Second-year corn after alfalfa often requires no fertilizer nitrogen

Terminated alfalfa (Medicago sativa L.) often provides N to at least 2 yr of subsequent corn (Zea mays L.) crops, but the variability in fertilizer N guidelines for second-year corn in the midwestern United States needs to be addressed. In several states, fertilizer N guidelines are the same for second-year corn after alfalfa as they are for continuous corn, whereas other states recommend reducing fertilizer N rates to second-year corn by 25 to 145 kg N ha^-1. Experiments were conducted at 28 farms in Iowa and Minnesota to determine fertilizer N requirements of second-year corn after alfalfa. Second-year corn in half of the trials showed no response to fertilizer N and the economically optimum N rate (EONR) for corn grain yield in the 14 responsive trials ranged from ≤67 to 196 kg N ha^-1. Furthermore, in 10 of 19 trials there was no response of grain yield to fertilizer N in both first- and second-year corn. These results indicate that current university fertilizer N guidelines for second-year corn after alfalfa often are unreliable. At the widely adopted critical concentration of 21 mg kg^-1, the pre-sidedress soil nitrate test (PSNT) correctly identified only 63% of the trials as responsive or nonresponsive to fertilizer N. When results of our experiments were combined with data from the literature, second-year corn grain yield responded to fertilizer N 55% of the time and the PSNT remained accurate only 65% of the time, indicating that more accurate forecasting of second-year corn N requirements is needed.