EFFECT OF ANNUAL MEDIC SMOTHER PLANTS ON WEED-CONTROL AND YIELD IN CORN

Using spring-seeded smother plants for weed control could reduce the e nvironmental impact of corn (Zea mays L.) production. Research was con ducted to determine whether currently available medic (Medicago spp.) cultivars are adapted for use as smother plants in corn. In field expe riments in 1992 at Becker and Rosemount, MN, Medicago scutellata (L.) Mill. cv. Sava and Kelson were interseeded with corn st 0, 85, 260, or 775 seeds m(-2). In 1993, Sava and Kelson, along with M. polymorpha L . cv. Santiago and M. lupulina L. cv. George,were interseeded with cor n at 260 seeds m(-2) and N fertilizer was applied at 0, 84 (56 at Rose mount), or 168 kg ha(-1). Land equivalent ratios for corn and medic in tercrops germ in 1992 were not > 1, indicating that corn and medics co mpeted strongly for resources. Medics seeded with corn at a rate high enough to consistently suppress weeds (260 seeds m(-2)) reduced weed d ry weight 14 wk after corn emergence by 69% at Becker and by 41% at Ro semount compared with monoculture corn. The same seeding rate reduced corn grain yield in weed-free plots by 21% at Becker and 15% at Rosemo unt compared with monoculture yields. In 1993, medic smother plants re duced weed dry weight more when grown in the 0 kg ha(-1) N plots than in the 168 kg ha(-1) N plots. Corn yield losses, however, were less se vere in the 168 kg ha(-1) N treatments than with 0 N. Annual medics ma naged as smother plants in corn effectively reduced weed biomass; howe ver, additional research is needed to identify medic genotypes and smo ther plant management systems that reduce corn yields less than those we evaluated, and that provide more consistent weed suppression across enviromnents.