Sunn hemp (Crotalaria juncea L.) is a tropical legume that produces plant biomass and nitrogen (N) quickly. Our objectives were to assess the growth of a new sunn hemp cultivar breed to produce seed in a temperate climate and determine the residual N effect on a rye (Secale cereale L.) cover crop in east-central Alabama from 2007 to 2009. Plant populations, plant height, stem diameter, biomass production, and N content were determined for two sunn hemp planting dates, following corn (Zea mays L.) and wheat (Triticum aestivum L.) harvest, across different seeding rates (17, 34, 50, and 67？kg/ha). Rye biomass was measured the following spring. Sunn hemp biomass production was inconsistent across planting dates, but did relate to growing degree accumulation. Nitrogen concentrations were inversely related to biomass production, and subsequent N contents corresponded to biomass levels. Neither planting date nor seeding rate affected rye biomass production, but rye biomass averaged over both planting dates following wheat/sunn hemp averaged 43% and 33% greater than rye following fallow. Rye biomass following corn/sunn hemp was equivalent to fallow plots. Early planting dates are recommended for sunn hemp with seeding rates between 17 and 34？kg/ha to maximize biomass and N production. 1. Introduction Nitrogen (N) fertilizer costs are volatile and alternative N sources (such as legumes) are receiving renewed interest. In the southeastern United States, winter annual legumes are commonly incorporated into crop rotations and can contribute considerable biomass and N during the traditional winter fallow period. Legume biomass production is a major factor determining legume N contribution [1]. Nitrogen accumulation is highly variable and dependent upon environment, legume selection, planting date, termination, growth stage, and management strategy [1–3]. Throughout the Southeast, warm-season legumes, such as cowpea (Vigna unguiculata L.), sericea lespedeza (Lespedeza cuneata L.), and soybean (Glycine max L.) are currently available to producers for use as warm season cover crops, but they cannot produce substantial biomass or N in a short period of time. Sunn hemp, a nontraditional tropical legume, can also be used as a warm season cover crop across the Southeast. “Tropic Sun”, the most extensively studied cultivar, has been shown to produce 5.9？Mg/ha biomass and contribute 134–145？kg/ha N in a 9–12-week period [4]. This rapid biomass production enables sunn hemp to serve as a summer cover between warm-season harvest and cool-season planting that could also reduce
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