Biotic Factors - Agroecology: The Ecology of Sustainable Food Systems - page 159

FIGURE 11.5 Cacao pod hulls used as allelopathic mulch, Tabasco, Mexico. The dark cacao hulls, seen between rows of zucchini,
suppress weed growth.

Crop Inhibition of Weeds

degrade relatively quickly, as a rule. However, breakdown
rates do differ. For this reason, the timing of mulch appli-
cation as well as the amount and age of the mulch must
be carefully considered so as to maximize weed inhibition
and limit the effect on crops.
An excellent example of allelopathically active
mulch is dried and crushed cacao pods, obtained in the
cocoa production process after the seeds and pulp have
been removed from the pods. Spread over the surface of
the soil or between established crop plants, the crushed
pods leach tannic substances that can inhibit the germi-
nation and establishment of weeds (Figure 11.5). Labo-
ratory bioassays of water extracts of the pod material
show considerable allelopathic potential. Other types of
crop and processing residue with allelopathic potential
include coffee chaff from the dried beans, almond hulls,
rice hulls, apple pomace, and grape skins and seeds.
Walnut hulls were one of the plant parts initially stud-
ied in detail for allelopathic potential, since it was noted
long ago that very few other plants (especially weeds)
would grow under walnut trees where the outer covering
over the walnut nut fell during fruit maturation.

When a crop plant itself is able to inhibit weeds through
allelopathy, farmers have a very important tool to add
to their toolbox. Several crops are known to be effective
in suppressing weeds that grow near them (Batish et al.,
2001). The list includes beets ( Beta vulgaris ), lupine
( Lupinus sp.), corn, wheat, oats, peas, buckwheat
( Fagopyrum esculentum ), millet ( Panicum sp.), barley,
rye, and cucumber ( Cucumis sativa ). Allelopathy can
be implicated in all cases, but research needs to thor-
oughly determine the role phytotoxins play in relation
to other forms of interference. In some cases, the inhi-
bition appears to occur from substances released by the
living crop plants, but in others it appears that the effect
is left over from decomposition products of crop resi-
dues incorporated into the soil at the end of the crop
cycle. Care has to be taken to keep these inhibitory
effects on weeds from affecting the crops that follow.
Mixtures of these crops might express even greater
allelopathic activity through complementary combining
of phytotoxins.