Crop diversification for weed management - Ecological Management of Agricultural Weeds - page 339

and peanut). Under both weed-free and weed-infested conditions, intercrop-
ping legumes with sorghum increased crop leaf area, light interception,
macronutrient (N, P, and K) uptake, dry matter production, and seed yield
above levels obtained from sole-cropped sorghum. Intercropping also
reduced the amount of light available to weeds, decreased weed macronutri-
ent uptake, and reduced weed dry matter production up to 76%, compared
with the sorghum sole crop. Figure 7.5 illustrates these effects for the
sorghum/fodder cowpea intercrop. Similar patterns of resource use, crop
yield, and weed suppression have been observed for intercrops and sole crops
of sorghum and pigeonpea (Natarajan & Willey, 1980 a , 1980 b ; Shetty & Rao,
1981).
A resource-based approach for studying weeds in intercropping systems
can provide mechanistic insights into why certain crop combinations are
more weed-suppressive than others. For example, based on data presented in
Tables 7 and 8 of Abraham & Singh's (1984) study, 90% of the variation

among cropping systems in final weed biomass was predictable as a linear
function of light penetration to ground level at 25 days after planting.That is,
the mixtures of crop species that were most effective in suppressing weed
growth were those that intercepted the most light early in the growing season.
Efforts to improve weed suppression by sorghum-based intercropping
systems might therefore focus on identifying management practices and cul-
tivars that increase early canopy development and light interception. Similar
resource-based approaches could be pursued for improving weed manage-
ment in other systems.

Crop diversity and density
One of the important issues emerging from studies of intercrop/weed
interactions is whether weeds are suppressed by increasing crop diversity per
se , or by the combined effects of increasing crop diversity and density.
Mixtures of crop species that complement or facilitate each other's use of
resources are often sown at greater total densities than those used for sole
crops.The most common way to do this is to add all or part of the normal sole-
crop population density for one crop into the normal sole crop density of
another crop.Thus, crop diversity and density effects are often confounded in
intercropping experiments.
To separate the effects of crop diversity from those of crop density requires
the use of experimental designs in which overall density is maintained con-
stant between sole crop and intercrop treatments.Suppose,based on the study
by Bulson, Snaydon & Stopes (1997), that the “normal” sowing densities for
sole crops of wheat and fava bean are 250 and 50 plants m 2 ,respectively.For a