Grain yield was georeferenced using a yield monitor system (grain mass flow and moisture sensors). Site coordinates for each yield measurement were determined with a differentially corrected GPS (OMNISTAR signal) Trimble 132 receiver with 5 cm accuracy. The SMSTM software version 3.0 of AgLeader Technology was used to read raw yield data (expressed at 13.5 % dry matter). Yield data were then processed to eliminate outliers with values lower than 500 kg ha-1 and greater than 6000 kg ha-1. Yield maps were obtained by plotting the yield data, at the nodes of a regular grid of 5 m spatial resolution. The yield maps were then georeferenced and projected in UTM WGS 84 zone 33 N. N use efficiency (NUE) was calculated using the partial balance approach. Inputs to this method consist of grain yield, and the amount of N applied. It is calculated as follows: