Alfalfa autotoxicity: Effects of reseeding delay, original stand age, and cultivar

Autotoxicity may reduce plant population and productivity of alfalfa (Medicago sativa L.) reseeded into winterkilled alfalfa stands; however, the interaction of important variables such as reseeding delay, stand age, and cultivar with autotoxicity has not been evaluated. We determined the effects of stand age (1-, 2-, or 3-yr-old alfalfa stands), reseeding delay (none or 2-wk delay) after plowing the original stands in May, and cultivar reseeded ('5262' or 'Wrangler') on populations and productivity of reseeded alfalfa. Oat (Avena sativa L.), corn (Zea mays L.), or tall rescue (Festuca arundinaceae Schreb.) served as controls. There was no consistent evidence for autotoxicity in the reseeding year, as populations and forage yields were similar when seeding followed alfalfa or the control crops, regardless of the age of the previous alfalfa stand or the alfalfa cultivar reseeded. Delaying seeding had inconsistent effects on alfalfa plant population but consistently reduced yields in the seeding year, with yields averaging 4.9 and 6.3 Mg ha^-1 with and without a 2-wk reseeding delay, respectively. In three of six experiments, yields at the first harvest in the year following reseeding averaged 15% lower following alfalfa than a control crop (3.6 and 4.2 Mg ha^-1, respectively), suggesting that alfalfa autotoxic response may be delayed. The lack of consistent evidence of autotoxicity, coupled with yield reductions in the seeding year associated with delayed seeding, suggest that the recommendation to delay alfalfa reseeding 2 wk following plowing of winterkilled stands may be unjustified.