Optimum field element size for maximum yields in winter wheat, using variable nitrogen rates

The resolution at which variability in soil test and yield parameters exist is fundamental to the efficient use of real-time sensor-based variable rat e technology. This study was conducted to determine the optimum field eleme nt size for maximum yields in winter wheat (Triticum aestivum L.), using va riable nitrogen (N) rates based on sensor readings. The effect of applying N at four different resolutions (0.84, 3.34, 13.38, and 53.51 m(2)) on grai n yield, N uptake and efficiency of use was investigated at Haskell, Hennes sey, Perkins, and Tipton, Oklahoma. At Feekes growth stage 5 an optical sen sor developed at Oklahoma State University measured red (670 +/- 6 nm) and near-infrared (NIR, 780 +/- 6 nm) reflectance in each subplot. A normalized -difference-vegetative-index (NDVI) was calculated from the sensor measurem ents. Nitrogen was applied based on a NDVI-N rate calibration. Nitrogen rat e, yield, N uptake, and efficiency of use responses to treatment resolution and applied N fertilizer differed in the 3 years of this experiment. In th e first year, no significant influence of resolution on N rate, yield, N up take, or efficiency of use was: observed, likely a result of a late freeze that drastically reduced yields. In the second year of the experiment, ther e was a trend for a lower N rate and a higher efficiency of use for the 0.8 4 m(2) resolution. In the third year of this study, there was a trend for a higher yield and a higher efficiency of use for the 53.51 m(2) resolution at both sites. In general, the finer resolutions tended to have increased e fficiency of use in high yielding environments (> 2300 kg ha(-1)), and decr eased yields in low yielding environments. This study indicates that applic ation of prescribed fertilizer rates based on spatial variability at resolu tions liner than 53.51 m2 could lead to increased yields, decreased grower costs, and decreased environmental impact of excess fertilizers.