The objectives of this project were: i) to evaluate on-farm the effect of N fertilization on corn grain (GY) and ethanol yield (EY) across the Mid-West, and ii) to analyze the spatial variability of GY and EY responses to N fertilizer within fields. Experiments were conducted in 21 production fields located in IN, IL, IA, MN, and ND. Six N rates (0 to 280 in 56 kg N/ha increments) were applied in 6 blocks within these fields using farmer's equipment. Grain yield was obtained with calibrated yield monitors and grain samples were collected and analyzed for EY (L of ethanol/Mg grain). Grain and EY responses to N fertilizer were analyzed with linear-plateau or linear models. Grain yield increased with N fertilization in all fields and the N rate required to maximize it varied from 56 to 190 kg N/ha. Ethanol yield decreased with N fertilization in 19 fields, 6 of which showed a linear response and in all others it reached a plateau. The N rate at which ethanol yield reached the minimum was higher than the N rate required to maximize yield in all fields except 2 where EY did not respond to N. At the N rate that maximized GY in each field, EY averaged436 l/Mg and ranged from 426 to 449 l/Mg, whereas GY ranged from 9.4 to 12.7 Mg/ha. Grain yield and EY responses to N also varied within fields. The N rate at which EY reached its minimum ranged substantially in several fields (i.e. more than 120 kg N/ha) and tended to be higher than the N rate required to maximize GY. The results also indicate that the application of the optimal N rate required to maximize GY would reduce the variability in EY within fields