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 rate technology. This study was conducted to determine the optimum field element size for maximum yields in winter wheat (Triticum aestivum L.), using variable 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 m²) on grain yield, N uptake and efficiency of use was investigated at Haskell, Hennessey, Perkins, and Tipton, Oklahoma. At Feekes growth stage 5 an optical sensor 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 measurements. Nitrogen was applied based on a NDVI–N rate calibration. Nitrogen rate, yield, N uptake, and efficiency of use responses to treatment resolution and applied N fertilizer differed in the 3 years of this experiment. In the first year, no significant influence of resolution on N rate, yield, N uptake, or efficiency of use was observed, likely a result of a late freeze that drastically reduced yields. In the second year of the experiment, there was a trend for a lower N rate and a higher efficiency of use for the 0.84 m² 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 m² resolution at both sites. In general, the finer resolutions tended to have increased efficiency of use in high yielding environments (>2300 kg ha^?1), and decreased yields in low yielding environments. This study indicates that application of prescribed fertilizer rates based on spatial variability at resolutions finer than 53.51 m² could lead to increased yields, decreased grower costs, and decreased environmental impact of excess fertilizers.