Agriculture Reference
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Table 12.5 Yield reduction of four winter wheat cultivars
differing in growth habit when competing with 150 downy
brome plants per square meter.
14,000
12,000
10,000
Yield Reduction (%)
1989
Height
(cm)
Growth
Habit
8,000
6,000
Cultivar
1990
1991
Norstar
110
Spreading
36
24
51
4,000
Redwin
90
Erect
39
33
52
Archer
70
Erect
42
40
63
Y = 2313.58 + (12279.17 - 2313.58)
× (1 + exp(4.268 (log X) - log(140.65))) -1 ;
R 2 = 0.99
2,000
0
Norwin
60
Spreading
53
62
81
0
50
100 150
Wheat seed rate (kg ha -1 )
200
250
300
350
Source: Adapted from Blackshaw (1994b).
Fig. 12.1. Redstem fi laree seed bank as affected by wheat
seed rates applied in four consecutive years. [Source: Black-
shaw et al. (2000b). Used with permission from the Weed
Science Society of America and Allen Press Publishing.]
with early emergence, rapid leaf expansion
forming a dense canopy, increased plant height,
early vigorous root growth, and increased root
size (Seavers and Wright 1999; Lemerle et al.,
2001). Thus, wheat competitive ability can also be
enhanced by seeding into a fi rm seedbed at an
optimum depth of 3 to 5 cm. Packing the seed row
will improve soil-to-seed contact and promote
rapid wheat emergence.
The establishment of a wheat crop with a more
uniform and dense plant distribution can increase
its ability to suppress weeds (Mohler 2001). This
is due to more rapid canopy closure that better
shades weeds and to better root distribution that
improves access to soil nutrients and water.
Increasing seeding rate of wheat is one means of
increasing its competitive ability with weeds
(Walker et al., 2002; Lemerle et al., 2004;
O'Donovan et al., 2005). A 4-year study found
that an increase in wheat seeding rate from 50 to
300 kg ha −1 reduced redstem fi laree [ Erodium cicu-
tarium (L.) L'Her. ex Ait.] biomass by 53% to
95% and increased wheat yield by 56% to 498%
(Blackshaw et al., 2000b). Additionally, redstem
fi laree in the soil seed bank for future weed infes-
tations was reduced by 79% (Fig. 12.1).
There is less potential to manipulate row spacing
in wheat than in traditional row crops such as
maize ( Zea mays L.) or soybean, which are com-
monly grown in wider rows than the 15- to 30-cm
row spacing common with wheat. Blackshaw et al.
(1999) found that a decrease in wheat row spacing
from 30 to 20 cm had little effect on foxtail barley
biomass or crop yield in western Canada. However,
Mertens and Jansen (2002) reported that reducing
wheat row spacing from 30 to 10 cm in The
Netherlands consistently reduced weed biomass
but wheat yield remained constant.
Many agricultural weeds are high consumers of
nutrients and therefore are capable of reducing
available nutrients for crop growth (Di Tomaso
1995). Additionally, growth and competitive
ability of many weed species is enhanced by
higher soil nutrient levels. Research has deter-
mined that fertilizer timing and application
method can markedly affect wheat-weed compe-
tition. Spring-applied versus fall-applied fertil-
izer often reduced weed biomass and increased
spring wheat yield (Blackshaw et al., 2004, 2005a).
Nitrogen fertilizer placed as subsurface bands,
rather than surface broadcast, reduced the com-
petitive ability of several weed species in wheat
(Kirkland and Beckie 1998; Blackshaw et al.,
2004). A fi eld study utilizing 15 N-enriched liquid
nitrogen fertilizer clearly documented greater
nitrogen uptake by wheat, and often lower nitro-
gen uptake by weeds, when nitrogen was placed
10 cm below the soil surface (away from surface-
germinating weeds) compared with surface broad-
cast (Blackshaw et al., 2002). Weed seed-bank
data in a multiyear study indicated that the nitro-
gen fertilizer application method not only affects
wheat-weed competition in any given year but is
a critical component of long-term weed manage-
ment (Fig. 12.2).
 
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