Effect of row spacing, growth stage, and nitrogen rate on spectral irradiance in winter wheat

Soil reflectance affects spectral irradiance measurements taken in winter w heatat early stages of growth when percent cover is low. The objective of t his study was to determine the critical percent vegetation coverage needed for forage nitrogen (N) uptake calibration with indirect spectral irradianc e measurements. Two field experiments were conducted at Tipton and Perkins, OK in October 1996. The effect of row spacing (15.2, 19.0, 25.4, and 30.5 cm) and growth stage (Feekes 4 and 5) under various N fertilizer rates (0, 56, 112, and 168 kg N ha(-1)) on spectral irradiance measurements from whea t was evaluated. The normalized difference vegetative index (NDVI) was used to characterize wheat canopy irradiance. In general, NDVI decreased with i ncreasing row spacing and increased with N fertilizer rate at Feekes growth stage 4. Row spacing and N rate were independent of each other since no si gnificant interaction was found. High correlation (r=0.81-0.98) was observe d between NDVI and vegetation coverage. Percent vegetation coverage was a g ood predictor of the other dependent variables including forage dry matter, and total N uptake, which could indirectly be determined using NDVI. The c oefficients of variation (CV's) from NDVI values decreased with increasing vegetation coverage suggesting that less variable NDVI values (CV less than 10%) might be obtained from plots where vegetation coverage exceeds 50%.