Suitability of sunn hemp as an alternative late-summer legume cover crop

The tropical legume 'Tropic Sun' sunn hemp (Crotalaria juncea L.) may have potential as an alternative legume cover crop or as forage for cattle in southern temperate regions. This study determined dry-matter production, chemical composition, and N release from sunn hemp residue under conventional and no-tillage systems as might be used in corn (Zea mays L.) production. Sum hemp was sown in mid-August and mowed in early December on a Norfolk sandy loam (fine-loamy, siliceous, thermic Typic Kandiudult) and a Lucedale fine sandy loam (fine-loamy, siliceous, thermic Rhodic Paleudult) in Alabama (1991-1992). Mesh bags were used to determine residue decomposition and N release. Average dry-matter production was 5.9 Mg ha(-1) 9 to 12 wk after planting. At mowing, residue N content averaged 126 kg ha(-1). Residue overwintered on the soil surface until early April. During the first 4 wk following mowing, N release from residue was 50%. In April, N remaining in overwintered residue was only 38% of that after mowing in December (45 kg N ha(-1)). Nitrogen release from residue during the subsequent corn growing season was 13% in no-tillage and 43% in conventional tillage. Sunn hemp produced sufficient dry matter to cover and protect the soil from erosion and provided sufficient N to benefit a succeeding summer crop. In addition, forage quality of leaves was suitable to provide late summer and fall grazing. Sunn hemp has potential to be managed as an alternative to winter legume cover crops in warm temperate regions.