Zane T. Walker, 1991 Upper Buford Cr., University of Minnesota, St. Paul, MN, Matt A. Yost, USDA-ARS Cropping Systems & Water Quality Research Unit, Columbia, MO, Jeffrey A. Coulter, Agronomy and Plant Genetics, University of Minnesota, St. Paul, MN and Michael P. Russelle, USDA-ARS, St. Paul, MN
Alfalfa (Medicago sativa L.) typically is grown in rotation with other crops, and corn (Zea mays L.) frequently is both the first and second crop selected to follow alfalfa termination. In the majority of reported cases, there is little need for additional fertilizer N to optimize yield of first-year corn following alfalfa, and current extension guidelines adequately address these conditions. For second-year corn following alfalfa, fertilizer N has been needed to optimize yield only in about one-half of cases and the economic optimum N rate has varied greatly in responsive fields. In contrast, current extension guidelines for second-year corn generally suggest an alfalfa N credit that is about one-half of that for first-year corn. There currently are no validated means of predicting either the likelihood of yield response to N or the economically optimum N rate. Inadequate crediting of alfalfa N supply can result in significant over-application of additional N in manure and fertilizer, with greater risk of environmental degradation. A number of soil- and plant-based indexes have been proposed for use in this rotation. In this work, we summarize the literature and unpublished results to evaluate potential candidates for use in improving N credit recommendations.