Crop diversification for weed management - Ecological Management of Agricultural Weeds - page 351

it.As more rows of peanut replaced millet to form various crop mixtures,both
Celosia density and biomass increased markedly, but only within the peanut
rows. These results suggest that the composition of weed communities in
intercrops can be predicted from the proportions of the component crops.
Second, greater evenness in the relative importance of different weed
species can be promoted by cropping practices that increase crop biomass and
reduce total weed biomass.Whether or not the weed species found in an inter-
crop grow in more balanced proportions than those in sole crops will then
depend on which cropping system provides the greatest degree of overall
weed suppression. This point is illustrated by data from Mohler & Liebman
(1987), who grew barley and pea sole crops and intercrops in replacement
series at different densities. High-density crops produced more biomass than
normal-density crops, and for a given density, crop biomass was greatest in
sole-cropped barley, least in sole-cropped pea, and intermediate in the inter-
crop. For both intercrops and sole crops, increased planting density led to
higher crop biomass production. These more productive crops had fewer
weeds,lower total weed biomass,and reduced relative importance of the dom-
inant weed species, Amaranthus retroflexus or Brassica kaber , depending on the
site. Other weed species, such as Ambrosia artemisiifolia , also produced less
biomass,but comprised a larger fraction of the weed community as total weed
biomass decreased. Thus, with greater crop competition, the composition of
the intercrop and sole crop weed communities shifted from dominance by
Amaranthus retroflexus or B.kaber to more mixed assemblages.This shift implies
that the weed communities were structured more by crop resource use than by
crop diversity.
Given the general dearth of data concerning weed species abundance in
intercrops, it is difficult to assess the prevalence of these impacts of intercrop-
ping on weed community organization. However, as noted in Chapter 10, a
better understanding of how cropping practices affect weed communities is
needed to anticipate shifts in the abundance of resident weed species and
prevent invasions by dispersing weed species.
Agroforestry
Why farmers use agroforestry systems
The use of trees by temperate and tropical farmers ranges from forest
fallow systems and traditional dooryard gardens to modern fruit orchards,
plantation tree crops, and alley cropping. Depending on the species and man-
agement strategies employed, agroforestry systems may be labor efficient or