Velasco, J. L., Rozas, H. S., Echeverría, H. E. and Barbieri, P. A. 2012. Optimizing fertilizer nitrogen use efficiency by intensively managed spring wheat in humid regions: Effect of split application. Can. J. Plant Sci. 92: 847-856. Efficient N fertilizer management is critical for the economical production of wheat and the long-term protection of the environment. Six experiments were conducted at three locations in the south-east of the province of Buenos Aires (SE), Argentina, during a 4-yr period, on Typic Argiudoll and Petrocalcic Paleudoll. The study was designed to evaluate the effects of splitting nitrogen (N) fertilizer on N use efficiency (NUE) in wheat (Triticum aestivum L.). Rates of 0 to 150 kg N ha^-1 were used, applied at tillering (Z24) or split between Z24 and flag leaf (Z39). The experimental design was a randomized complete block with three replications. Grain yield ranged from 3522 to 8185 kg ha^-1, according to N availability and application time. In the experiments without water stress (three out of six), average grain yield (across experiments) was 6669 and 6989 kg ha^-1 for full and split fertilization, respectively. In four out of six experiments, average N in above-ground biomass (NAB), N recovery fraction (NRF), and grain protein content (GPC) for split N application were greater than for full N at Z24 (NAB, 176 and 157 kg N ha^-1; NRF, 66 and 51%; GPC, 100 and 92 g kg^-1, for split and full N application, respectively). In years without water stress, splitting N between Z24 and Z39 is an appropriate strategy to improve NRF, reducing N losses, and minimizing the environmental impact of fertilization.