Enhancing the competitive ability of crops - Ecological Management of Agricultural Weeds - page 276

Pantone & Baker,1991; O'Donovan,1994),although more complicated model
forms that allow for the nonequivalence of the weed and crop are needed to fit
the data. In these studies, usually weed and crop biomass are plotted against
crop density alone (Figure 6.3) rather than the combined density of the crop
and weed. The increased importance of crop density for weed suppression at
high weed densities is evident in such plots: in the crop density range over
which the weed-free crop gives approximately full yield,the slopes of both the
weed biomass and crop biomass curves are steeper when weed density is
higher. This indicates that the incremental effect of additional crop density is
greater when weed density is high, as stated above.
Examining Figures. 6.2 and 6.3 further reveals that the yields of both the
crop and the weed change most rapidly when the crop is at low densities.That
is,the higher the crop density,the smaller is the effect on both the crop and the
weed of raising the crop density further by some small increment. The
optimal crop density is the density at which a further increment of seed costs
more than the expected increase in yield is worth.Charts similar to Figure 6.3
showing the relation of yield and weed biomass to crop density could be
useful in helping growers determine appropriate planting densities for their
typical or expected levels of weed pressure.
Whether greater productivity at higher crop density results in greater yield
depends on the nature of the crop. First, probability of lodging and disease
increases with crop density for some crops (Hartwig, 1957; Felton, 1976;
Appleby & Brewster, 1992), and the seriousness of potential losses must be
balanced against any potential gain in yield due to improved weed manage-
ment. Second, the form of the yield-density curve varies among crop species.
Although the relation expressed in equation 6.1 usually holds for crop
biomass, the effects of intraspecific competition frequently depress harvest
index when the crop is grown at high densities. For cereal grains and other
crops in which the number of harvested units changes with density (e.g.,
seeds, berries, cotton bolls), the response of yield to density often approxi-
mates a hyperbolic curve, or has a wide plateau with yield reduction only at
very high densities.The curve is likely to be more rounded for crops in which a
substantial reproductive structure has to be built in order to produce seeds
(e.g., maize, sunflower). Finally, for root crops and the many vegetable crops
for which small produce is unmarketable, the curve of yield in response to
density is usually quite peaked. At high density, resources are divided among
many small individuals, each of which makes one small root or fruit (Willey &
Heath, 1969). For this latter type of crop, density is likely to have limited use-
fulness as a weed management strategy, and few density/weed management
studies exist for these crops (Table 6.1). For many seed crops, however, the