Crop diversification for weed management - Ecological Management of Agricultural Weeds - page 348

with or without tillage. Late-season weed biomass in the maize/subterranean
clover intercrop managed without herbicides or tillage was 54% to 90% lower
than in sole-cropped maize managed with herbicides and conventional
tillage, and 70% to 96% lower than in sole-cropped maize with herbicides but
without tillage. Ilnicki & Enache (1992) attributed weed suppression by sub-
terranean clover to the physical barrier created by the dead mulch layer and
suggested that the mulch could have allelopathic effects. Competition by
clover plants that germinated during the summer also may have been impor-
tant for late-season weed suppression.
Subterranean clover is well suited for use as a living mulch with soybean,
squash, cabbage, snap bean, and tomato in New Jersey (Ilnicki & Enache,
1992), but it is poorly adapted for many other environments because of insuf-
ficient winter hardiness, more synchronous and competitive growth with
main crops, and other factors. The concept of temporal complementarity
between living mulches and main crops is nonetheless intriguing and should
be explored further.
Although smother crops planted at the same time as main crops can be
used to suppress weeds, they can also reduce main crop yields unacceptably
(De Haan et al ., 1994; De Haan, Scheaffer & Barnes, 1997). However, certain
species may provide weed control without a loss of main crop yield. This may
be possible if (i) resource depletion by minor crops occurs at times when it has
no effect on main crops; (ii) the minor crops avoid competition for N because
they are legumes, or avoid competition for light and water because they are
shorter than main crops (shading by the main crop decreases transpiration by
the minor crop); and (iii) the rapid early growth or physiological shade toler-
ance of minor crops allows them to produce an understory canopy that sup-
presses weed germination and establishment.
Examples of smother crops that can be planted simultaneously with main
crops to suppress weed growth while maintaining crop yields can be drawn
from both tropical and temperate regions. In Andhra Pradesh, India, Shetty &
Rao (1981) found that adding smother crops of cowpea or mungbean to main
crops of sorghum or pigeonpea reduced early-season weed growth and
allowed a reduction of hand-weeding from twice to once without a reduction
in main crop yield.In Maine,USA,intercropping red clover with spring barley
had no effect on barley yield but reduced weed biomass at the time of barley
harvest and up to nine months thereafter (Figure 7.8) (M. Liebman, unpub-
lished data). Similarly, in England, intercropping red clover with barley
reduced growth of the perennial grass weed Agropyron (Elytrigia) repens from
both seeds and rhizome fragments (Williams, 1972; Dyke & Barnard, 1976).
Red clover has a low light compensation point of about 140 mol s 1 m 2