Nitrogen requirements of first-year corn following alfalfa were not altered by fall-applied manure

Although farmers in the midwestern United States oft en 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 oft en does not require supplemental N for maximum grain yield, and that more accurate methods are needed to predict fertilizer N response.