Synopsis Plant size and yields of both alfalfa and birdsfoot trefoil were influenced by weed control treatments and seeding rates during the season of establishment. Pre‐emergence control of weeds increased first year yields of both legumes, had no effect on plant numbers but increased size of root‐crowns. Plants were larger at low seeding rates, particularly where weeds were controlled, and initial yields were comparable to those at higher seeding rates. Second year yields were influenced more by plant numbers than size of root‐crowns.