Agriculture Reference
In-Depth Information
capacity to cause large reductions in wheat grain
yield. For example, Anderson (1993) showed that
18 plants m
−2
of jointed goatgrass reduced wheat
grain yield by 27% in eastern Colorado. Simi-
larly, Stougaard and Xue (2004) showed that
grain yield of wheat declined by 54% as wild oat
density increased to >400 plants m
−2
.
In some weed species the expression of crop
yield loss is heavily season-dependent. In a study
over 3 years, Peterson and Nalewaja (1992)
showed that wheat grain yield loss from competi-
tion with green foxtail [
Setaria viridis
(L.) Beauv.]
at a similar density over seasons ranged from 0%
to 47%. Environmental factors (early season tem-
perature, precipitation, and soil texture) that
infl uence seedling emergence pattern in green
foxtail were found to be extremely important in
determining the competitive impact of this weed
species on wheat. Earlier, Blackshaw et al. (1981)
also reported high year-to-year variability in the
competitive effects of green foxtail on wheat
yield.
Crop seeding rate, which impacts crop density,
can also infl uence the competitive ability of
wheat against weeds and can therefore be a useful
weed management tool. In a recent study from
Denmark, Olsen et al. (2005) showed that increas-
ing wheat seeding rate from 200 to 700 seeds m
−2
reduced weed biomass in all three spatial patterns
investigated. They also found that weed biomass
was lower and crop biomass was higher in plant-
ing patterns that represent a lower level of spatial
aggregation. However, such results need to be
interpreted with caution because there is evidence
that at least in some situations, spatially aggre-
gated systems (e.g., wide crop rows sown with
discs) can result in much lower and delayed estab-
lishment of weed seedlings that tend to be less
competitive with the crop (Chauhan et al.,
2006a).
Stougaard and Xue (2004) showed that larger
wheat seed size and higher density contributed to
increased competitive ability of wheat with wild
oat in Montana. The combined use of larger seed
size and higher seeding rate resulted in a more
competitive wheat crop, improving grain yields
by 30%. Benefi ts from higher wheat density can
also be related to suppression in weed reproduc-
tive output and lower grain contamination. In
some experiments in the US Great Plains, Kappler
et al. (2002) found that grain contamination
(dockage) from jointed goatgrass was reduced at
the rate of 6% for every 10 additional wheat plants
above the threshold wheat density of 70 plants
m
−2
. Based on this research they concluded that
increased seeding rates may be a good long-term
investment as part of an integrated jointed goat-
grass control program in winter wheat.
Martin et al. (1987) in New South Wales
(Australia) showed that the optimum seeding rate
of wheat increased in the presence of wild oat.
However, they concluded that the gains from
increasing wheat density beyond the weed-free
optimum (100 plants m
−2
) were too small to make
it a viable alternative to herbicide use or crop
rotation for wild oat control. More recently,
Lemerle et al. (2004) reported that doubling
wheat plant density from 100 to 200 plants m
−2
halved rigid ryegrass (
Lolium rigidum
Gaudin)
shoot dry matter.
Wheat grain quality and marketability
The preponderance of research reporting the
effects of weeds on wheat have dealt with the
impact of weed competition on grain yield. There
are few reports in the literature on the effects
of weeds on grain quality or nutritional value
(Zimdahl 1990). The few reports that do exist
paint a mixed picture of the importance of weeds
to wheat grain quality.
Grain protein is an important quality factor in
wheat. Several studies, including fi eld and green-
house studies, found no effect of weed competi-
tion on wheat grain protein or nitrogen content
(Bell and Nalewaja 1968; Rooney 1991; Das and
Yaduraju 1999). Other fi eld studies found weeds
reduced wheat grain protein in some years or
locations (Young et al., 1994; Mason and Madin
1996). Still other studies noted signifi cant
increases in spring wheat protein content in some
years or locations (Friesen et al., 1960; Nakoneshny
and Friesen 1961). In a greenhouse study con-
ducted at ambient and elevated carbon dioxide
