Medic planting date effect on dry matter and nitrogen accumulation when clear-seeded or intercropped with corn

Intercropping annual forage legumes with row crops has been proposed as a strategy to control erosion, suppress weeds, and contribute biological N to companion or subsequent crops. This experiment evaluated the effect of planting date of two annual medic species [burr and snail medic, Medicago polymorpha L. and M. scutellata (L.) Mill.] on yield and N accumulation of both medic and interseeded corn (Zea mays L.). Medics were either clear-seeded (i.e., monocropped) or interseeded into corn at five planting dates from 7 May to 13 July in each of two years at East Lansing, MI, on a Capac loam (fine-loamy, mixed, mesic Aeric Endoaqualt). Clear-seeded medic produced up to 3 Mg ha^-1 of dry matter at 60 d after planting. In the interseeding system, medic dry matter yields were only 50% of yield in a clear-seeded system. Average aboveground medic N concentration was 50 and 20 kg ha^-1 in clear-seeded and interseeded systems, respectively. Interseeding at corn planting produced the greatest medic dry matter, but also was associated with the greatest reduction in corn yield. Corn yields were not reduced by interseeding medic 28 d after corn planting. Interseeding medic into corn was associated with a positive yield response (3-25%) of subsequent corn grain. Medics reduced fertilizer needs of corn in the subsequent year by 37 kg N ha^-1.