Field-specific fertilizer nitrogen requirements for first-year corn following alfalfa

Current fertilizer N guidelines in the midwestern and northeastern United States may not adequately account for variable N supply to corn (Zea mays L.) grown after alfalfa (Medicago sativa L.). Nitrogen supplied by alfalfa often is adequate for optimum grain yield of the subsequent corn crop, but fertilizer N sometimes is needed. Using 259 site-years of data from midwestern and northeastern United States literature and multiple logistic regression analysis, we successfully identified binary response categories (statistically significant or nonsignificant) to fertilizer N in first-year corn following alfalfa in 89 to 97% of the cases on fine-textured soils and on medium-textured soils with spring-terminated alfalfa, and in 73 to 90% of the cases on medium-textured soils with fall-terminated alfalfa. Only 1 of 11 site-years on coarse-textured soils did not require fertilizer N, preventing separation of responsive and nonresponsive site-years. Using multiple linear regression analysis, ≥68% of the variation in actual economically optimum nitrogen rate (EONR) across a range of price ratios (PRs) (4.48-11.2 as US$ kg^-1 N/US$ kg^-1 grain) was explained on fine-, medium-, and coarse-textured soils by simple predictors-alfalfa termination time, alfalfa stand age, and weather conditions from October through spring. Although independent data are needed to validate the equations, this approach to field-specific fertilizer N guidelines for first-year corn following alfalfa appears, in many cases, to identify when no N is needed and provide improved rate estimates when N response is expected. This approach to fertilizer N recommendation may prove effective for other rotations.