Field-Specific Fertilizer Nitrogen Requirements for Second-Year Corn Following Alfalfa.

The most extreme cases of excessive N fertilization in corn (Zea mays L.) occur when it is grown as the first and second crop following alfalfa (Medicago sativa L.). Past research suggests that second-year corn following alfalfa requires fertilizer N only one-half of the time to economically optimize yield, yet surveys indicate that many growers still hesitate to reduce fertilizer N applications. This may be due to the inability to identify which fields will require additional N. A meta-analysis was conducted using plot-level data from 212 site-years of research mainly in Iowa, Minnesota, and Wisconsin to identify: 1) which site-years of second-year corn responded to fertilizer N; 2) the economically optimum N rate at various fertilizer N/corn grain price ratios for responsive site-years; and 3) combinations of variables(soil texture, alfalfa termination timing, alfalfa stand age, and weather conditions) that predict corn response to fertilizer N. Only 32% of the 212 site-years responded to fertilizer N. Of those that responded, EONRs ranged widely from 48 to ≥269 kg N ha^-1 at the price ratio of 5.6 (US$ kg N^-1/US$ Mg^-1; 0.10 lb^-1 N/$ bu^-1). Preliminary regressions appear to correctly identify which fields will respond to N about two-thirds of the time when using combinations of alfalfa stand age and monthly precipitation and air temperature prior to the beginning of second-year corn (October – May). Refinement and addition of soil, crop, and weather variables should further improve predictions and allow for a more field- or site-specific approach to N management for corn following alfalfa.