Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 233

crop was established.One of the few examples of such a system involves use of
certain cultivars of subterranean clover in a rather limited area of the eastern
USA (Chapter 7) (Enache & Ilnicki, 1990; Ilnicki & Enache, 1992). Breeding
efforts might increase the number of early maturing cover crops that could be
used for mulch systems in other regions.
In warm regions with enough moisture to support the production of two
or more crops per year, more options exist for the use of cover crops before,
during, and after the production of main crops (Pieters, 1927, pp. 283-311;
Miller, Graves & Williams, 1989; Reijntjes, Haverkort & Waters-Bayer, 1992,
pp. 168-73). The cover species may be killed or suppressed by tillage, herbi-
cides, or slashing with hand tools.
Slash/mulch ( tapado ) systems are used in many tropical countries to
produce bean, maize, taro, and other staple foods in residues of forest, scrub,
or weedy fallow vegetation (Thurston, 1997). The mulches used in such
systems apparently suppress weeds considerably (Moreno & Sánchez, 1994;
Thurston, 1997, pp. 26, 45-8), but these effects have not yet been studied
systematically. Efforts by both farmers and researchers are under way to
improve slash/mulch systems by introducing legume cover crops into fallow
vegetation (Buckles et al ., 1994; Madrigal, 1994; Moreno & Sánchez, 1994; see
also Chapters 3 and 7).
Allelopathy
Many plant species produce and release chemicals that are toxic to
other plants, a phenomenon referred to as allelopathy. Allelochemicals may
also be produced by microbes that transform plant products during residue
decomposition. Living crops can have direct allelopathic effects on weeds (see
Chapter 6), and live and decomposing weeds can reduce crop performance
(Bhowmik & Doll, 1982, 1984; Putnam & Weston, 1986), but the most impor-
tant application of allelopathy involves the use of crop residue to suppress
weed germination, establishment, and growth.
Studies of allelopathic effects of crop residue on weed and crop species typ-
ically comprise a description of the symptoms and injuries present in target
plants exposed to residue or extracts from it, isolation of the putative causal
agent(s), and application of the isolated agent(s) to healthy plants to deter-
mine whether similar damaging effects can be reproduced. Using chemical
isolation and bioassay techniques in laboratory and glasshouse experiments,a
number of classes of chemicals have been identified as allelopathic agents.
Those found frequently include alkaloids, coumarins, cyanogenic glucosides,
flavonoids, phenolic acids, polyacetylenes, quinones, and terpenoids
(Einhellig & Leather, 1988; Worsham, 1989; Rice, 1995).