ALFALFA YIELD COMPONENT RESPONSES TO SEEDING RATE SEVERAL YEARS AFTER ESTABLISHMENT

Little information is available regarding the relationships of alfalfa (Medicago sativa L.) forage yield with yield components in seed-established stands grown under dryland conditions. Field experiments that included two cultivars and 15 seeding rates ranging from 2.2 to 33.6 kg pure live seed ha-1 were established in 1985 at Brookings and Highmore, SD. The objective was to determine seeding rate effects on components of alfalfa forage yield and on plant morphology several years after establishment. In the seeding year, plant density increased linearly with seeding rate at both locations. Yield components were determined in 1988 and 1989. Mean alfalfa yield during 1988 and 1989 was optimum at the 13.4 kg pure live seed ha-1 rate at Brookings, but did not respond to seeding rate at Highmore. Plant density continued to be affected by seeding rate 4 yr after establishment, but greater mortality at high seeding rates caused the response to deviate from linearity. Shoots per plant were negatively correlated with plant density (r = -0.73, P = 0.01). Shoot weight did not respond to seeding rate at Brookings but declined linearly with seeding rate at Highmore. Nevertheless, path analysis suggested that shoot weight was the major influence on alfalfa yield at both locations. Leaf-to-stem ratio and shoot length were not affected by seeding rate. Alfalfa yield components are affected similarly by plant density in seed-established and transplanted stands. Although yield may not be affected by plant density, shoot weight may be affected because of differences in plant competition.