Aerial color infrared photography to optimize in-season nitrogen fertilizer recommendations in winter wheat

Remote sensing in the form of aerial color infrared (CIR) photography has been shown to be a useful tool for in-season N management in winter wheat (Triticum aestivum L.). The objectives of this study were (i) to develop a methodology for predicting in-season optimum fertilizer N rates for winter wheat at growth stage (GS) 30 directly from aerial CIR photography and (ii) to quantify how the relationships between these optimum N rates and spectral indices respond to different levels of biomass of the wheat crop. Field studies were conducted for three winter wheat growing seasons (2002-2004) over a wide range of soil conditions across North Carolina using a split-split plot randomized complete block design. Different planting date-seeding rate (PDSR) combinations were applied to create a range of biomass levels at GS 30. Different levels of N were applied at GS 25 (N-25) to create a range of N supply and winter wheat radiance, and at GS 30 (N-30) to measure grain yield response to N-30. Aerial CIR photographs were obtained at each site at GS 30 before N applications. Significant biomass response to PDSR and yield response to N-25 and N-30 were observed. Optimum N-30 ranged from 0 to 124 kg ha(-1) with a mean of 55 kg ha(-1). Better prediction of optimum N-30 rates were obtained with spectral indices calculated relative to high-N reference strips compared to absolute bands or spectral indices. Biomass measured at GS 30 influenced the strength of the relationship between optimum N-30 and spectral indices. When the GS-30 biomass was >1000 kg ha(-1), the best predictor of optimum N-30 (R-2 = 0.85) was a quadratic model based on measured winter wheat radiance relative to mean radiance in the G band for the high N reference strip (Rel Gs).