With escalating nitrogen (N) fertilizer costs and continuing concerns about environmental impacts of N losses from cropland, there is substantial interest in improving the efficiency of nitrogen use by crops. In the Midwest, this interest is focused on corn since that crop receives most of the fertilizer N and most of the non-fertilizer N inputs from manures and previous legume crops. Nitrogen use efficiency (NUE) refers to the proportion of available or applied N that is taken up by the crop. Alternatively, NUE can also be expressed in terms of yield produced per unit of applied N (e.g., bu/lb N). The interest in improving NUE employs the rationale that enhanced NUE could allow less N to be applied without reducing yield, and if less N is used more efficiently, then losses of excess N to the environment should be lowered.