Nitrogen use efficiency (NUE) may be improved by using a sensor-based nitrogen rate calculator (SBNRC). The objective of this study was to correlate NDVI readings with spring wheat grain yield. Cultivar and water availability effects were evaluated on NDVI readings and grain yield. Three experiments (E) were conducted during 2005 in Paraná, Argentina. (31.5ºS). E1 included three N levels (0; 88 and 166 kg N ha-1); applied early in the season (at three leaves, Zadocks 13); and an additional treatment was set to test the SBNRC at visible first node (Zadocks 31) on the plots where 88 kg N ha-1 was applied. E2 was conducted to evaluate the SBNRC at Zadocks 31 with no N applied at planting. E3 included two N levels (0 and 120 kg N ha-1); three water availability levels (full irrigation, irrigation during vegetative period only, and irrigation during grain filling only); and three short season and three full season cultivars. Results showed that NDVI readings with a Green Seeker™ sensor at Zadocks 31 were a good tool to predict wheat yield (r2>0,8). In E1 a great biomass and yield variability was detected and sensor based variable rate management improved NUE. NDVI response index and grain yield response index were significantly (p<0.05) correlated (r2= 0,46). In E2, the SBNRC was useful to anticipate crop yield N needs and grain yield response. Wheat yields obtained with the SBNRC, irrespective of N rate at planting, were similar to crop yield of the reference plots. In E3, the effect of water availability was more important on NDVI than on grain yield. Different NDVI and grain yield relationships were fitted for short and full season spring wheat cultivars. Results showed that sensor-based N prescription tools are promissory to improve NUE. Additional extensive on-farm studies will be conducted in the future.