1. INSTITUTO NACIONAL DE TECNOLOGIA AGROPECUARIA SECRETARIA DE AGRICULTURA, GANADERIA, PESCA Y ALIMENTACION "Wheat yield prediction by using an active sensor in the Northern Argentinean Pampas." "Wheat yield prediction by using an active sensor in the Northern Argentinean Pampas." Ricardo Melchiori 1, Octavio Caviglia 1, Agustín Bianchini 2, Nelson Faccendini 3, Susana Albarenque 3 and William Raun 4 1 INTA EEA Paraná, Argentina 2 AAPRESID, 3 FCA-Univ. Nac. de ER, 4 Oklahoma State University. rmelchiori@parana.inta.gov.ar Objectives: To correlate NDVI readings with spring wheat grain yield. To evaluate cultivar and water availability effects on NDVI and grain yield. Materials and methods 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 wheat cultivars. Normalized difference vegetative index (NDVI) measurements were taken using a Green Seeker Sensor (NTech Industries, Inc., CA, USA) during de season. At maturity, grain yield, and N concentration in grain were recorded. Nitrogen use efficiency (NUE) of N applied was calculated as grain yield / N applied. Results Green Seeker™ NDVI readings at Zadocks 31 were a good tool to predict wheat yield (r2>0,80). In E1 a great biomass and yield variability was detected and sensor based variable rate N management (SBNRC) 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 predict 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 (166 kg N ha-1). In E3, the effect of water availability was more important on NDVI than on grain yield. Different NDVI and grain yield relationships were obtained 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. Conclusions Sensor based diagnostic tool, can help to predict wheat yield, calculate N fertilization rates and to improve nitrogen use efficiency. Acknowledgments: This work was possible by the financial support from PASA FERTILIZANTES Figure 2. Relationship between NDVI (Green Seeker) at first visible node (GS 31) and wheat yield in short and full season wheat cultivars Figure 3. Water availability effects on NDVI vs yield relationships. Figure 1. Relationship between NDVI (Green Seeker) at first visible node (GS 31) and wheat yield