Crop diversification for weed management - Ecological Management of Agricultural Weeds - page 358

barteri hedgerows increased cowpea yield by 20% to 57% and maize yield by
11% to 132% compared to plots without hedgerows. Jama, Getahun & Ngugi
(1991) also obtained higher maize yields from mixtures with L. leucocephala
than from maize sole crops.Budelman (1990 b ) reported yam yields were 135%
higher from yam/ Gliricidia sepium mixtures than from yam sole crops. Salazar,
Szott & Palm (1993) found, however, that hedgerows of Inga edulis , L. leucoce-
phala ,and Erythrina spp.spaced 4 m apart reduced yield of rice growing within
1.5 m of the tree bases. Lal (1989) found that maize and cowpea yields were
lower in associations with L. leucocephala and G. sepium at 2-m and 4-m spacing
than in a no-tillage treatment without hedgerows, especially in seasons with
below average rainfall. He attributed the negative effect of the hedgerows on
maize and cowpea to competition for soil moisture.
Using herbaceous vegetation to manage weeds in tree crops
In systems where trees function as main crops,herbaceous species can
be planted or allowed to volunteer from the soil seed bank to produce an
understory layer that reduces soil erosion and compaction, adds organic
matter, improves soil fertility, limits damage by insect and mite pests, and
suppresses weeds (Altieri & Schmidt, 1985; Hogue & Neilsen, 1987; Bugg et
al ., 1991; Bugg & Waddington, 1994; Prokopy, 1994).
In an early review of cover cropping practices for closed-canopy tropical
plantation crops, such as coconut, rubber, and oil palm, Sampson (1928) noted
that various legume species were useful for suppressing Imperata cylindrica and
other weeds,while young trees were being established.Research focused on the
use of two legume cover crops, Pueraria phaseoloides and Desmodium ovalifolium ,in
Honduran oil palm plantations indicated that they greatly reduced labor
requirements for weed suppression (Centro Internacional de Información
Sobre Cultivos de Cobertura, 1994). Obiefuna (1989) found that intercropping
plantain with egusi melon,a fast-spreading vine,suppressed weed biomass pro-
duction, reduced weeding requirements for young plantains, and increased
plantain yield as much as 27% above the level obtained from a weed-free treat-
ment without egusi melon. In addition to weed suppression, Obiefuna (1989)
noted that intercropped egusi melon enhanced soil microbial activity and
nutrient supply, moderated soil temperature, and increased soil moisture
content, all of which could improve plantain performance.
For many tree crops, the canopy never closes, even when each individual
plant is at mature size and in full production. A mosiac of light conditions
exists on the ground,with shade patches beneath trees interspersed with light
gaps between tree rows and sometimes within rows (Vandermeer, 1989, pp.
106-26). Capture of nutrients and water by trees is greatest beneath their