Sensor Based Nitrogen Rate Calculator Validation for Wheat in Argentina.

The use of new tools to prescribe Nitrogen (N) fertilization rates based on wheat canopy reflectance is promissory in areas where rainfall is certain during late tillering and jointing. The objective of this study was to validate a Sensor Based N Rate Calculator (SBNRC) as an N diagnostic tool at end of tillering. Six experiments were conducted in 2007 at INTA Parana Research Station, Entre Ríos, Argentina (31.5º S 60.3º W). Strip trials were used to evaluate grain yield response to N rates at planting (N0, N92 y N184 kg ha^-1) and to mid season prescriptions using a SBNRC (N0+ and N92+). A GreenSeeker sensor was used to obtain NDVI readings at first visible node (GS 31) and a locally developed equation was used (SBNRC) to elaborate prescriptions. Average grain yield was 2777, 4639 and 5210 kg ha^-1 for N0, N92 and N184, respectively. Grain yield response to N prescriptions ranged from 1303 to 3268 kg ha^-1. SBNRC was able to determine variable crop N needs, and N rates were 69 to 15 kg N ha^-1 for N0+ and N92+. The response index (NDVI_Nrich/NDVI_treatment) varied from 29 to 98% for N0+ and from 1 to 18% for N92+. Yield predictions were overestimated in 31 and 42% at planting or at planting + mid season fertilization. Wheat grain yield for N0+ was 23% lower than the N rich strip (NRS), whereas in N92+ grain yield reduction was only 7%. Total N applied (planting + mid season) for N0+ and N92+ was 39% and 58% of the NRS (184 kg N ha^-1). SBNRC improved N use efficiency (NUE) from 13 kg grain kg N^-1 in NRS, to 20 and 19 for N0+ and N92+, respectively. These results show that although some refinement is needed to yield predictions, SBNRC significantly improved NUE.