Weed management is a major cost and constraint in both organic and conventional cropping systems (Walz, 1999). Although critical during the growing season, weed management during fallow periods is also important to avoid weed infestations in following cash crops. In Florida, vegetable crops are grown during the fall, winter, and spring, but during the hot, rainy summer months, the land is often left fallow (Wang et al., 2003). The type of fallow is specific to each grower depending on costs, management, and resources of each individual. The integration of a cover crop fallow into a cropping system as an alternative to a weedy fallow can incur additional production costs but can also provide beneficial agroecosystem services. In addition to suppressing weed populations and limiting or preventing additions to the weed seed bank, a cover crop can provide soil stability, improve nutrient cycling, reduce leaching and runoff of nutrients and pesticides, and provide nutrients such as nitrogen, thus serving as a green manure.
Cover crop species are most effective in smothering and suppressing weed populations when they emerge and establish rapidly and produce a large amount of shoot biomass (Teasdale, 1998). Cover crops reduce weed pressure through physical interference with weed seed germination and seedling growth through alteration of light quantity, light quality, temperature, soil moisture content, and nutrient availability (Teasdale and Mohler, 1993). Some cover crops adversely affect weed seed germination and growth through chemical interference (allelopathy). Various types and amounts of allelochemicals are produced and can be released from the plant through leaching, volatilization, root exudation, death, and decay of plant parts, aided by various biotic and abiotic factors (Rice, 1984). Scientists agree that allelopathy is an aspect of plant interference; however, there is a lack of consensus on the importance of allelopathy in weed suppression and information on the modes of action (Marcias et al., 2007; Putnam and Tang, 1986).
Sunn hemp has rapid stand establishment and shoot biomass accumulation and thus is a suitable cover crop for weed suppression. A leguminous cover crop, sunn hemp obtains nitrogen through biological fixation, and the cultivar Tropic Sun has been shown to suppress root-knot nematodes (Rotar and Joy, 1983). As a tool for weed suppression, sunn hemp is low maintenance and requires no attention after planting until the time of harvest (White and Haun, 1965). Sangakkara et al. (2006) investigated crops, including sunn hemp, which could be used in place of weedy fallows in tropical farming systems. Sunn hemp was found to have the largest above-ground biomass and, therefore, the greatest weed suppression in comparison with other crops (Tithonia diversifolia, Phaseolus beans, and a natural fallow), suppressing up to 82% of the weeds. They also showed that a cash crop of mungbean (Vigna radiata L.) planted after the sunn hemp fallow had the lowest weed populations, indicating that a grower might see a continued benefit of reduced weed populations in cash crops that follow a sunn hemp cover crop.
Collins et al. (2007) suggested that branching in an erect cover crop, including sunn hemp, could increase leaf area of the cover crop and thereby increase the competitive nature of the cover crop with weed species. Additionally, Collins et al. (2007) suggested that cover crop suppression of weeds can be enhanced by choosing cover crop cultivars with growth habits that reduce light penetration to the soil surface and reduce weed seed germination. Branching in sunn hemp depends on the planting density and apical dominance of the plant. Planting density can influence the height and formation of primary and secondary branches (Rotar and Joy, 1983) and low planting densities can increase the number of branches (Abdul-Baki et al., 2001).
The flowers of sunn hemp appear in racemes on the terminal 15 to 20 cm of secondary branches, and an increase in branches could also mean an increase in flowering (Abdul-Baki et al., 2001). Nanda (1962) noted the importance of apical dominance on flower bud formation and that terminal flower budbreak always preceded the breaking of lateral flower buds.
Abdul-Baki et al. (2001) reported that cutting the main stem at 90 cm to break apical dominance reduced plant height at maturity, but increased stem diameter, the number of branches, and flowers per plant. The results in that study indicated that the most responsive morphological changes from cutting the main stem were the increases in plant biomass and numbers of primary and secondary branches compared with uncut plants. Most farmers will not have equipment that can remove main stems of 90-cm-tall sunn hemp. However, mowers can be used for this purpose earlier in the season.
The objectives of this study were to determine the effect of planting densities and early removal of apical dominance of sunn hemp on weed suppression, branch formation, and seed production. The hypothesis was that seeding rate and cutting to remove apical dominance would impact plant morphology, weed suppression, and seed production of sunn hemp.
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