Crop diversification for weed management - Ecological Management of Agricultural Weeds - page 356

While offering the possibility of shorter, but more effective fallow periods,
the I. edulis/D. ovalifolium combination also demonstrated potential problems:
more labor was needed for establishment of the legumes, D.ovalifolium tended
to become a weed in later crop production periods, and growth of native tree
and shrub species was suppressed (Staver, 1989 b ). Farmer-scientist collabora-
tions could be highly productive for addressing these types of problems and
developing management strategies for native and introduced plants that
would provide the benefits of accelerated succession, while minimizing labor
requirements and negative effects on crops.
Alley cropping and weed dynamics
As arable land has become scarce and the length of forest fallow
periods has declined, simultaneous associations of trees and annual crops
have been investigated as alternatives to shifting cultivation systems. Alley
cropping (or “hedgerow intercropping”) is a management-intensive agrofo-
restry practice in which rows of trees or shrubs, usually rapidly growing
legumes, are interspersed with annual crops and pruned before and during
crop production. Prunings can be applied to the field as mulch and organic
fertilizer, fed to livestock, or used as firewood. Farmers have been slow to
adopt alley cropping, except for soil conservation purposes on sloped lands
(Thurston, 1997), but experimental studies provide insights into how man-
agement practices and environmental conditions affect relationships among
weeds, crops, and trees. Five points that emerge from alley cropping studies
merit attention here.
First, during fallow periods preceding annual crop production, hedgerow
trees and shrubs can suppress weeds through shading and other forms of com-
petition. At the end of a four-month fallow preceding taro production in
Western Samoa, Rosecrance, Rogers & Tofinga (1992) found photosyntheti-
cally active radiation (PAR) levels between Calliandra calothyrsus and Gliricidia
sepium hedgerows were only 2% to 10% of values measured in control plots
without the trees. In southwestern Nigeria, Anoka, Akobundu & Okonkwo
(1991) observed that rhizome biomass of Imperata cylindirica declined by 96%
three years after planting G. sepium hedgerows, and by 90% after planting
Leucaena leucocephala , whereas it increased in a hedgerow-free control treat-
ment. Death of I. cylindrica rhizomes was attributed to the nearly complete
canopy cover produced by the trees during the rainy season.Yamoah,Agboola
& Mulongoy (1986) compared different legumes for use as hedgerows in alley
cropping systems and found that Cassia siamea intercepted more PARand sup-
pressed weed biomass production more effectively than Flemingia congesta or G.
sepium. Greater PAR interception and weed suppression by C. siamea were
attributed to its planophile leaves and extensive side branches.