Field-Specific Nitrogen Recommendations for Second-Year Corn Following Alfalfa.

When alfalfa (Medicago sativa L.) is terminated, N from soil organic matter and decomposing alfalfa shoots, crowns, and roots becomes available for at least two years of subsequent corn (Zea mays L.) production. Current university recommendations reflect that fertilizer N requirements have been studied far less for second-year corn following alfalfa than for first-year corn. In several states, fertilizer N recommendations are the same for continuous corn and second-year corn following alfalfa, while other states recommend reducing the amount of N applied to second-year corn by 25 to 145 kg N ha^-1. The variability and inconsistency in N recommendations for second-year corn needs to be addressed. On-farm experiments were conducted at 28 farms in Minnesota and Iowa to determine fertilizer N requirements of second-year corn following alfalfa. Surprisingly, half of the farms required no fertilizer N to increase grain yield. The remaining 14 responsive farms had economically optimum N rates ranging from ≤67 to 196 kg N ha^-1. These results indicate that the second-year N credit is highly variable and that book value N credits for second-year corn are not precise. A suite of predictor variables (soil, weather, and first-year corn characteristics) will be used to identify second-year corn fields that do not require fertilizer N to increase grain yield, and to predict the economically optimum N fertilizer rate on responsive sites. Field-specific N recommendations for second-year corn would greatly improve N management by increasing net returns for growers and reducing the environmental impacts of over-fertilization.