Current tools for making crop N fertilizer recommendations are primarily based on plot and field studies that relate the recommendation to the economic optional N rate (EONR).  Some tools rely entirely on localized EONR (e.g., MRTN). In recent years, tools have been developed or adapted to  account for within-field variation in crop N need or variable within season factors. Separately, attention continues to elevate for how N fertilizer recommendations might account for environmental impacts. Efficiently using plant-available N from any source (fertilizer, manure, irrigation water, and mineralization) is a challenge because nitrate is soluble in water and thus is subject to losses via runoff from agricultural fields and leaching through porous soils. In addition, N can be lost from waterlogged soils via denitrification. One suggested way to consider the effect of N fertilizer on the environment is through crop N use efficiency (NUE) indices.  Studies indicate global NUE is only 30-40% for corn, which implies that fertilizer N application rate is 2-3 times more than the N contained in the grain. Using datasets of grain yield in response to N fertilizer we evaluated what NUE looks like incrementally (iNUE). For those last units of N fertilizer applied to reach economic optiomal N rate, iNUE as determined by the N removed with the grain is less than 10%. Our analysis also shows how temporatal and spatial soil and weather factors impact NUE. The objective of this paper will be to show how iNUE helps expose the problem of N fertilizer management yet also guide site-specific N management approaches.