. Crop and Weed Biomass
Cover crops on average reduced weed biomass by 86% (p < 0.05) compared with
control across both the Lansdowne and Bringelly field sites (Figure 2A,B). At Bringelly,
buckwheat and millet reduced weed biomass by 65% and 77%, respectively, compared
with the control, with teff achieving weed biomass suppression of 95% (Figure 2A). Millet
and teff both had relatively high biomass,1.5 and 2.9 t ha−1 , respectively compared with
buckwheat (0.4 t ha−1 ) at Bringelly. Despite having higher (p < 0.05) emergence counts early
in the trial (Table 2), millet biomass production was relatively low (1.5 t ha−1 ) at Bringelly
site compared with Lansdowne (3.6 t ha−1 ) (Figure 2A). Poor buckwheat crop establishment
resulted in low biomass production at Bringelly (Table 2, Figure 2A). However, despite
this low crop density, buckwheat reduced (p < 0.05) weed biomass by 65% compared with
control (p < 0.05) (Figure 2A). Similarly, at Lansdowne, all three cover crop treatments
reduced (p < 0.05) weed biomass compared with the control treatment, with buckwheat,
millet and teff crops suppressing weeds by 94%, 92% and 90%, respectively (Figure 2B).
Buckwheat and millet yielded 92% and 59% higher biomass, respectively, at Lansdowne
than at Bringelly (Figures 1 and 2). Teff biomass production (2.9 t ha−1 ) was consistent
across both sites (Figure 2), indicating that it could be a successful inclusion in farming
systems across different soil types, such as those at Bringelly and Lansdowne.
3.3. Crop Height
The heights of all cover crop species were lower (p < 0.05) at Bringelly than at Lans-
downe (Figure 3). Teff was taller (p < 0.05) than other cover crop treatments at Bringelly
and taller (p < 0.05) than buckwheat at Lansdowne (Figure 3).
3.4. Wheat and Weed Emergence
At both Bringelly and Lansdowne, wheat emergence was consistent across all treat-
ments, averaging 100 plants m−2 (Figure 4). Summer-grown cover crop residues did not
interfere with wheat crop establishment as there were no differences (p > 0.05) in wheat
plant emergence between the cover crop and bare fallow treatments (Figures 4 and 5).Agronomy 2022, 12, x FOR PEER REVIEW 6 of 10
Agronomy 2022,
Agronomy 12, 12,
2022, x FOR
1831PEER REVIEW 6 of 10 6 of 9
7 7
6 A 6 a B
7 7 a
5 5
biomass (t ha−1)
A B
biomass (t ha−1)
6 4 6 a
b 4
a a
x
5 3 x 5
biomass (t ha−1)
3
biomass (t ha−1)
b
4 2 b 2 4
a y a
x
3 1 x y 1
z 3 y
b y y
0 0
2 2
Control Buckwheat Millet Teff Control Buckwheat Millet Teff
a y
1 cover crop weed y 1 cover crop weed
z y y y
0 0 weeds produced at Bringelly (A) and Lansdowne
Control Figure 2. Average
Buckwheat dry biomassTeff
Millet of cover crops and
Controlbefore
(B). The cover crop and weed biomass data were transformed Buckwheat Millet Teff
analysis, and backtransformed
data for both cover crop weed biomasses are presented. Error bars represent approximate standard
cover crop weed cover crop weed
errors of mean calculated based on backtranformed values. Different letters show significant differ-
ences (p < 0.05) (ab for weed biomass and xyz for crop biomass).
Figure
Figure 2. Average
2. Average dry biomass
dry biomass of cover
of cover crops
crops and and weeds
weeds producedproduced at Bringelly
at Bringelly (A) and (A) and Lansdowne
Lansdowne
3.3.(B).
(B). Crop
The The cover
Height
cover crop crop and weed
and weed biomassbiomass datatransformed
data were were transformed before analysis,
before analysis, and backtransformed
and backtransformed
datadata
for
Theboth
for cover
both
heights ofcrop
cover weedweed
all crop
cover biomasses
crop are
biomasses
species presented.
werearelower (pError
presented. bars
at represent
Error
< 0.05) approximate
bars represent
Bringelly than Lans- standard
approximate
at standard er-
errors
downe of mean
(Figure calculated
3). Teff based
was on
taller backtranformed
(p < 0.05) than values.
other Different
cover crop letters
treatments show
at significant
Bringelly
rors of mean calculated based on backtranformed values. Different letters show significant differences differ-
ences
and (p < 0.05) (ab for weed biomass at and xyz for crop biomass).
(ptaller
< 0.05)(p 0.05) in wheat
0 0
plant emergence between the cover crop and bare fallow treatments (Figures 4 and 5).
Control Buckwheat Millet Teff Control Buckwheat Millet Teff
At Bringelly, millet and teff cover crop residues had the lowest weed emergence com-
Wheat
pared buckwheat and control, averaging 58 and 25Wheat
with Weed Weed m−2, respectively.
weed plants
Density
Figure4.4.Density
Figure of of wheat
wheat andand weed
weed plants
plants at Bringelly
at Bringelly (A) and
(A) and Lansdowne
Lansdowne (B). Error
(B). Error bars show
bars show
standard error. Different letters show significant differences (p < 0.05) (ab for weed density
standard error. Different letters show significant differences (p < 0.05) (ab for weed density andand x x for
for wheat density).
wheat density).
A B0 0
Control Buckwheat Millet Teff Control Buckwheat Millet Teff
Wheat Weed Wheat Weed
Agronomy 2022, 12, 1831 Figure 4. Density of wheat and weed plants at Bringelly (A) and Lansdowne (B). Error bars show 7 of 9
standard error. Different letters show significant differences (p < 0.05) (ab for weed density and x
for wheat density).
A B
Figure 5. Wheat crop establishment and the emergence of weeds in the control (A) and teff (B) plots
Figure 5. Wheat crop establishment and the emergence of weeds in the control (A) and teff (B) plots
at Bringelly site in winter (June) 2021.
at Bringelly site in winter (June) 2021.
At Bringelly, millet and teff cover crop residues had the lowest weed emergence
compared with buckwheat and control, averaging 58 and 25 weed plants m−2 , respectively.
Teff suppressed weed emergence by 74% and 73% (p < 0.05) compared with the buckwheat
and control treatments, respectively (Figures 4A and 5).
The Lansdowne results were less defined. Any cover crop seedlings that emerged
in the wheat crop were counted as weeds, and a high number of volunteer buckwheat
seedlings resulted in high weed densities (157 plants m−2 ) in these treatments. The number
of weeds in the buckwheat residues were 240% higher than those in the control plots,
highlighting the importance of proper cover crop desiccation prior to viable seed production.
However, millet and teff had 67% and 26% less weed emergence than the control plots and
86% and 69% less than the buckwheat plots, respectively (Figure 4B).
4. Discussion
Cover crop species that established well and produced the highest levels of biomass
substantially restricted the emergence and growth of weeds in field trials at Bringelly
and Lansdowne. For instance, millet with higher emergence and biomass at Bringelly
provided greater levels of weed suppression, whilst those crops that were less established,
were less competitive and allowed more weeds to persist. These results aligned with
those of Creech [19], who stated that, “A large part of successful weed suppression lies
in rapid and complete cover crop establishment”. Similarly, low initial weed counts in
well-established treatments, particularly teff, correlated with Brown [16], who found that
teff was able to supress weed emergence even when the weed seed bank levels were
considered high, which was the case at the Bringelly site. Gebrehiwot et al. [17] had similar
findings on the importance of having well-established teff treatments, concluding that a teff
cover crop with suitable plant establishment had 41% less overall weed cover compared
with those that failed to successfully establish. Our findings, therefore, align with other
studies that attribute strong initial weed suppression levels to cover crops that possess fast
establishment rates and early ground cover [14,16].
The direct relationship between cover crop biomass and weed suppression rates was
evident in this study, with the highest biomass-yielding crops being teff at Bringelly and
buckwheat at Lansdowne; these produced the greatest restrictions on weed growth. Sim-Agronomy 2022, 12, 1831 8 of 9
ilarly, previous studies by Smith et al. [20] and Alonso-Ayuso et al. [12] determined that
cover crop biomass is the major influence on weed suppression. These results also corre-
sponded with the findings of Teasdale et al. [10], who described the negative correlation
between crop and weed biomass in studies on a range of cover crop species and weeds.
Cover crops offer the potential for two periods of weed control, first when cover crops
are actively growing and second after their termination; then, the residues act as a mulch
on the soil surface and interfere with weed emergence in the following crops. In our study,
the cover crops planted not only suppressed the weed emergence and growth during the
summer fallow (Feb-Apr), but their residue also suppressed the emergence of weeds in the
subsequently planted wheat crop in autumn (May). Teff and millet both suppressed the
weed emergence in following wheat crop by more than 70% at Bringelly, a site with a high
density of summer weeds (1116 ± 197 weed plants m−2 ). Cover crop mulch reduces the
quantity and quality of light and is a physical barrier to weed seedling emergence [21,22],
especially those of small-seeded species. Wheat crop establishment was, however, not
affected by the presence of any cover crop residue or its absence (fallow control). These
results have important implications for the northern grain region, where early weed control
in winter crops is critical for reliable crop yields.
5. Conclusions
The strong correlation between cover crop biomass and weed growth suppression
was consistent across all trials, indicating that high cover crop biomass is imperative when
selecting cover crop species. Teff has the potential to become a widely incorporated cover
crop in the northern region as it proved itself a fast-establishing, high biomass-producing
and weed-suppressive species that performs in a range of soil types. However, more
research is required to determine the mechanisms of weed suppression by different cover
crops, especially studies aiming to disentangle the physical (competition) and chemical
(allelopathy) nature of interference.
Supplementary Materials: The following supporting information can be downloaded at: https:
//www.mdpi.com/article/10.3390/agronomy12081831/s1, Table S1: The normality test results of
original and transformed data of biomass of cover crop and weeds at Bringelly and Lansdowne sites.
Author Contributions: Conceptualization, L.H., A.S. and M.W.; methodology, L.H.; software, A.S.;
formal analysis, A.S. and L.H.; investigation, L.H.; resources, M.W.; data curation, L.H.; writing—
original draft preparation, L.H. and A.S.; writing—review and editing, M.W and A.S.; supervision,
A.S. and M.W; project administration, M.W.; funding acquisition, M.W. All authors have read and
agreed to the published version of the manuscript.
Funding: Part of this study was made possible thanks to the Grains Research Development Coopera-
tion (GRDC US00084).
Data Availability Statement: Data available on request.
Acknowledgments: The authors would like to thank Dr Peter Thompson for his help in statistical analyses.
Also thank you to Paul Lipscombe for helping in field trial planting and management. Part of this study
was made possible thanks to the Grains Research Development Cooperation (GRDC US00084).
Conflicts of Interest: The authors declare no conflict of interest.
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