| Autotoxicity of alfalfa (Medicago sativa L.) is a form of allelopathy in which established alfalfa plants produce a chemical(s) that inhibits establishment and yield of new alfalfa reseeded too soon following alfalfa. Effects of rotation intervals, soil texture, and the zone of influence of old alfalfa plants were evaluated to develop appropriate management strategies for reseeding old alfalfa stands. Experiments were conducted at three locations for rotation interval and zone of influence studies. For the rotation interval experiment, old alfalfa was killed with herbicides at different time intervals before a common spring planting date. Plant density of alfalfa at intervals of 12-mo, 6-mo, 0.75-mo, and 0.5-mo after killing old alfalfa with herbicides was 5.9, 2.2, 13.3, and 19.2% lower, respectively, than that for the control rotation interval of 18 mo. Dry matter yield was 0.5, 6.5, 2.9, and 7.8% lower, respectively, than the 18-mo control. Relative rankings of rotation intervals for both plant density and dry matter yield were consistent for 3 yrs after seeding. Granular chlorpyrifos insecticide treatment at planting did not significantly affect plant density or yield, but seed treatment with metalaxyl fungicide improved plant density by 5% over all five rotation intervals. Establishment and yield of new alfalfa within a 20-cm radius of an old alfalfa plant were reduced by 26% and 56%, respectively, compared to the control distance of 80-100 cm from an old plant, but yield of alfalfa seedlings between 40 and 80 cm away from the old plants was increased by 12% to 15% compared with the control. Autotoxic chemical(s) in water extracts from alfalfa herbage required 50% more water to move through 10-cm soil columns of Carlow silty clay loam than through Sarpy fine sandy loam. Results suggest that a 12-mo rotation interval is adequate for reseeding alfalfa with minimal autotoxicity; establishment and yield of new alfalfa would be inhibited at old plant densities as low as 8 plants m{dollar}sp{lcub}-2{rcub}{dollar} due to the autotoxic zone of influence; and alfalfa may be reseeded after shorter rotation intervals on sandy soils than on heavy textured soils due to differential leaching of the autotoxic chemical(s) by rainfall or irrigation. |
