Annual Medicago as a smother crop in soybean

Use of conservation tillage and narrow row spacing in soybean [Glycine max (L.) Merr.] production has led to increased use of herbicides for weed control. Some producers are seeking alternative weed control methods, such as smother crops, that would reduce dependence on chemical weed control. A successful smother crop must compete strongly with weeds but minimally with the crop. In four environments, we intercropped three annual Medicago spp. (medics) with soybean to test their utility as a smother crop for weed control. Annual medics were intercropped with soybean at rates of 0, 85, 258, or 775 seeds m^-2, and the intercrops were grown with and without weed control. Increasing medic seeding rate decreased weed yields but also reduced soybean herbage and grain yields. For the weed-controlled treatment, average soybean grain yields declined 7 kg ha^-1 for every 10 seeds m^-2 increase in medic seeding rate. Soybean grain yield was lower when grown with Medicago scutellata L. cv. Sava than when grown with Medicago polymorpha L. cv. Santiago or Medicago lupilina L. cv. George. Soybean grain yield was negatively related (r = -81) to medic herbage production. In the autumn following soybean harvest, medic residue ranged from 200 to 3700 kg ha^-1 depending on the location and seeding rate. Medics provided residue for soil protection, suppressed weeds, but also reduced soybean yields.