Agriculture Reference
In-Depth Information
IMPACT OF WEEDS ON WHEAT
tion in weed interference due to N placement
(banding) is likely to be the result of improved
access to the nutrient by the crop.
Increasing nitrogen inputs under weedy situa-
tions can exacerbate crop-weed competition.
Carlson and Hill (1986) reported an increase in
wheat yield loss from wild oat in treatments where
preplant fertilizer nitrogen was applied (Table
12.1). It appears that the differences in respon-
siveness to nitrogen between the two species
intensifi ed competition for other growth factors
such as light and water.
Competition
Competition arises when two or more organisms
seek a common resource whose supply falls below
their combined demand (Donald 1963). The
central proposition is that each individual in a
population affects, and is affected by, other indi-
viduals in the population. Due to their presence
in crop communities, weeds consume essential
growth factors that would have been otherwise
available to the crop. Consequently, crops grow
more slowly in the presence of weeds, and they
produce lower biomass and grain yield, reducing
profi tability for the farmer. The balance between
supply and demand of various growth factors will
vary during the growing season; consequently,
the factor most actively competed for could
change during the growing season.
Light (shading)
In mixed plant communities, individuals of a
more competitive species can reduce the amount
of light reaching the leaves of neighboring plants
of other competing species as a result of having
greater height or leaf area. Due to interception by
plant leaves, light quantity and quality (ratio of
red to far-red wavelength) change as light passes
through the canopy. Cudney et al. (1991) con-
ducted a fi eld study on wild oat-wheat competi-
tion under nonlimiting nitrogen and moisture
conditions. They found that wild oat grew taller
than wheat and had a greater proportion of its
canopy above 60 cm at anthesis and maturity.
The mathematical model developed by Cudney et
al. (1991) predicted that competition from wild
oat was due to reduced leaf area of wheat at early
growth stages (possibly due to competition for
soil factors) and reduced light penetration to
wheat leaves at later growth stages (competition
for light).
Nutrients
Considerable evidence in the literature shows that
neighboring plants compete with each other for
the supply of essential mineral nutrients. Species
or individuals with larger and more effective root
systems are likely to absorb a greater share of the
nutrient pool, thereby affecting the growth of
other members of the plant community. However,
even when roots occupy the same depths, compe-
tition may be less intense for nutrients with low
mobility, for example, phosphate. In contrast,
depletion shells of nitrate ions around active roots
tend to be large (20-60 mm), and intermingling
roots of crops and weeds are likely to differen-
tially affect acquisition of this nutrient (Nye and
Tinker 1977).
Manipulation of nitrogen fertilization is a
promising cultural practice to reduce weed inter-
ference in crops. Nitrogen fertilizer placed in
narrow bands below the crop rows compared with
surface broadcast has been found to reduce the
competitive ability of wild oat (
Avena fatua
L.)
(Kirkland and Beckie 1998), foxtail barley
(
Hordeum jubatum
L.) (Blackshaw et al., 2000a),
and jointed goatgrass (
Aegilops cylindrica
Host)
(Mesbah and Miller 1999) with wheat. The reduc-
Table 12.1
Effect of wild oat density and the rate of
preplant nitrogen on yield loss of spring wheat.
Preplant N Rate (kg ha
−1
)
0
67
134
Wild Oat Density (plants m
−2
)
% loss
8
8
19
20
16
15
31
30
32
23
45
55
Source:
Adapted from Carlson and Hill (1986).
