Why is Corn Nitrogen Fertilization Above the Economic Optimal Rate a Bad Idea?

For decades, the gold standard used for how much N fertilizer should be applied to crops has been the economic optimal N rate (EONR). At this fertilization rate the increased return from grain sales equals the increased fertilizer cost (i.e., marginal profit equals zero). For corn especially, many tools have been developed throughout the US Midwest to help farmers determine the EONR for their fields. A comparison of how well these decision tools perform will be reviewed. Recent research has shown that integrating local soil and weather information into the tools can improve their performance. Notwithstanding all the research and outreach on corn N fertilization, farmers often apply N at rates greater than EONR. Uncertain weather is a big driver for adding extra N fertilizer as insurance. But the problem is that for those last pounds of N fertilizer applied to reach EONR, N use efficiency (NUE) as determined by the N removed with the grain is extremely low. And if low at EONR, the problem is worse when insurance N rates are applied. This point will be illustrated and discussed. Long-term, new strategies will be needed to better target N fertilization.