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species or biotypes from adapting to production systems. The following are specific
examples of integrated weed management systems used in dryland wheat produc-
tion systems in North America.
2.2.1 Winter Wheat-Summer Crop-Fallow in the U.S. Central Great
Plains
In the U.S. Central Great Plains, dryland agriculture has developed around winter
wheat production. Summer fallow, the practice of controlling all plant growth dur-
ing the non-crop season, was quickly adopted to stabilize winter wheat production
in the region. Winter wheat—fallow was the predominant crop rotation in the
Central Great Plains during most of the twentieth century (Baumhardt and Anderson
2006 ). Downy brome (Bromus tectorum L.), jointed goatgrass (Aegilops cylindrica
Host), and feral rye (Secale cereale L.) are weeds that cause significant economic
loss in winter wheat—fallow production regions of the Great Plains and western
USA, particularly where conservation tillage is used (Lyon and Baltensperger
1995 ). In addition to wheat yield loss, jointed goatgrass and feral rye seed fre-
quently contaminate winter wheat grain, resulting in economic loss from dockage
and grade reduction. These three winter annual grass weeds have a similar life cycle
and physiology to winter wheat, which limits effective control methods. While
plowing with a moldboard plow can effectively control these weeds (Donald and
Ogg 1991 ; Kettler et al. 2000 ; Stump and Westra 2000 ), tillage that buries nearly all
surface crop residues defeats the goal of no-till systems, i.e., maintaining crop resi-
dues on the soil surface (Unger et al. 2006 ). An IWM approach must balance crop
production, weed control, and soil and water conservation.
Several herbicides including sulfosulfuron, propoxycarbazone, pyroxsulam, and
imazamox can provide selective control of one or more of these grass weeds.
Imazamox can only be used with imazamox-tolerant wheat cultivars or serious crop
injury will occur. Concerns with these products include high cost, long soil residual
that restricts rotation flexibility, and development of weed resistance with frequent
use.
Crop rotation with late spring-planted crops effectively controls these winter
annual grass weeds in winter wheat (Daugovish et al. 1999 ). Growing a winter
wheat crop every three or four years rather than every other year promotes depletion
of the soil seed bank as long as no plants are allowed to produce seed during the
non-wheat portion of the rotation. This is easily accomplished if the rotational crops
are not growing during the early spring when the use of nonselective herbicides or
tillage can be used to kill emerged plants.
Seed production of winter annual grass weeds can be reduced by combining
cultural practices. Feral rye and jointed goatgrass seed production was reduced by
applying nitrogen fertilizer five months before wheat seeding, increasing the wheat
seeding rate, and planting a standard height cultivar (Anderson 1997 ). Standard
height cultivars frequently yield less than many semi-dwarf cultivars. If a semi-
N.C. Hansen et al.