Enhancing the competitive ability of crops - Ecological Management of Agricultural Weeds - page 270

Crop density

Many studies have demonstrated that weed biomass and other meas-
ures of weed abundance usually decrease as crop density increases (Table 6.1)
(Mohler, 1996). The pattern is remarkably consistent: of the 91 cases found in
the literature, only six failed to show decreasing weediness with increasing
crop density. Since most crops are sown in rows, variation in density involved
variation in crop arrangement in most of these studies. The effects of crop
arrangement in those studies in which arrangement was varied at constant
crop density are discussed in the following section.
The role of crop density in weed management is well illustrated by a study
of the interactions between safflower and Setaria viridis (Blackshaw, 1993).
Safflower was sown in early May at six rates that resulted in densities ranging
from 12 to 192 plants m 2 , and plots were maintained in either weedy or
weed-free conditions. S. viridis biomass declined with increasing crop density
(Figure 6.1a). Probably because safflower has a larger seed than S. viridis , the
crop was initially taller than the weed, and maintained this height difference
throughout the season (Figure 6.1b). As a consequence, by early July the crop
substantially shaded the weed, and the degree of shading increased with crop
density (Figure 6.1c).Due to the increased competitive pressure exerted by the
crop at higher densities, the plateau in yield was reached at substantially
higher crop density in the weedy, relative to the weed-free, condition (Figure
6.1d). Although the optimal crop density was much higher in the presence of
weeds, even at very high density yield was still less in the weedy condition.
The mathematical analysis of how plant density affects the competition
between species of annual plants has received considerable attention (De Wit,
1960; Håkansson, 1983; Spitters, 1983; Firbank & Watkinson, 1985). A brief
introduction to this work helps explain the conditions under which crop
density is most useful for weed management. Frequently, biomass of a crop,
Y C , can be expressed as
Y C N C / ( a bN C ) (6.1)

where N C is the density of the crop (Shinozaki & Kira, 1956; Harper, 1977, p.
156; Håkansson, 1983). As N C becomes large, Y C rises to an asymptote at 1/ b
(Figure 6.2a). In the absence of weeds, the change in Y C with density is negli-
gible beyond a certain range, and consequently the flat portion of the Y C curve
at high density is sometimes referred to as the “law of constant final yield”
(Kira, Ogawa & Shinozaki, 1953; Harper, 1977, p. 154). Now suppose the crop
competes with a weed that is very similar to it in all respects.Then,

Y C N C / [ a b ( N C N W )]

Y W N W / [ a b ( N C N W )]

Y C Y W ( N C N W ) / [ a b ( N C N W )]