Agriculture Reference
In-Depth Information
Water
Intermingled root systems at least partly exploit
the same pool of water. The main evidence for
this comes from calculations which show that the
water potential at the midpoint between neigh-
boring roots is similar to that at the root surfaces,
until most of the available water has been removed
from the soil (Lawlor 1972). Since plants suffer
from water defi cits in many parts of the world, at
least in some periods on some soils, competition
for water is likely to be widespread.
Convincing evidence for crop-weed competi-
tion comes from a study on soybean [ Glycine max
(L.) Merr.] in the US by Jones et al. (1997), in
which they measured the impact of weeds on the
sap fl ow through the crop. They found that weed-
induced yield and sap fl ow reductions were very
similar in magnitude, thereby indicating that
water deprivation was the primary cause of yield
reduction in soybean (Table 12.2). Although no
such information is available for wheat, it is plau-
sible that competition for water with weeds would
have some signifi cance, particularly during the
reproductive phase of the crop.
As stated earlier, the balance between supply
and demand of different growth factors will tend
to vary during the growing season; as a result, the
factor that is most actively competed for at the
start of the growing season may not be the same
factor most actively competed for later in the
season. In the example of Jones et al. (1997), com-
petition for water was the primary cause of weed
effects on soybean. In other studies, competition
for nitrogen has been shown to be the key factor
involved in competition. In most situations,
however, it is likely that competition is taking
place for several growth factors simultaneously.
Furthermore, the level of supply of one factor can
infl uence the severity of competition for another
factor. The example of Carlson and Hill (1986)
illustrates this principle quite well, where increas-
ing the supply of nitrogen aggravated competitive
effects of wild oat on wheat, possibly due to
greater competition for light and water.
Wheat grain yield
Weed species present and their density are the
major determinants of the impact of weeds on
wheat yield and quality. All available evidence
indicates that the relationship between weed
density and crop yield loss is hyperbolic or expo-
nential in shape. Absolute yield or the relative
yield (yield in the presence of weeds divided by
weed-free yield) decreases asymptotically with
increasing density of weeds. Therefore, all man-
agement programs endeavor to achieve weed-free
crops, but this is becoming increasingly diffi cult
because of the evolution of herbicide resistance in
many major weed species around the world.
At the same weed density, yield loss caused by
different weed species varies considerably. This
could be due to several factors intrinsic to the
weed species, including initial seedling size
(related to seed size), relative growth rate, leaf
area, canopy architecture, root growth, and dis-
tribution. A comparison of the impact of different
weed species on wheat yield loss in Australia
showed more than a 100-fold difference in per-
centage yield loss per weed plant (Gill and
Davidson 2000). Most agricultural weeds have a
Table 12.2 Sap fl ow and grain yield of soybean grown either in monoculture, with common cocklebur ( Xanthium
strumarium L.), or with sicklepod ( Cassia obtusitolia L.) at a fi xed density.
Measured
Species
Sap Flow
Sap Flow Reduction
Soybean Yield a
Yield Loss
Competitor Species
(kg ha −1 d −1 )
(%)
(kg ha −1 )
(%)
Soybean
None
48,000
1,553
33,100* a
Soybean
Common cocklebur
31
962*
38
Soybean
Sicklepod
22,700*
53
792*
51
Source: Adapted from Jones et al. (1997).
a Means followed by an asterisk are signifi cantly different ( p
<
0.05) from the corresponding value in the weed-free
soybean.
 
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