Alfalfa (Medicago sativa L. ‘Vernal′) seeded in April in rows 55 cm apart and kept free of weeds produced 820 kg/ha of seed during the year of seeding. Competition from a dense popualtion (40 plants/m of row) of mixed species of annual weeds reduced the seed yield to 45 kg/ha. Competition from a dense population (55 plants/m of row) of broadleaf weeds, a light population (4 plants/m of row) of broadleaf weeds, or a heavy population (75 culms/m of row) of barnyardgrass [Echinochloa crusgalli (L.) Beauv.] reduced yields to 80, 310, and 160 kg/ha, respectively. A weed management system, consisting of EPTC (S-ethyl dipropylthiocarbamate) or profluralin [N-(cyclopropylmethyl)-α,α,α-trifluoro-2,6-dinitro-N-propyl-p-toluidine] applied at or before planting, 2,4-DB [4-(2,4-dichlorophenoxy) butyric acid] applied postemergence, trifluralin (α,α,α- trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) incorporated with the soil when the alfalfa was 20 cm tall, interrow tillage, and a modest input of hand labor brought the crop to harvest free of weeds, and the yields of alfalfa seed were similar to those from plots kept weed-free by hand labor only. When weeds within the rows were controlled by hand labor only, labor inputs as great as 930 h/ha were required to bring the crop to harvest free of weeds, whereas labor inputs of 7 to 17 h/ha removed all surviving weeds when effective weed management systems had been applied.