Annual legumes as green manure and forage crops in winter canola (<i>Brassica</i> <i>napus</i> L.) rotations

Cropping systems that reduce chemical fertilizer N requirements in winter canola (Brassica napus L.) need further study. Studies were conducted in 1994/1995 and 1995/1996 at two locations in Michigan i) to quantify the N accumulated at plowdown by berseem clover (Trifolium alexandrinum L.), alfalfa (Medicago sativa L.) and three annual medic species (Medicago truncatula Gaertn., M. polymorpha L., and M. scutellata L.) when they were managed as green manure or managed as forage; ii) to compare N response of the winter canola crop following the legumes; and iii) to estimate the N fertilizer replacement 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 planting (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 treatment resulted in the highest winter canola yield in 1995, but treatment differences were not significant in 1996. A supplementary effect of some treatments on winter canola yield was observed in 1995. Fertilizer N had a similar effect on winter canola yield across all treatments, and resulted in a quadratic response in 1995 and a linear response in 1996. Winter canola grain yield following annual legumes managed as green manure or as forage were similar. Winter canola yield following berseem clover was higher than when following annual medics. No FRV of legumes on winter canola was observed. Key words: Annual medics, berseem clover, winter canola, Medicago spp., Trifolium alexandrinum L., Brassica napus L., green manure, crop rotation