In order to improve water use efficiency (WUE) and increase crop production, efficient irrigation management in conjunction with improved nitrogen fertilization is needed. The objective of this study was to determine the effects of irrigation and nitrogen fertigation on WUE and yield of maize grown for silage. The experiment was conducted in 2003 and 2004 in Iran and included four irrigation levels, consisting of two deficit irrigation levels 0.7 and 0.85 soil moisture deficit (SMD), a full irrigation level 1.0 SMD, and an over irrigation level 1.13 SMD, and three N fertilizer levels consisting of 0.0 kg N ha^-1 (N0), 150 kg N ha^-1(N150), and 200 kg N ha^-1 (N200). Soil water content was measured up to a depth of 80 cm using a standard gravimetric method in 2003 and up to a depth of 180 cm with a neutron probe in 2004. Crop evapotranspiration (ET_C) was measured using the soil-water balance method. Total crop biomass (TCB) was measured at the silage stage and grain yield was measured at final harvest. TCB and grain yield was affected by both nitrogen and irrigation, but there was no significant interaction between irrigation and nitrogen fertilizer. WUE increased in response to deficit irrigation. However, irrigation water use efficiency (IWUE) decreased due to deficit irrigation for silage maize. There was no significant difference between WUE for the different irrigation levels. Increasing the amount of nitrogen improved WUE from 2.14 kg m^-3 for the N0 treatment to 2.53 kg m^-3 for the N200 treatment. IWUE ranged from 1.38 to 1.76 kg m^-3 for silage maize. Based on the results of our study, deficit irrigation via sprinkler irrigation system with a short irrigation interval is not a good strategy to increase WUE for silage maize in arid and semi arid region due to the increased evaporation losses. 
Keywords: Water use efficiency, Silage maize, Deficit irrigation, Sprinkler irrigation, Fertigation, Nitrogen, Crop production