Annual legumes as green manure and forage crops in winter canola (Brassicanapus L.) rotations

Cropping systems that reduce chemical fertilizer N requirements in winter c anola (Brassica napus L.) need further study. Studies were conducted in 199 4/1995 and 1995/1996 at two locations in Michigan i) to quantify the N accu mulated at plowdown by berseem clover (Trifolium alexandrinum L.), alfalfa (Medicago sativa L.) and three annual medic species (Medicago truncatula Ga ertn., M. polymorpha L., and M. scutellata L.) when they were managed as gr een manure or managed as forage; ii) to compare N response of the winter ca nola crop following the legumes; and iii) to estimate the N fertilizer repl acement value (FRV) of the legumes. The legumes were seeded in early May in both years. Winter canola was planted after legume plowdown 90 d after pla nting (DAP). Four N rates (0, 50, 100, and 150 kg N ha(-1)) were applied to the winter canola in early spring. Herbage N yield at plowdown was highest (103.9 kg N ha(-1)) in berseem clover managed as green manure and this tre atment resulted in the highest winter canola yield in 1995, but treatment d ifferences were not significant in 1996. A supplementary effect of some tre atments on winter canola yield was observed in 1995. Fertilizer N had a sim ilar effect on winter canola yield across all treatments, and resulted in a quadratic response in 1995 and a linear response in 1996. Winter canola gr ain yield following annual legumes managed as green manure or as forage wer e similar. Winter canola yield following berseem clover was higher than whe n following annual medics. No FRV of legumes on winter canola was observed.