Crop diversification for weed management - Ecological Management of Agricultural Weeds - page 357

Second, narrower spacing between hedgerows promotes greater weed sup-
pression. Rosecrance, Rogers & Tofinga (1992) found weed coverage between
rows of Gliricidia sepium and Calliandra calothyrsus during the last four months
of taro production averaged 60% in 6-m alleys,but only 35% in 5-m alleys,and
15% in 4-m alleys. Jama, Getahun & Ngugi (1991) reported that shading by
Leucaena leucocephala during fallow periods between cropping seasons reduced
weed biomass by 74% in 8-m alleys, but by 84% in 4-m alleys, and 93% in 2-m
alleys, compared to a hedgerow-free control treatment. Improvements in
weed control and possible soil benefits from higher densities of trees must be
weighed against the loss of crop production area, however.
Third, tree prunings applied to the soil surface can suppress weeds. Effects
are species-specific and rate dependent. In Ivory Coast, Budelman (1988)
found that at an application rate of 5 Mg ha 1 , leaf mulch of Leucaena leucoce-
phala had no effect on weed biomass,whereas Gliricidia sepium mulch had some
suppressive ability and Flemingia macrophylla mulch was strongly weed-sup-
pressive.Leaf mulch decomposition was most rapid for G.sepium ,slowest for F.
macrophylla , and intermediate for L. leucocephala . The lack of weed suppression
from L. leucocephala , in spite of its moderate decomposition rate, was attrib-
uted to its small leaflets, which separated from the rachis as soon as drying
began after pruning. Flemingia macrophylla had a larger leaflet size and an
intact, although curled, leaf after drying. Weed suppression by F. macrophylla
increased as the amount of mulch material increased. Eleven weeks after
mulching with F. macrophylla , weed biomass was reduced about 70% by a 3 Mg
ha 1 application, about 85% by a 6 Mg ha 1 application, and virtually elimi-
nated by a 9 Mg ha 1 application, compared to an unmulched control treat-
ment. Budelman (1988) suggested that mulches were most effective against
annual weed species, but also hindered the emergence of Cyperus rotundus
shoots.They would not be effective against root and stump suckers,which can
easily pass through a mulch layer.
Fourth, alley cropping can change weed community composition. Siaw,
Kang & Okali (1991) observed that grass species were common in control
plots, whereas grasses were uncommon in plots with Leucaena leucocephala and
Acioa barteri hedgerows. They also found that L. leucocephala seedlings were
increasingly common in hedgerow plots. Rippin et al . (1994) found that grass
weeds were greatly reduced in Erythrina poeppigiana hedgerow and mulch plots
and that the grass weed Rottboellia exaltata was absent from hedgerow plots.
Fifth, alley cropping must be evaluated in terms of its impacts not only on
weed biomass and community composition, but also on crop production.
Alley cropping is clearly better suited for some crops, soils, and climates than
others, though more research is still needed to identify optimum combina-
tions. Siaw, Kang & Okali (1991) reported that Leucaena leucocephala and Acioa