SEEDING-YEAR CUTTING AFFECTS WINTER SURVIVAL AND ITS ASSOCIATION WITH FALL GROWTH SCORE IN ALFALFA

Evaluation of alfalfa (Medicago sativa L.) cultivar persistence normally requires at least 3 yr of testing in stressful environments. Fall growth score sometimes has been used to predict winter survival in the absence of long-term tests. Our objectives were to evaluate the effect of seeding-year cutting stress on alfalfa cultivar winter injury and plant survival in several Minnesota environments (locations), and to associate winter injury with cultivar fall growth score. Seeding-year cutting schedules consisted of harvesting at intervals of 24, 30, 35, and 45 d at each of five site-years. The locations were Lamberton in southwest, Waseca in southeast, Morris in west central, and Rosemount (2 yr) in east central Minnesota. Average alfalfa stands across cutting schedules were 93, 72, 46, and 55% in the year following seeding at Lamberton, Waseca, Morris, and Rosemount, respectively. No differences in winter injury due to cutting schedules occurred at Lamberton because of sufficient winter snow cover. At the other locations, cultivar differences in winter injury were least for the 45-d schedule. The 24-, 30-, or 35-d cutting schedules produced severe winter injury that differentiated cultivars at Morris, Waseca, and Rosemount. The most fall-dormant cultivar, Rambler, generally had the least winter injury, whereas nondormant 'Nitro' had the most winter injury. Fall growth score was correlated with winter survival in stressful environments and explained almost-equal-to 45% of the variation in winter injury, spring stands, or yields for the 24-d cutting schedule treatment. Winter survival of alfalfa for the 35- and 45-d cutting schedules in the most stressful environment, Morris, explained 42% of the variation in long-term cultivar winter survival, while fall growth score explained 51%. An index combining seeding-year cutting stress results with fall growth score was a better predictor of long-term cultivar winter survival than seeding-year results alone. Alfalfa winter survival is a complex trait that is difficult to predict.