In agricultural production systems, a variety of environmental conditions may be manipulated to sustain beneficial production. Nutrients and soil organic matter should be replenished; erosion prevented; beneficial physical properties of the soil maintained or improved; and pests, pathogens, and weeds managed or suppressed. Cover crops can contribute to the improvement of future agricultural production by providing some or all of these benefits and are particularly valuable for low-input and sustainable agrosystems (Bowman et al., 1998). In addition to protecting land from erosion, increasing soil organic matter and cation exchange capacity, improving moisture-holding characteristics, suppressing pests, pathogens, and weeds, reducing soil compaction, and serving as food, fiber, and forage, many legume species also have the potential to fix large amounts of nitrogen (Cherr et al., 2006; Creamer and Baldwin, 2000; Dabney et al., 2001).
Cover crops can suppress weeds in cropping systems by competing with weeds for available resources and by promoting conditions that are unfavorable for weed germination and establishment (Teasdale, 1998). The latter mechanism includes allelopathy, which is the inhibitory or stimulatory effect of a plant on another species as a result of the release of chemicals into the environment (Putnam and Tang, 1986). Allelopathy of crop species is currently underused for weed management in agricultural systems. However, its role can be expanded through the use of allelopathic cultivars of cash crops (Wu et al., 1999) and by preceding cash crops with cover crops that exude allelochemicals or produce residues that decompose to release allelochemicals that are phytotoxic to weeds (Batish et al., 2006).
Recently, there has been increased interest in using leguminous cover crops in sustainable and organic cropping systems in Florida (Abdul-Baki et al., 2005; Collins, 2004; Scholberg et al., 2006). Species such as cowpea, sunn hemp, and velvetbean can be used during summer fallow periods to suppress weeds through resource competition (Collins, 2004); however, it is likely that weed suppression by these cover crops may also be in part the result of allelopathy.
Allelopathy in velvetbean has been attributed to L-3-[3,4-dihydroxyphenylalanine (L-DOPA)] (Fujii et al., 1991), which is exuded from leaves and roots (Fujii, 1999; Nishihara et al., 2005). It is estimated that velvetbean can contribute 200 to 300 kg·ha−1 of L-DOPA to the soil each year (Fujii et al., 1991). Aqueous extracts of velvetbean have been shown to inhibit radicle elongation in germinating seeds of Amaranthus hypochondriacus L., barnyardgrass [Echinochloa crusgalli (L.) P. Beauv.], tomato, and cabbage (Brassica oleracea var capitata L.) (Caamal-Maldonado et al., 2001; Gleissman, 1983). In addition, the growth of corn (Zea mays L.), beans (Phaseolus vulgaris L.), and cabbage also was inhibited by aqueous extracts of fresh velvetbean leaves (Gleissman, 1983). L-DOPA suppression of germination has been shown to be selective. Inhibition of radicle growth was most pronounced in Cerastium glomeratum Thuill, Lactuca sativa L., Linum usitatissimum L., and Spergula arvensis L., whereas species from the Poaceae and Fabaceae families were the least affected (Fujii et al., 1991).
Sunn hemp is a fast-growing, 1- to 3-m tall, leguminous cover crop that generates a large quantity of biomass and a considerable amount of nitrogen (Akanvou et al., 2001; Mansoer et al., 1997). A previously unknown, nonprotein amino acid isolated from the seeds of sunn hemp and other Crotalaria species (Pant and Fales, 1974) can be considered to be a candidate allelochemical of sunn hemp. It was identified as delta-hydroxynorleucine (5-hydroxy-2-aminohexanoic acid) (Pilbeam and Bell, 1979) and demonstrated to be phytotoxic to lettuce (Wilson and Bell, 1979). Extracts of sunn hemp seeds were also reported to have reduced growth in 12 weeds tested, generally causing greater reduction in growth with greater concentration (Cole, 1991). Aqueous extracts of sunn hemp leaves applied to wheat plants decreased root length but did not affect plant height, number of leaves, or shoot and root dry weights (Ohdan et al., 1995).
Cowpea has been shown to suppress weeds when used as a cover crop, intercrop, or organic mulch (Hutchinson and McGiffen, 2000; Ngouajio et al., 2003; Unamma et al., 1986). Aqueous cowpea extracts were recently reported to reduce radicle growth in crop and weed seeds (Hill et al., 2006). However, no putative allelochemicals have as yet been reported for cowpea. The present study was intended to complement cover crop/weed competition studies with cowpea, sunn hemp, and velvetbean (Collins, 2004). The objective was to compare the allelopathic potential of cowpea, sunn hemp, and velvetbean leaf extracts and dried ground residue on germination and growth of weed and crop species.
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