Agriculture Reference
In-Depth Information
distribution may be used to improve the estima-
tion of weed distribution by use of co-kriging
methods. The prediction variance of
Lamium
spp.
was improved 11% when silt content was included
in co-kriging compared to kriging weed sample
data alone (Heisel et al., 1999), indicating a pref-
erence of different weed species for certain soil
characteristics.
Site-specifi c weed control has great potential to
reduce herbicide use, but camera and application
technologies, along with image analysis algo-
rithms, must be improved before it is commer-
cially viable (Gerhards and Oebel 2006). Herbicide
savings with site-specifi c management must com-
pensate for the costs of weed sampling, weed
mapping, data processing, decision making, and
site-specifi c application technology if it is to
replace whole-fi eld spraying (Timmermann et al.,
2003).
Plowing with a moldboard plow can effectively
control these weeds (Donald and Ogg 1991;
Kettler et al., 2000; Stump and Westra 2000),
but it buries nearly all surface crop residues.
Maintaining crop residues on the soil surface
protects soil from wind and water erosion and
increases the storage of soil water (Unger et al.,
2006).
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 prod-
ucts include high cost, long soil residual that
restricts rotation fl exibility, 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).
Table 12.7 shows the impact of adding a late
spring-planted crop to the winter wheat-fallow
rotation on jointed goatgrass seedling density,
wheat grain contamination, and the soil seed
bank. Similar results were reported for feral rye.
Growing a winter wheat crop every 3 or 4 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 nonwheat
portion of the rotation. This is easily accom-
plished if the rotational crops are not growing
during the early spring when the use of nonselec-
tive herbicides or tillage can be used to kill
emerged plants.
The intensifi cation and diversifi cation of the
winter wheat-fallow rotation has not only helped
to control winter annual grass weeds in wheat,
but it has also increased water storage and water-
use effi ciency by reducing the length of the 16-
month fallow period to 10 or 11 months and by
replacing soil evaporation by crop transpiration
(Baumhardt and Anderson 2006). Compared
to winter wheat-fallow, winter wheat-summer
crop-fallow has increased annualized grain and
forage yields, increased net profi tability, increased
potentially active surface-soil organic C and N,
and reduced yield loss in wheat due to soilborne
disease.
PUTTING IT ALL TOGETHER:
EXAMPLES OF EFFECTIVE SYSTEMS
Winter wheat in North America: Winter
wheat-summer crop-fallow
In the US Great Plains dryland agriculture has
developed around wheat production. Summer
fallow, the practice of controlling all plant growth
during the noncrop 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 20th century (Baumhardt and Anderson
2006).
Downy brome, jointed goatgrass, and feral rye
cause signifi cant economic loss in winter wheat-
fallow production regions of the western US,
particularly where conservation tillage is used
(Lyon and Baltensperger 1995). In addition to
wheat yield loss, jointed goatgrass and feral rye
seed frequently 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.
