423dwarf cultivar is used, row spacing can be reduced to help compensate for the loss
in weed competitiveness.
By combining crop rotation, cultural practices, and herbicides, winter annual
grass weeds are effectively controlled in winter wheat and the entire cropping sys-
tem is made more sustainable. The winter wheat—summer crop—fallow system is
an example of an effective IWM system with implications beyond weed control.
2.2.2 Zero-till Spring Wheat in North America
Zero tillage (no-till) has become a widely adopted agronomic practice in the spring
wheat production areas of Canada and the northern USA. Research has shown that
some weed species may become more prevalent with zero tillage, but overall weed
densities decline with time (Derksen et al. 2002 ; Anderson 2003 ; Blackshaw 2005 ).
Weed seed mortality tends to be greater when weed seeds are left on the soil surface
compared to when buried in the soil with tillage. Additionally, crop residues on the
soil surface may inhibit weed germination and growth through physical suppression
and/or allelopathic interactions. Thus, zero tillage has contributed greatly to
improved weed management as well as higher spring wheat yields.
Improved soil moisture conservation with zero tillage has allowed a greater vari-
ety of crops to be grown in recent years in the semi-arid Canadian Prairies. Wheat-
based rotations now include more oilseeds [e.g., canola (Brassica napus L.), flax
(Linum usitatissimum L.)] and pulses [e.g., field pea, lentil, and faba bean]. Inclusion
of forages such as alfalfa (Medicago sativa L.) or red clover (Trifolium pretense L.)
in rotation with spring wheat, with the main goal of managing weeds, is gaining
acceptance in areas where forage demand is high. Survey results indicate that 83 %
of farmers had lower weed densities after two to four years of forage production
(Entz et al. 1995 ). Diverse crop rotations have resulted in lower weed populations in
spring wheat.
Weed management in spring wheat can be improved by including fall-seeded
crops in rotation (O’Donovan et al. 2007 ; Beres et al. 2010 ). Many spring-
germinating weeds emerge after canopy closure of fall-seeded crops, which makes
them noncompetitive. Winter wheat, winter rye, and winter triticale (Triticosecale
spp.) are being more widely grown on the Canadian Prairies. Systematically chang-
ing planting dates and crop species prevents any one weed species from developing
into a major problem (Derksen et al. 2002 ; Harker et al. 2016 ).
Spring wheat farmers in Canada are slowly but surely adopting IWM systems.
Foxtail barley (Hordeum jubatum L.) is an example of a weed species that became
a greater problem with zero tillage (Blackshaw 2005 ). However, Blackshaw et al.
( 1999 ) determined that good control of this weed could be attained by combining
crop rotation, higher wheat seeding rates, banded nitrogen fertilizer, and timely her-
bicide use in a multi-year approach. Farmer adoption of an IWM system for foxtail
barley was one of the first success stories, and it occurred in part because the need
was so great. Farmers will readily adopt new practices when they perceive a need
for change and when those practices are effective and affordable.
Dryland Agriculture in North America