Simple, Effective Predictors of Nitrogen Need for Corn Following Alfalfa.

The most extreme cases of excessive N fertilization in corn (Zea mays L.) often occur when following alfalfa (Medicago sativa L.). Past research has shown that first-year corn following alfalfa often requires no fertilizer N, yet surveys indicate that many growers still hesitate to reduce fertilizer N applications for first-year corn. This may be due to the inability to identify which fields will need additional N, and which will not. We conducted an analysis of 259 site-years from the literature to determine: 1) which site-years of first-year corn responded to fertilizer N; and 2) the economically optimum N rate (EONR) at various fertilizer N/corn grain price ratios for responsive sites. Multiple logistic and linear regression were used to identify combinations of simple predictors (soil texture, alfalfa termination timing, alfalfa stand age, and weather conditions) that identified responsiveness to N in 73 to 90% of the cases and accounted for ≥68% of the variation in EONR at various price ratios. Contrary to most university fertilizer N guidelines, alfalfa stand condition measurements or alfalfa termination timing were not reliable predictors of fertilizer N response in first-year corn. Response to N and EONR for first-year corn following alfalfa varied widely across soil, alfalfa management, and weather conditions, indicating the necessity of field-specific fertilizer N recommendations. This novel field-specific approach should help reduce risk of under- or over-application of fertilizer N, improve net returns to growers, and reduce the environmental harm of excessive N loss. These models appear to be robust, but should be independently validated for their predictive capacity. Intuitively, this field-specific approach may help identify fertilizer N responsiveness and requirements for crops in other rotations.