Agriculture Reference
In-Depth Information
Prairies (Table 12.8). Wheat-based rotations now
include more oilseeds [e.g., canola, fl ax (
Linum
usitatissimum
L.)] and pulses [e.g., fi eld pea and
lentil (
Lens culinaris
Medik.)]. 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 2 to 4 years
of forage production (Entz et al., 1995). Diverse
crop rotations have resulted in lower weed popu-
lations in spring wheat.
Weed management in spring wheat can be
improved by including fall-seeded crops in
rotation (O'Donovan et al., 2007). Many spring-
germinating weeds emerge after canopy closure of
fall-seeded crops, which makes them noncom-
petitive. Winter wheat, winter rye, and winter
triticale (
Triticosecale
spp.) are being more widely
grown on the Canadian Prairies. Systematically
changing planting dates and crop species prevents
any one weed species from developing into a
major problem (Derksen et al., 2002).
Spring wheat farmers in Canada are slowly but
surely adopting IWM systems. Foxtail barley 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 multiyear approach. Farmer adop-
tion of an IWM system for foxtail barley was one
of the fi rst 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.
Another multiyear 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 fi eld 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 crop-
ping 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 competi-
tive 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).
Spring wheat in Australia
Widespread development of herbicide resistance
in several important weed species has forced Aus-
tralian farmers to reassess their approach to weed
management. A 2003 survey across the Western
Australian wheatbelt showed that only 8% of
rigid ryegrass and 17% of wild radish populations
were susceptible to all the most commonly used
in-crop selective herbicides (Owen et al., 2007;
Walsh et al., 2007). Even with such widespread
resistance, herbicides have remained an impor-
tant component of weed management; but
much greater thought is given to integration
of herbicides with nonchemical weed control
tactics.
Ley farming systems, in which pastures were
rotated with wheat, was the most widely used
farming system in Australia for many years.
Although the system has lost some of its popular-
ity due to changes in commodity prices, pastures
are still rotated with wheat production over large
areas of farm land. In this system, pastures offer
an excellent opportunity to control herbicide-
resistant rigid ryegrass with grazing by sheep and
with nonselective herbicide application soon after
rigid ryegrass fl owers (Gill and Holmes 1997).
This practice, referred to as
spray-topping
, has
played an important role in the management of
herbicide-resistant weed populations in pasture-
wheat rotations in southern Australia. Gill and
Holmes (1997) reported that the use of spray-
