USE OF SPECTRAL RADIANCE FOR CORRECTING IN-SEASON FERTILIZER NITROGENDEFICIENCIES IN WINTER-WHEAT

Variable rate application technology based on spectral radiance has no t previously been used for correcting in-season winter wheat nitrogen (N) deficiencies. Soil and yield mapping has been used to recommend va riable amounts of applied fertilizer in crop production, however both are restricted by the time required to obtain results and their utilit y is bound by the year in which they were generated. The objectives of this study were to determine the relationship between spectral radian ce at specific wavelengths with wheat forage yield and forage N uptake , and to evaluate the potential use of spectral radiance measurements for correcting in-season wheat N deficiencies using sensor-based varia ble rare technology, Five studies were conducted, three in farmer fiel ds where variable soil N deficiencies were present and two On Oklahoma Agricultural Experiment Station land. Spectral radiance readings for red and near-infrared (NIR) wavelengths were obtained in wheat between Feekes physiological stages 4 and 6 using photodiode-based sensors fi tted with interference filters and interfaced to an embedded microcont roller Correlation between a plant nitrogen spectral index (PNSI), a v ariation of the normalized-difference-vegetative-index (NDVI), and tot al N uptake in wheat forage was then established. Based on the PNSI re adings, a variable 0 to 112 kg N hal topdress N rare was determined fo r 3 x 3 m plots and N as urea ammonium-nitrate (UAN) applied according ly (variable rate). In addition to the variable rate treatment, a fixe d rate and a check plot (no N applied) were evaluated in a randomized complete block experiment The PNSI was highly correlated with estimate s of wheat forage N uptake at all locations and stages of growth. Whea t grain yields increased significantly as a result of applying topdres s N in both the fixed rate and variable rate treatments when compared to the check (no topdress N applied). However no significant differenc es in wheat grain yield were found when comparing the fixed and variab le rate treatments. Variable N rate treated plots (based on PNSI) resu lted in a total N savings between 32 and 57 kg N ha(-1) when compared to the fixed topdress N rates. In addition to improving sire-specific N use efficiency, this technology will likely decrease the risk that o verfertilization poses to the environment.