Enhancing the competitive ability of crops - Ecological Management of Agricultural Weeds - page 281

Lowering weed density also has a beneficial effect when the crop is a good
competitor. When the crop is a competitive species, the combined effects of
thinning weeds and increasing crop density are smaller than when the crop is
a poor competitor, but yield may approach that for the weed-free condition
(Figure 6.4a). Thus, enhanced crop density is likely to be an effective compo-
nent of many integrated weed management programs.
Although increased planting density is clearly advantageous when weeds
are present, exactly how much density should be increased depends on both
the nature of the crop and the density of weeds present.Consequently,specific
recommendations are difficult to obtain. In general, (i) a moderate increase in
density above the accepted optimum for weed-free conditions is usually bene-
ficial for those crops whose weed-free yield is relatively insensitive to density,
(ii) an increase in density of 20% to 100% is likely to improve weed control
measurably, and (iii) density increases greater than 50% to 100% are likely to
result in lodging, disease, and other problems. Even for growers who often
achieve good weed control, some increase in density may provide relatively
cheap insurance against failures of other weed management measures.

Crop spatial arrangement

Row spacing and random versus regular planting patterns
Both theoretical and empirical studies indicate that crop planting
pattern has a substantial effect on the competitive balance between crops and
weeds. Fischer & Miles (1973) explored the effects of planting pattern using a
stochastic model in which plants grew radially until every point in the field
was occupied by either a crop plant or a weed without overlap. They assumed
that the weeds were randomly positioned by a Poisson process, and examined
a variety of plant arrangements for the crop. Several of their results warrant
comment here. First, they showed that the percentage of the field surface
occupied by weeds increased as the “rectangularity” of the crop increased
(Figure 6.5), where rectangularity is the row spacing, b , divided by the within-
row plant spacing, a . Second, random sowing of the crop was always inferior
to a square lattice planting, and in fact, was equivalent to a rectangular plant-
ing arrangement in which b / a 3.537 (Figure 6.5). Although a triangular

lattice was superior to a square lattice, the difference was too small to have
practical importance in field conditions. Finally, they found that grouping
crop plants into clusters greatly decreased their ability to occupy space relative
to the weeds. Although the percentage of space occupied by weeds in the
model of Fischer & Miles (1973) varied with crop density,and with the relative
emergence times and growth rates of the weeds and crops,the relative ranking