Estimating winter wheat tiller density using spectral reflectance sensors for early-spring, variable-rate nitrogen applications

Adequate tiller density is critical for attaining optimum grain yield in winter wheat (Triticum aestivum L.). To ensure maximum tiller development, several states in the Mid-Atlantic recommend split-applying N in the spring based on tiller density at Zadoks Growth Stage 25. However, this strategy requires that several labor-intensive measurements be made in each field. Recent work has suggested that remote sensing might eliminate this problem. The objectives of this study were to estimate winter wheat tiller density using an on-the-go, spectral reflectance sensor and to determine the effect on grain yield of tiller density-based, variable-rate N applications at a 1-m(2) resolution. Twenty-two site-years of data were collected from diverse locations across Virginia from 2000 to 2002. The normalized difference vegetation index (NDVI) was a reliable predictor of tiller density across environments (0.67 less than or equal to r(2) less than or equal to 0.99), with 18 of 22 sites having slopes and intercepts that were not different from one another. Nitrogen fertilizer rates and grain yields resulting from using sensor-based estimates of tiller density were not different from those when using the standard practice for the Mid-Atlantic region at four out of six locations. At two of the six sites, sensor-based N recommendations were 11 kg N ha(-1) lower than standard recommendations with no effect on grain yield, resulting in higher N use efficiencies at these locations. These results show that on-the-go, optical sensor technology can be used to accurately estimate winter wheat tiller density for determining and applying appropriate N fertilization rates at a 1-m(2) resolution with minimal ground truthing required (one physical tiller count for each major soil type in a field).