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Another multi-year study conducted at three locations assessed the merits of
combining several crop production practices to manage weeds in the context of full
or reduced herbicide rates in spring wheat and other major field crops of the
Canadian Prairies (Blackshaw et al. 2005a, b). Factors included in the study were
crop rotation, seeding date, seeding rate, fertilizer timing, and herbicide rate. The
combination of earlier seeding date (3 weeks earlier), higher crop seeding rate (50
% higher), and spring-applied subsurface-banded fertilizer resulted in the most
competitive cropping system. Weeds were controlled with this IWM approach and
it is notable that the weed seed bank was not greater after four continuous years of
using 50 % herbicide rates in a competitive cropping system at two of three sites.
Farmers were impressed with the level and consistency of weed control in this study
but were only truly convinced of the merits of these IWM systems when they were
shown to be economically viable (Smith et al. 2006 ).
These examples illustrate that weed populations adapt to changes in farm man-
agement. Over-reliance on herbicides for weed control has led to weed resistance
and weed shifts that complicate weed management. With a limited array of avail-
able herbicide classes, weed resistance will remain a challenge to wheat production
in the future. Farmers must strive to alter management practices and maximize the
competitive advantage of the crop at the expense of weeds. The ubiquitous use of
glyphosate jeopardizes the sustainability of no-till systems that have proven so
effective for diversifying and intensifying wheat production systems in semi-arid
environments. Proper glyphosate stewardship is critical to maintaining these effec-
tive production systems. Sustainable weed management in dryland wheat produc-
tion will best be achieved through continued development and adoption of integrated
weed management and crop production practices.
3 Diversifying Rotations to Include Forages and Short
Season Crops
Throughout the dryland regions of North America, a 2-year rotation of wheat-
summer fallow was commonplace during the previous century and continues to be
widely practiced. A two-year rotation with one crop functions in drylands because
(1) the early maturing wheat avoids the late-season heat and drought common in
North American dryland regions, and (2) the fallow period stabilizes yields and
reduces the incidence of crop failure (Greb et al. 1970 ; Baumhardt and Anderson
2006 ). While the rotation can function in the short term, many long-term problems
have been documented with the wheat-summer fallow rotation, including increased
soil erosion, saline seep formation, reduced precipitation use efficiency, decreased
soil organic C and N, and reduced annualized crop yield and economic returns
(Black et al. 1981 ; Janzen 1987 ; Campbell et al. 1990 ; Wienhold et al. 2006 ; Sainju
et al. 2015a). Wheat-summer fallow cropping systems are unlikely to maintain soil
quality, much less improve the quality of degraded soils. For example, in a 19-year
N.C. Hansen et al.