Yield goal versus delta yield for predicting fertilizer nitrogen need in corn

Fertilizer N needs of corn (Zea mays L.) vary widely both among and within fields. Many states use yield goal to help determine differences in fertilizer N need, but some states have questioned yield goal-derived recommendations because of the poor correlation of yield with fertilizer N need. In this study, data from 298 previously reported experiments in five states (Illinois, Minnesota, Missouri, Pennsylvania, and Wisconsin) were combined to evaluate fertilizer N response of corn. Corn grain yield at the economically optimum N rate (EONR) was positively but poorly correlated with EONR (r(2) = 0.02). This was consistent with others who have observed that maximum or optimum economic yield is a poor predictor of EONR. Our analysis indicated N supplied by the soil and previous management reduced N need from that predicted by yield alone at most locations. Delta yield (grain yield at optimum N rate minus grain yield of the control) was a much better predictor of EONR at these same locations (r(2) = 0.47). A theoretically derived equation based on the delta yield concept was similarly capable of predicting EONR for corn. These results indicate that fertilizer recommendation systems that rely solely on yield or ignore yield entirely are limited to explaining less than 50% of the variation in EONR for corn. Farmers should be encouraged to monitor delta yield as a more effective indicator of EONR than actual yield. Greater understanding of the delta yield concept is needed before relying on it to predict fertilizer N requirements.