Aerial Color Infrared Photography for Determining Late-Season Nitrogen Requirements in Corn

Fast and accurate methods of determining in-season corn (Zea mays L.) N requirements are needed to provide more precise and economical management and potentially decrease groundwater N contamination. The objectives of this study were (i) to determine if there is a response to late-season N applied to corn at pretassel (VT) under irrigated and nonirrigated conditions, and (ii) to develop a methodology for predicting in-season N requirement for corn at the VT stage using aerial color infrared (CIR) photography. Field studies were conducted for 3 yr over a wide range of soil conditions and water regimes in the North Carolina Coastal Plain. Different fertilizer N rates were applied (i) at planting (NPL) to create a range of N supply, corn color, and near-infrared (NIR) radiance; and (ii) at VT (NVT) to measure yield response to NVT. Aerial CIR photographs were obtained for each site at VT before N application. Significant grain yield responses to NPL and NVT were observed. Economic optimum NVT rates ranged from 0 to 224 kg ha^-1 with a mean of 104 kg ha^-1. Better prediction of economic optimum NVT rates was obtained with spectral band combinations rather than individual bands, and improved when calculated relative to high-N reference strips measured at VT. The best predictor of economic optimum NVT (R^2 = 0.67) was a linear-plateau model based on corn color and NIR radiance expressed using the Green Difference Vegetation Index (GDVI) relative to high-N reference strips (Relative GDVI, RGDVI).