Enhancing the competitive ability of crops - Ecological Management of Agricultural Weeds - page 279

The above discussion begs the question of whether the yield advantage of
increased crop density in the presence of weeds is greater for competitive
crops or for relatively noncompetitive crops. The general relationships
involved can be visualized by means of two hypothetical examples (Mohler,
1996). Suppose a field has a weed density of 100 plants m 2 . Further suppose
that it is planted with a crop having a yield potential of 1000 kg ha 1 in weed-
free conditions, and that the crop is usually planted at 100 plants m 2 . Thus,
doubling planting density from 100 to 200 m 2 will increase the proportion
of crop plants from 50% to 66.7%. The effect of such a change in crop density
can be explored using a replacement series diagram in which yields of the two
species are plotted against their relative densities (De Wit, 1960). If the crop is
a better competitor than the weed, it will have a convex curve, whereas the
weed will have a concave curve (Figure 6.4a).Figure 6.4 is drawn such that the
relative percentages of the crop and weed are in the usual linear scale.
However, since planting density does not affect initial density of the weed,
weed density is fixed at 100 m 2 , and crop density therefore varies hyperboli-
cally. For simplicity, competition is assumed to follow a simple replacement
process in which the two species have equal growth potential and combined
yield of the crop and weed is the same for all mixtures.
If the crop is competitive (Figure 6.4a), doubling density increases yield
from 750 to 850 kg ha 1 . Even this is less than the potential yield of 1000 kg
ha 1 possible in weed-free conditions, but if control measures that allow full
yield are unavailable or prohibitively expensive,as in the case of safflower dis-
cussed above (Blackshaw,1993),the increased planting density may be a viable
tactic. Nevertheless, the percentage increase in yield due to elevated planting
density is moderate - only 13% more than could be obtained by planting at the
usual density.
The same replacement series curves can be inverted to examine the interac-
tion between a highly competitive weed and a less competitive crop (Figure
6.4b). In this case, doubling crop density increases yield from 250 to 410 kg
ha 1 , an increase of 64%. Although the percentage increase in yield is large,
even the yield at twice normal density is so far below the crop's potential that
either other weed management methods would be needed or the crop would
probably not be grown. Low yield of noncompetitive crops under weedy con-
ditions may be the reason why most studies of crop density involve large-
seeded species (Table 6.1) that have an initial size advantage over weeds
during establishment (see Chapter 2).
Note, however, that combining increased crop density with other weed
control methods can be very effective when the crop is a poor competitor. For
example, if the weed is thinned from 100 to 50 plants m 2 at the outset, then