Agriculture Reference
In-Depth Information
Table 12.3
Effect of two cropping sequences on downy
brome plant density.
Table 12.4
Effect of underseeded sweetclover in 1993 on
weed growth during fallow in 1994 and spring wheat yield
in 1995.
Continuous Winter
Wheat (plants m
−2
)
Winter Wheat-Canola
(plants m
−2
)
Year
Weed Biomass
(September 1994)
Wheat Yield
(1995)
1988
30
28
Crop Treatment
(g m
−2
)
(kg ha
−1
)
1989
54
25
1990
190
35
Field peas (1993)
446
2,970
1991
400
70
Mustard (1993)
156
2,160
1992
920
38
Field peas
sweetclover
(1993-1994)
+
14
3,750
1993
740
40
Mustard
sweetclover
(1993-1994)
+
3
3,520
Source:
Adapted from Blackshaw (1994a).
Source:
Adapted from Blackshaw et al. (2001b).
could effectively be controlled through use of
diverse crop rotations.
Fallow is often included in rotation with wheat
in the semiarid Great Plains of North America.
Fallow can effectively reduce weed populations
(Blackshaw et al., 2001a; Anderson 2003), but it
can negatively affect soil quality and expose the
soil to erosion. Research has examined the useful-
ness of cover crops and green manure crops as
partial fallow replacements. Moyer et al. (2000)
documented that a winter rye (
Secale cereale
L.)
cover crop planted after harvest of summer crops
suppressed weed growth in the fall and early
spring. Winter rye residue, after terminating the
crop at heading in June, continued to suppress
weeds for the remainder of the fallow period,
likely due to combined physical and allelopathic
effects (Teasdale 1996; Weston 1996). Another
study found that underseeded biennial sweetclo-
ver [
Melilotus offi cinalis
(L.) Lam] reduced weed
establishment after harvest and in the following
spring before being terminated at the 90% bloom
stage in late June (Blackshaw et al., 2001b). Sweet-
clover residue provided excellent weed suppres-
sion throughout the remaining portion of the
fallow year. Wheat yield in the subsequent pro-
duction year was higher due to fewer weeds and
greater nitrogen availability from sweetclover
nitrogen fi xation (Table 12.4).
Diverse crops grown in rotation with wheat
allow for greater herbicide choice over years and
may avoid continuous use of the same herbicide
with inadvertent selection for weed resistance.
Additionally, crop diversity encourages opera-
tional diversity that, in turn, can facilitate
improved weed management. Different crops are
naturally planted and harvested at different times
of the year. If suffi cient differences exist in ger-
mination requirements between the rotational
crop and potential weed species, then seeding
date can be manipulated to benefi t the crop. Early
sown spring crops may out-compete weeds that
require warmer soil temperatures for germina-
tion. For example, densities of the C
4
species
green foxtail have declined in early planted spring
crops such as canola or fi eld pea (
Pisum sativum
L.) in zero-tillage systems that often have lower
soil temperatures (Blackshaw 2005). Conversely,
delayed seeding can be used to manage early
spring germinating weeds such as kochia [
Kochia
scoparia
(L.) Schrad.]. Alternating seeding dates
over years is a desired weed management practice
and one that farmers should try to implement.
Wheat cultivars can vary considerably in their
competitiveness with weeds (Hucl 1998; Lemerle
et al., 2001). Winter wheat cultivars have been
identifi ed that differ in their competitive ability
with downy brome (Blackshaw 1994a). Wheat
yield reductions caused by downy brome varied
by as much as 30% depending on the cultivar
grown (Table 12.5). Tall (non-semidwarf) culti-
vars (90-110 cm) had a height advantage over
downy brome (70 cm) and cultivars with a spread-
ing growth habit provided better interrow shading
of downy brome. Increased competitive ability of
wheat, or crops in general, has been associated
