Rotation Interval and Pesticide Effects on Establishment of Alfalfa after Alfalfa

Published intervals for avoiding autotoxicity when reseeding alfalfa (Medicago sativa L.) after alfalfa range from 2 wk to more than 1 yr. Intervals between killing old alfalfa stands with herbicides and reseeding alfalfa were compared in field experiments established in 1992 (Locations 1 and 2) and 1993 (Location 3) near West Plains, MO. Soils were Poynor cherty silt loam (loamy‐skeletal over clayey, siliceous, mesic Typic Paleudult), Ashton silt loam (fine‐silty, mixed, mesic Mollic Hapludalf), and Waben very cherty silt loam (loamy‐skeletal, siliceous, mesic Ultic Hapludalf), respectively. Main plots were 18‐, 12‐, 6‐, 0.75‐, and 0.5‐mo rotation intervals before a common May planting date. Subplots were insecticide and fungicide treatments at planting. Insecticide or fungicide treatment did not affect plant density or yield during the establishment year or two subsequent years. Among years, plant densities for the 0.75‐ and 0.5‐mo rotation intervals ranged between 12 and 19.2% lower (P < 0.05) than that of the 18‐mo control interval, and yields for the 6‐ and 0.5‐mo intervals ranged between 6.3 and 10.3% lower than that of the control (P < 0.05). Although plant density of the 0.75‐mo rotation interval was lower, yield was similar to the 18‐mo control due to incomplete kill of old plants. Plant density and dry matter yield remained ranked according to rotation interval from the seeding year through the third year, suggesting an autoconditioning effect during establishment that was retained at the population level. A rotation interval of at least 12 mo is suggested before reseeding alfalfa after alfalfa in the lower Midwest.