Crop diversification for weed management - Ecological Management of Agricultural Weeds - page 345

Figure 7.7 Effect ofpea cultivar identity on (a) above-ground biomass of Brassica
hirta and (b) seed yields ofbarley and pea in mixtures ofthe three species grown
under well-watered,low nitrogen conditions,in an experiment conducted in
California,USA.(Adapted from Liebman,1989.)
Crop genotype
Similar to the situation for sole crops (see Chapter 6),large differences
in weed growth have been observed between different cultivars of crops sown
in mixtures. Weed biomass was 49% to 56% lower when sorghum was inter-
cropped with a cowpea cultivar used for fodder than when it was intercropped
with one used for seed production (Abraham & Singh, 1984). Samson, Foulds
& Patriquin (1990) found significant differences in weed suppression ability
between cultivars of both perennial and annual ryegrasses intercropped with
maize. Brassica hirta biomass was 44% lower when barley was intercropped
with a long-vined, large-leafed pea cultivar ('Century') rather than a shorter-
vined, smaller-leafed pea cultivar ('Alaska') (Figure 7.7a) (Liebman, 1989).
Generally, intercrops are more weed-suppressive when they include culti-
vars that have rapid initial growth and dense, planiform canopies. Choosing a
cultivar for its weed suppression characteristics may conflict with other objec-
tives, however. Intercropping barley with 'Century' pea rather than 'Alaska'
pea gave better suppression of B. hirta (Figure 7.7a) and increased pea seed
yield by 45% (Figure 7.7.b), but also reduced barley seed yield by 88% (Figure
7.7b). In this case, better weed suppression and greater production of the
legume would have to be weighed against the reduced yield of the cereal.