Nitrogen Requirements of First‐Year Corn following Alfalfa Were Not Altered by Fall‐Applied Manure

Although farmers in the midwestern United States often apply livestock manure when terminating alfalfa ( Medicago sativa L.), there are no published reports on direct effects of fall manure application on alfalfa N credits to first‐year corn ( Zea mays L.). Therefore, eight on‐farm experiments were conducted in Minnesota to test whether manure applied during alfalfa termination in fall affects the rate of fertilizer N needed for the subsequent corn crop. Manure application rates were determined by cooperating growers and N fertilizer rates were 0, 45, 90 or 179 kg N ha –1 at planting and 45 kg N ha –1 as a sidedress. At five locations, neither manure nor fertilizer N increased grain or silage yield. At three remaining locations, N fertilizer was needed to economically optimize grain (98 kg N ha –1 at two locations) or silage yield (113 kg N ha –1 at three locations), but manure did not alter the response to fertilizer N. The presidedress soil nitrate test (PSNT) predicted the need for N fertilizer to optimize grain yield 63% of the time and the basal stalk nitrate test (BSNT) identified the need for N fertilizer 75% of the time. When results of this study were combined with data from the literature, the PSNT was accurate only 55% of the time and the BSNT could not separate responsive and nonresponsive sites. These results confirm that first‐year corn following alfalfa often does not require supplemental N for maximum grain yield, and that more accurate methods are needed to predict fertilizer N response.