Weed management using crop competition 
in the United States: A review
Authors: Prashant Jha, Vipan Kumar, Rakesh K. 
Godara, and Bhagirath S. Chauhan
NOTICE: this is the author’s version of a work that was accepted for publication in Crop 
Protection. Changes resulting from the publishing process, such as peer review, editing, 
corrections, structural formatting, and other quality control mechanisms may not be reflected in 
this document. Changes may have been made to this work since it was submitted for publication. 
A definitive version was subsequently published in Crop Protection, (July 2016). 
DOI# 10.1016/j.cropro.2016.06.021
Jha, Prashant, Vipan Kumar, Rakesh K. Godara, and Bhagirath S. Chauhan. "Weed management 
using crop competition in the United States: A review." Crop Protection (July 2016).
Made available through Montana State University’s ScholarWorks 
scholarworks.montana.edu 
Weed management using crop competition in the United States: A
review
Prashant Jha a, *, Vipan Kumar a, Rakesh K. Goda
a Montana State University-Bozeman, Southern Agricultural Research Center, Hunt
nova
dev
see
in
id d
quen
the
nop
on a
tha
ressure for development of herbicide-resistant weed populations in a cropping
outcome, the over reliance on the same-site-of-action herbicide in
a cropping system has resulted in an increased development of
herbicide-resistant (HR) weed populations worldwide (Heap,
2016). Globally, the US ranks first, with more than 150 unique
n (Gossypium spp.)
and its increasing
Duke and Powles,
species over the
t al., 2009; Beckie
6). In addition, HR
heat (Triticum aes-
ent reports on HR
stainability of the
st of managing HR
weeds in the absence of any new site-of-action herbicide discovery
over the last two decades further exacerbates the problem (Duke,
2012). Current data and future projections on HR weeds strongly
suggest the need for development and implementation of inte-
grated weed management (IWM) strategies in cotton, corn, soy-
bean, and wheat to curb the resistance problem (Norsworthy et al.,
2012; Vencill et al., 2012).
Developing cost-effective and sustainable weed management
strategies further necessitates the in-depth understanding ofWeeds adversely affect the crop growth and yield by competing
with crops for limiting resources such as light, water, and nutrients
(Harper, 1977; Swanton et al., 2015). The intensity and duration of
the crop-weed competition determines themagnitude of crop yield
losses (Swanton et al., 2015). Avoiding or reducing crop yield losses
due to weed competition requires the utilization of diverse and
effective weed management programs (Chauhan and Ope~na, 2013;
Swanton et al., 2015). Herbicides are the dominant tools used for
weed control in global agriculture, and an annual worldwide her-
bicide sale is estimated to be US $27 billion (Kraehmer, 2012). As an
in soybean [(Glycinemax (L.) Merr.] (1996), cotto
(1997), and corn (Zea mays L.) (1998) in the US,
use for weed management in these GR crops (
2009) has resulted in evolution of 15 GR weed
last 20 years in the North America (Johnson e
et al., 2014; Kumar et al., 2014, 2015; Heap, 201
weed populations have also been reported in w
tivum L.) grown in the US (Heap, 2016). Insurg
weed populations pose a serious threat to the su
US cropping systems. Moreover, an increasing co1. Introduction cases of HR weed evolution, followed by Europe and Australia
(Heap, 2016). Commercialization of glyphosate-resistant (GR) traitreducing the selection p
system.emergence herbicide application. Furthermore, reduced row spacing, increased seeding rates, and weed-
competitive cultivars are effective in reducing reliance on a single site-of-action herbicides, therebyb Crop Protection Division, Monsanto Company, St. Louis, MO, USA
c The Centre for Plant Science, Queensland Alliance for Agriculture and Food In
Australia
a b s t r a c t
Exploiting the competitive ability of crops is essential to
management practices. Reduced row spacing, increased
cultivars can potentially manage cropeweed competition
cultural weed management practices facilitate a more rap
affect the emergence, density, growth, biomass, and subse
growing season. These cultural practices can also favour
influencing the canopy architecture traits (plant height, ca
development, and leaf distribution). These crop-competiti
crop yield losses due to interference from weed cohortsra b, Bhagirath S. Chauhan c
ley, MT, USA
tion (QAAFI), The University of Queensland, Toowoomba, Queensland,
elop cost-effective and sustainable weed
ding rates, and selection of competitive
cotton, soybean, wheat, and corn. These
evelopment of crop canopy that adversely
tly the seed production of weeds during a
weed suppressive ability of the crop by
y density, leaf area index, rate of leaf area
ttributes can potentially reduce the risk of
t escape an early- or a late-season post-
concepts and factors involved in crop-weed competitive in-
teractions (Blackshaw et al., 2000; Swanton et al., 2015). The
competitive ability of a crop depends on various physiological and
morphological attributes that allow the crop to utilize light, water,
nutrients, and other limited resources effectively in the presence of
the weed pressure. A variety of cultural practices such as crop
planting dates, competitive cultivars, seeding rates, row widths,
cover crops, nutrient management, and irrigation strategies can
manage crop-weed competition and favour the crop competitive-
ness against weeds (Chauhan and Ope~na, 2013; Swanton et al.,
2015). Furthermore, the use of reduced crop row spacing,
increased seeding rates, and competitive cultivars in a cropping
system can potentially minimize reliance on herbicides with the
same site of action and manage HR weed seed banks (Norsworthy
et al., 2012; Vencill et al., 2012). In the U.S., soybean, cotton, corn,
and wheat account for more than 70% of all crop acres planted
annually (Price et al., 2011). Therefore, the aim of this review article
is to highlight the importance of weed management using crop
competition though manipulations in crop row spacing and seed-
ing rates and use of competitive cultivars in these crops grown in
the US.
2. Row spacing
2.1. Cotton
Traditionally, cotton in the US is grown in wide rows spaced 76-
to 102-cm apart. The concept of Ultra Narrow Row (UNR) spacing
originated in 1990s, with cultivation of cotton in narrow rows (twin
rows) with a spacing of 19e38 cm (Reddy, 2001; Wilson et al.,
2007). The UNR system allows the use of higher seeding rates of
cotton per unit area. The major goals of adopting the UNR cotton
system are to reduce production cost, improve weed control, and
increase yield as well as economic return by increasing the plant
population (Parvin et al., 2000; Nichols et al., 2004). Early and rapid
canopy closure under the UNR cotton system also helps conserve
soil moisture and suppress weeds early in the season (Reddy, 2001;
Molin et al., 2004). Several studies conducted in the US have shown
amixed response onweed control under the UNR vs. the traditional
wide row (76e102 cm) cotton production system (Parvin et al.,
2000; Bryson et al., 2003; Molin et al., 2004). Rogers et al. (1976)
reported that only 6 weeks of weed-free period was needed to
obtain high yields in cotton grown in narrow rows (53 cm);
whereas, 10e14 weeks of weed-free period was required to obtain
similar yields inwide rows (79e106 cm). A four-year study showed
that Sida spinosa L. under the UNR cotton system had 74e82% less
total dry weight, and 71e90% less number of capsules per plant,
comparedwith thewide rowcotton system (Molin et al., 2004). The
study also reported a 67e85% decline in the total dry weight
plant
1 of Euphorbia hyssopifolia L. under the UNR vs. the wide row
cotton system (Molin et al., 2004). Stephenson and Brecke (2010)
found that cotton planted in 19-cm twin rows, with each set of
twin rows being 76-cm apart, had greater control of Commelina
benghalensis L., Senna obtusifolia L. Irwin & Barneby, and Jacque-
montia tamnifolia L. compared with the single-row (76 cm apart)
planting pattern of cotton. In addition, the end-season total weed
dry biomass was reduced by 35% in the twin-row (two rows 38 cm
apart on 102-cm beds) compared to the single-row (on 102-cm
beds) cotton planting system (Reddy and Boykin, 2010). Aulakh
et al. (2011) observed that sequential post-emergence applica-
tions of pyrithiobac at 2- to 4-leaf stages of conventional cotton
increased the S. obtusifolia control in 38-cm compared to 102-cm
wide rows. In contrast, Miller et al. (1983) observed no differ-
ences in total density of grass or broadleaf weed species with tillage
plus trifluralin and prometryn, and tillage plus trifluralin andfluometuron treatments, in cotton grown under narrow (51 cm) vs.
wide (102 cm) rows. Similarly, a minimal effect on annual grass and
broadleaf weed control was observed with various pre-emergence
followed by post-emergence herbicide programs in glufosinate-
resistant cotton planted in 38-cm vs. 97-cm rows (Wilson et al.,
2007).
2.2. Soybean
Numerous studies conducted in the US have reported the po-
tential benefits of the narrow-row spacing onweedmanagement in
soybean. The benefits are mainly attributed to the early canopy
closure in the soybean planted in narrow (19- or 38-cm wide) that
enhances the competitive ability of the crop against weeds,
compared to wide (76 cm or more) rows (Steckel and Sprague,
2004; Jha et al., 2008; Jha and Norsworthy, 2009). Shibles and
Weber (1966) reported that the 95% solar light interception
occurred 17 d earlier in the 25-cmwide rows compared to the 102-
cm wide rows. In addition, soybean canopy closure occurred 40 d
earlier in soybean planted in 25-cm vs. 76-cm wide rows
(Mickelson and Renner, 1997). Reducing soybean row spacing from
76 to 19 cm delayed the critical timing of weed control (CTWR)
from the first-trifoliate to the third-trifoliate stage of soybean,
indicating enhanced weed competitive ability of the narrow-row
soybean (Knezevic et al., 2003). Reducing the soybean row
spacing from 91 to 23 cm reduced the weed density from 16 to 2
plants m
2 and the aboveground biomass from 141 to 33 g m
2
(Yelverton and Coble, 1991). There was a greater control of Setaria
faberi Herrm. and Amaranthus rudis Sauer., and an increase in
soybean yield in the GR soybean planted in narrow (19- or 38-cm)
vs. wide (78-cm) rows (Young et al., 2001). Similar results on the
suppressive effect of the narrow-row soybean onweed density and
biomass were evident for several other weeds including, other
Amaranthus species, Chenopodium album L., S. obtusifolia, Ipomoea
species, Xanthium strumarium L., and Ambrosia artemisiifolia L.
which are predominant in the US soybean production (Legere and
Schreiber, 1989; Mickelson and Renner, 1997; Buehring et al.,
2002; Steckel and Sprague, 2004; Hock et al., 2006; Harder et al.,
2007). A single application of glyphosate at the V3 stage of soy-
bean eliminated seed production from Amaranthus palmeri S. Wats
plants in 19-cm soybean rows, compared with 600 seeds m
2
produced by plants grown in 97-cm soybean rows (Jha et al., 2008).
Furthermore, in majority of these studies, soybean yields were
greater in narrow compared to wide rows.
A dense canopy of soybean when planted in narrow rows
(19 cm) resulted in decreased photosynthetic active radiation (PAR)
and red/far-red ratio of light available to seed on or near the soil
surface under no-tillage conditions (Norsworthy, 2004). This
resulted in reduced emergence of X. strumarium, S. obtusifolia, and
A. palmeri by 33, 68, and 76%, respectively, in plots in the presence
compared to those in the absence of soybean (Norsworthy, 2004;
Jha and Norsworthy, 2009). Thus, the early canopy formation trait
of narrow-row soybean can potentially reduce weed resurgence
and reduce reliance on multiple post-emergence glyphosate ap-
plications in GR soybean (Steckel and Sprague, 2004; Jha et al.,
2008).
2.3. Wheat
There is relatively limited research conducted in the US on
assessing the effect of row spacing on cropeweed competition in
wheat. Nalewaja and Arnold (1970) reported that reducing the row
spacing could enhance the wheat competitiveness against weeds,
and improve wheat yields. There was a 12% increase in the yield of
hard red winter wheat, with a decrease in the row spacing from 23
to 7.5 cm in a field infested with Bromus secalinus L. (Solie et al.,
1991). Koscelny et al. (1990) found a 20% increase in wheat yield
and a 16% reduction in seed yield of B. secalinus, with a decrease in
wheat row spacing from 23 to 8 cm. Similarly, reducing the wheat
row spacing from 30 to 10 cm decreased Aegilops cylindrica Host.
spikelet yield from 513 to 391 kg ha
1 (Kelley, 1998). Increased
wheat yield and competitiveness in wheat planted in narrow rows
were due to an increase in the number of tillers per unit area
(Koscelny et al., 1990). A delayed canopy closure in wheat planted
with wider row spacing provides opportunity for emergence of
late-season weed cohorts and substantial late-season weed
competition, resulting in reduced grain yield (Koscelny et al., 1990;
Kelley,1998). A narrow (11e15 cm) compared to awide row (23 cm)
spacing can result in amore uniform plant distribution, growth, leaf
and root architecture of wheat plants, allowing them to more
efficiently utilize PAR and available water and nutrients from the
soil (Koscelny et al., 1990; Kelley, 1998). In the northeastern Mis-
souri, US, light interception and leaf area index of wheat were
greater when planted in 19-cm than in 38-cm wide rows (Sandler
et al., 2015).
2.4. Corn
Studies conducted in the US have documented the superior
weed suppression ability and yield of corn planted in narrow rows
(19 or 38 cm) vs. wide rows (76 cm) (Teasdale, 1995; Murphy et al.,
1996; Dalley et al., 2004). Early canopy closure and increased
shading of weeds resulted in an increased crop competitiveness
and a reducedweed growth in narrow-vs. wide-rowcorn (Teasdale,
1995; Dalley et al., 2004). In a 3-year field study, Murphy et al.
(1996) reported a 20% less weed biomass production in the corn
planted in 50-cm compared to 75-cmwide rows, implying a greater
competitive ability of corn planted in narrow rows. Up to 60% re-
ductions in weed biomass have been reported in corn planted in
38-cmvs. 76-cm row spacing (Shrestha et al., 2001; Tharp and Kells,
2001). Furthermore, narrow rows (38 cm) coupled with a higher
plant population (90,000 to 124,000 plants ha
1) resulted in an
adequate weed control even at the reduced rates of herbicides in
corn (Teasdale,1995). The risk of yield loss due toweed interference
from the delayed post-emergence glyphosate treatment in GR corn
was reduced when the corn was planted in 38-cm vs. 76-cm row
spacing (Dalley et al., 2004). In a study conducted in Michigan, US,
corn row spacing did not affect grain yield; nevertheless, a more
efficient utilization of soil moisture by corn plants grown in
competition with C. album and S. faberii was evident in 36-cm
compared to 76-cm rows (Dalley et al., 2006). However, a little to
no effect of row spacing on corn-velvetleaf competition was re-
ported (Teasdale, 1998). Similarly, the row spacing had no effect on
emergence or growth of weeds and the critical period of weed
control (CPWC) in corn, especially at high weed densities (Johnson
and Hoverstad, 2002; Dalley et al., 2006). Environmental condi-
tions, weed density, species composition, and herbicide application
timings can potentially mask the effect of row spacing on crope-
weed competition (Hall et al., 1992; Dalley et al., 2006).
3. Seeding rate
3.1. Cotton
There is a limited research conducted on the effect of cotton
seeding rates on cropeweed competition in the US. In a study
conducted in Florida, US, control of C. benghalensis and S. obtusifolia
increased with an increase in the cotton density from 7 to 26 plants
m
2 under low levels of weed management (glyphosate plus S-
metolachlor) (Stephenson and Brecke, 2010). However, the effect ofcotton density on weed control was not evident at the medium
[glyphosate þ S-metolachlor followed by (fb) glyphosate] and high
(glyphosate þ S-metolachlor fb glyphosate fb MSMA þ prometryn)
weed management inputs (Stephenson et al., 2011). Total nodes,
plant height, and total bolls per plant of cotton decreased with an
increase in cotton density from 7 to 15 plants m
2 (Stephenson
et al., 2011); nevertheless, the planting density had a minimal ef-
fect on cotton seed and lint yield and on fiber quality (Jones and
Wells, 1997; Stephenson et al., 2011). A planting density of 13
plants m
2 in a single-row planting pattern was needed to obtain
similar yield and weed control provided by a twin-row cotton
planted at 7 plants m
2 (Stephenson and Brecke, 2010).
3.2. Soybean
Seeding rate is an important cultural practice that influences the
crop-weed competition, thereby influencing weed management
(Nice et al., 2001; Arce et al., 2009). Increasing the soybean-seeding
rate from 124,000 to 494,000 seeds ha
1 reduced weed interfer-
ence and increased soybean yield by 48% (Oplinger and Philbrook,
1992; Harder et al., 2007; Arce et al., 2009). Nice et al. (2001) also
reported a reduction in the density and biomass of S. obtusifolia and
Solanum ptycanthum L. with an increase in soybean population
from 245,000 to 481,000 or 676,000 plants ha
1 in the narrow
rows. Similar suppressive effects of increased seeding rates on
cropeweed competition have been reported for several other
problem weed species in soybean, including Solanum ptycanthum
L., Ipomoea lacunosa, Sesbania exaltata (Raf.) Cory., Amaranthus
species, and S. obtusifolia (Buehring et al., 2002; Norsworthy and
Oliver, 2002a, 2002b; Rich and Renner, 2007; Place et al., 2009).
An early canopy development is an important component of
cultural weed control in soybean (Buehring et al., 2002;
Norsworthy and Oliver, 2002a, 2002b; Jha et al., 2008). An in-
crease in the seeding rate from 148,200 to 469,300 seeds ha
1
increased the cumulative intercepted photosynthetic active radia-
tion (CIPAR) during the critical period of weed control (CPWC) in
soybean by 76e172% (DeWerff et al., 2014). An early canopy
development with an increase in soybean population from 247,000
to 729,000 plants ha
1 improved I. lacunosa and S. exaltata control
by >30%, with a glyphosate application at the V2 stage of GR soy-
bean (Norsworthy and Oliver, 2002a). Also, a lack of complete
canopy formation at the time of an early post-emergence applica-
tion of glyphosate in GR soybean increased the risk of late-season
weeds and consequent crop yield loss; the effect was more pro-
nounced in soybean planted at 240,000 and 300,000 compared to
420,000 seeds ha
1 (Arce et al., 2009). A greater competitiveness of
soybean planted at a high density can not only suppress late-
emerging weed cohorts, but also those that survive an early-
season post-emergence herbicide treatment (Young et al., 2001).
This could potentially improve the consistency of weed manage-
ment and reduce reliance on post-emergence herbicides for weed
control (DeWerff et al., 2014).
3.3. Wheat
Several researchers in the US have documented the benefits of
increasing seeding rate or plant density on increased competitive
ability of wheat against weeds. For instance, a four-year study
concluded that increasing the seeding rate of wheat from 50 to
300 kg ha
1 reduced the Erodium cicutarium L. biomass by 53% and
seed production of E. cicutarium by 95%, and increased wheat yields
by 56e498% over the four years of the study (Blackshaw et al.,
2000). Avena fatua and Aegilops cylindrica are the most trouble-
some weeds in wheat production systems in the Great Plains and
Pacific Northwest regions of the US (Heap, 2016). In Montana, US,
increasing the spring wheat seeding rate from 175 to 280 plants
m
2 increased the wheat grain yield by 12% and reduced the
biomass and seed production of Avena fatua L. by 20% (Xue and
Stougaard, 2002; Stougaard and Xue, 2004). Similarly, Carlson
and Hill (1985) observed an increase in competitive ability of
wheat against A. fatua and up to 23% increase in grain yield, by
increasing the wheat density from 175 to 700 plants m
2. An in-
crease inwinter wheat population density from 40 to 60 plantsm
1
row also reduced A. cylindrica biomass by 27%, weed head density
by 37%, and weed spikelet biomass by 47% (Yenish and Young,
2004). In a study conducted in Maine, US, increasing the spring
wheat planting density from the standard practice of 400 plants
m
2 to 600 plants m
2 reduced Sinapis alba L. density by 30%;
nevertheless, it reduced the grain protein by 5% (Kolb et al., 2012).
Optimizing the wheat-seeding rate is, therefore, also important
from a grain quality standpoint (Kolb et al., 2012). With a limited
information available from the US, some insights can be gained
from other countries. In UK and Europe, higher wheat seeding rates
enhanced the flag-leaf senescence and negatively affected the
grain-filling stage and protein concentration in the wheat kernels
(Gooding et al., 2002). In addition, high seeding rates can reduce
wheat grain protein due to an increased intraspecific competition
for available soil nitrogen (Gooding et al., 2002; Arduini et al.,
2006).
3.4. Corn
Several studies conducted in the US demonstrated the impor-
tance of seeding rate as a cultural weed management tool in corn
production. Increasing the corn seeding rates enhanced the corn
leaf area and rate of canopy closure, which further improved the
ability of corn plants to suppress Abutilon theophrastiMedik., one of
the major problem weeds in corn production in the US (Lindquist
et al., 1998). Corn planted at a high density (100,000 plants ha
1)
in 38-cm narrow rows reduced early-seasonweed interference, and
also aided in controlling late-emerging weed cohorts (Shrestha
et al., 2001). Weil (1982) observed a negative correlation between
corn plant density and total weed biomass accumulation. The
greater competitiveness and an early canopy development by
increasing the population density of corn reduced the biomass of
late-emerging weed cohorts by 41% (Murphy et al., 1996; Williams
and Boydston, 2013). Similarly, an increase in corn density reduced
canopy-transmitted photosynthetic photon flux density (PPFD),
and consequently the total dry matter accumulation (reduced by
89%) of Amaranthus retroflexus L. (McLachlan et al., 1993). Further-
more, corn-induced shading caused a relatively greater allocation
of dry matter to main stem than to branches of A. retroflexus,
thereby influencing theweed canopy architecture (McLachlan et al.,
1993). Teasdale (1998) observed a 99% reduction in seed production
of A. theophrasti in corn planted at 128,000 compared to 64,000
plants ha
1 seeding rate. Increasing the corn population from
59,300 to 72,900 plants ha
1 reducedweed seed production by 50%
and improved corn yields (Tharp and Kells, 2001). Williams and
Boydston (2013) also observed a decline from 72 to 27% in seed
production of Panicum miliaceum L. with an increase in corn
seeding rate from 35,000 to 105,000 seeds ha
1, respectively.
4. Competitive cultivars
4.1. Cotton
Only a few field studies have been conducted in the US to test
the weed competitive ability of cotton cultivars. Like corn and
soybean, the differential competitive ability of cotton cultivars is
associated with differences in canopy development characteristicsand time of maturity (Andries et al., 1974; Chandler and Merredith,
1983). Andries et al. (1974) evaluated three near-isogenic strains of
cotton including okra-, super okra-, and normal-leaf type, under
three different row spacings (25-, 50-, and 100-cm rows), and with
various combinations of trifluralin and flumeturon for weed control
and observed that okra or normal-leaf type cotton had a better late-
season weed control than the super okra-leaf strain. Chandler and
Merredith (1983) found that the early-maturing cotton cultivar
(‘DES 21326-04’) was less tolerant to early-season competition
from Anoda cristata (L.) Schlecht than the late-maturing cultivar
(Deltapine 16 or Stoneville 213). However, Bridges and Chandler
(1987) reported no differences in the weed competitive ability
between three cotton cultivars (‘1209-619-7’, ‘TAMCOT SP37-H’,
and ‘Stoneville 213’) with different plant heights, when grown in
competition with Sorghum halepense (L.) Pers. at increasing
densities.
4.2. Soybean
Soybean cultivars grown in the US are divided into different
maturity groups, from Group 0, best adapted to region north of
46N, to Group IX, best adapted to southern regions like southern
Florida and Texas (Zhang et al., 2007). Soybean cultivars can differ
in their ability to compete with annual grass and broadleaf weeds,
and can adapt to different environments by changes in plant
morphology, canopy architecture, and yield variables (Burnside,
1972; McWhorter and Hartwig, 1972). Bussan et al. (1997)
observed differences in weed competitive abilities between 16
soybean genotypes that were mostly associated with the differ-
ences in canopy architecture, height, and volume of the plants. For
instance, a greater biomass reduction in S. obtusifolia occurred in
the presence of taller soybean cultivars (maturity Group VI)
(Shilling et al., 1995). Similarly, the tall soybean cultivars being
more competitive, reduced seed production and seed weight of
I. lacunosa by 48 and 38%, respectively, compared to the short
cultivars (Bennett and Shaw, 2000). Jordan (1992) found that the
short-statured soybean cultivars reduced X. strumarium growth
early in the season, while the tall cultivars had a greater suppres-
sive effect on the weed growth late in the season. Late-maturing
soybean cultivars had earlier emergence, greater seedling vigour,
more rapid canopy development, and a greater competitive ability
against A. theophrasti and Setaria italica (L.) Beauv. compared with
the early-maturing cultivars (Rose et al., 1984; Nordby et al., 2007).
In contrast, early-maturing soybean cultivars reduced seed pro-
duction and seed weight of I. lacunosa by 76 and 49%, respectively,
compared with the late-maturing cultivars by allowing harvest
before the physiological maturity of the weed (Bennett and Shaw,
2000). However, James et al. (1988) observed a lack of consistent
effect of cultivar selection on S. obtusifolia interference in soybean.
Similarly, Monks and Oliver (1988) did not find differences in grain
yield or weed biomass between the soybean cultivars, ‘Forrest’
(Group V cultivar) and ‘Centennial’ (Group VI cultivar). Rose et al.
(1984) proposed that soybean cultivars with a weed suppressive
ability through allelopathy would be an added advantage to
growers.
4.3. Wheat
Competitive cultivars apparently offer a cheap tool to consider
in IWM programs. The efficiency of light interception by a partic-
ular crop cultivar contributes to its competitive ability against
weeds (Holt,1995). Crop cultivars that rapidly shade the soil surface
are more competitive in suppressing the weed growth than the
slow growing cultivars (Holt, 1995). Plant height, tiller number,
early vigour, and leaf size strongly correlate with the suppressive
ability of a wheat cultivar against weeds (Blackshaw et al., 1981;
Mason et al., 2007). Tall spring wheat cultivars (83 cm in
height) were found to have a more pronounced suppressive effect
on weeds than the shorter cultivars (73e78 cm) (Blackshaw et al.,
1981; Wicks et al., 1986). Tall, hard red winter wheat cultivar
‘Turkey’ (>90 cm tall with higher number of stems/tillers m
2) was
found to be the most competitive wheat cultivar for suppressing
annual grasses including Bromus tectorum L. inwheat production in
Nebraska, US (Challaiah et al., 1986; Wicks et al., 2004). Similarly,
taller, soft winter wheat cultivars had a greater grain yield and a
lower seed production of A. cylindrica, when grown in competition
(Ogg and Seefeldt, 1999). Among the eight different winter wheat
cultivars tested in competition with A. cylindrica and B. secalinus,
the two shorter cultivars (‘TAM 107’ and ‘2180’) were the least
competitive (Kelley et al., 1999). The greater weed suppressive
ability of the taller cultivars was due to their greater ability to
intercept PAR (Kelley et al., 1999). In contrast, 14e30% more grain
yield reductions in shorter compared to taller winter wheat culti-
vars occurred in the presence of the season-long interference from
B. tectorum (Blackshaw, 1994). There was also a negative relation-
ship between wheat plant height and B. tectorum seed yield
(Challaiah et al., 1986). In addition, the ability of competitive cul-
tivars to reduce the fecundity of a weed species should be an
important consideration for managing weed seed banks and
reducing reliance on herbicides.
4.4. Corn
Differences in weed suppression ability among genotypes is an
important trait exploited both in conventional and modern corn
cultivars. Plant height, canopy density, leaf area index, leaf up-
rightness, rate of leaf area development, and leaf distribution are
the important canopy architecture traits that can determine the
crop tolerance to weed interference (Lindquist et al., 1998;
Lindquist and Mortensen, 1998; Sankula et al., 2004). So et al.
(2009) tested 25 commercial sweet corn hybrids in the presence
of P. miliaceum competition, and revealed that the canopy archi-
tecture was positively associated with the competitive ability of the
hybrids. On the other hand, Staniforth (1961) found an early-
maturing corn hybrid to be more tolerant to a high density of
Setaria glauca (L.) Beauv compared with a late-maturing hybrid.
Begna et al. (2001) also observed that early-maturing, leafy reduced
stature (LRS) and conventional Pioneer (‘P3979’) corn hybrids, had
29e44% less total weed biomass compared with the late-maturing,
big leaf (LMBL) corn hybrids, when grown under a high weed
pressure. Furthermore, the differential yield loss in response to a
high weed density of a modern vs. old corn hybrid was due to the
differences in leaf area index, late-season PPFD, and nitrogen-use
efficiency between the two hybrids (Tollenaar et al., 1994). Con-
cerning reductions in weed biomass and fecundity, the corn hybrid
‘GH2547’ was 25e31% and 70e91% more competitive than the
‘Spirit’ hybrid against P. miliaceum in trials conducted in Wash-
ington and Illinois, respectively (Williams et al., 2008). Lindquist
and Mortensen (1998) observed a greater reduction in the num-
ber of capsules of A. theophrasti by old-corn compared to modern-
corn hybrids. In another study, it was reported that the ‘Pioneer
3260’ corn hybrid with a horizontal leaf architecture, had lower
weed density, weed biomass, intercepted PAR, and weed seed
production, compared to the ‘Pioneer 3394’ corn hybrid, with an
upright leaf architecture (Sankula et al., 2004).
5. Conclusions
The high rate of adoption of transgenic GR soybean, cotton, and
corn in the US since 1996, has meant unprecedented and often anexclusive use of this relatively simple, reliable, and cost-effective
glyphosate-based weed control in these crops. Consequently,
there are now evolved GR populations of A. palmeri, A. artemissifolia
L., Ambrosia trifida L., Amaranthus rudis JD Sauer, and various Conyza
and Lolium spp. in the US corn, soybean, and cotton production
(Heap, 2016). As GR-crop technology will dominate and remain
popular with growers, it is anticipated that other GR weeds will
evolve in the near future, posing a greater risk to the sustainability
and continued success of glyphosate and GR crops. It is to be noted
that GR weeds are not yet a problem in many parts of the world,
and lessons can be learnt to add weed control diversity by adopting
ecological weed management approaches, thereby, maintaining
the sustainability of glyphosate and GR crops for future harvests.
Ecological weed management strategies including reduced row
spacing, higher seeding rates, and competitive cultivars can provide
an early-season competitive advantage to the crop, facilitate an
early crop canopy development, and maximize the resource utili-
zation by the crop to reduce weed density, biomass, and seed bank
additions. These strategies can potentially manage crop-weed
competition in cotton, soybean, wheat, and corn. These crop
competition variables can be used as an integral component of the
best management practices to reduce herbicide usage and mini-
mize herbicide selection pressure for the evolution of HR weed
populations in farm fields. Reduced row spacing, increased seeding
rates, and competitive cultivars can be potentially integrated into
the current glyphosate resistance management programs to reduce
survival and fecundity of GR weeds in GR corn, soybean, or cotton
production systems; however, there is a limited research on un-
derstanding how these cultural weed control strategies relate to
management of GR weeds. Future research efforts should also
explore the allelopathic potential of competitive cultivars to sup-
press weeds in an agroecosystem.
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