Alfalfa N Credit to First-Year Corn: No-Tillage and Manure Effects.

Alfalfa (Medicago sativa L.) N credits to first-year corn (Zea mays L.) need to be validated for no-tillage and fall manure applications. To determine whether adjustments to the alfalfa N credit are needed for these factors, experiments were conducted at 6 farms with no-tillage corn after alfalfa and on 9 farms with fall manure application. The reliability of the presidedress soil nitrate test (PSNT) to identify the need for fertilizer N in first-year corn was also tested. Fertilizer N was not required to maximize corn yield across 6 no-till farms, confirming that alfalfa N credits alone can supply the entire N requirement of no-till corn following alfalfa. At 6 of the farms with manure treatments, neither manure nor fertilizer N was needed to economically optimize grain yield. At the other 3 farms, manure increased the economically optimum N rate (EONR) for grain yield to between ≤45 to ≥180 kg N ha^-1. Sidedress N applications of 45 kg N ha^-1 were nearly twice as efficient in increasing corn yield as N applied at planting. Manure or fertilizer N increased silage yield by 3.1 or 9.5 Mg ha^-1 (4 or 11%), respectively, on the 5 farms where silage yield was measured. Based on critical concentration of 21 mg NO₃-N kg^-1, the PSNT was accurate in only 40% of the cases, suggesting that improved methods or tests are needed to accurately predict when fertilizer N is needed in corn following alfalfa.