Crop diversification for weed management - Ecological Management of Agricultural Weeds - page 342

Figure 7.6 Above-ground biomass ofweeds present in different cropping system
treatments 87 days after crop emergence,in an experiment conducted in Rio
Grande do Sul,Brazil.(Adapted from Fleck,Machado & De Souza,1984.)
the sole-cropped cereal, but less than in the sole-cropped legume. It is unclear
why intercropping failed to provide a weed control advantage over sole crop-
ping in these studies, but did provide an advantage in the studies by Sharaiha
& Gliessman (1992) and Fleck, Machado & De Souza (1984). To answer this
question, the influence of environmental conditions and species-specific
resource use and growth characteristics needs to be investigated in more
detail.
In contrast to the variable results of studies testing the effects of crop diver-
sity on weeds, studies focused on crop density consistently show that increas-
ing intercrop density reduces weed biomass (Bantilan, Palada & Harwood,
1974; Shetty & Rao, 1981; Tripathi & Singh, 1983; Mohler & Liebman, 1987;
Lanini et al ., 1991; Weil & McFadden, 1991; Bulson, Snaydon & Stopes, 1997).
Increases in intercrop density can also have important beneficial effects on
crop yield (Willey, 1979 b ). Bulson, Snaydon & Stopes (1997) observed that
yields of wheat and fava bean sole crops failed to increase when densities
exceeded 100% of recommended levels; in contrast, yield of a wheat/fava bean
intercrop, expressed as a land equivalent ratio (LER), continued to rise when
total crop density exceeded the level used for a normal density sole crop (Table
7.2). A wheat/fava bean intercrop with each component sown at 75% of its