Variable nitrogen rate determination from plant spectral reflectance in soft red winter wheat

Variable rate nitrogen (N) application based on in-season remote sensing can potentially improve wheat (Triticum aestivum L.) N management and N use efficiency (NUE). The goal of this study was to evaluate the potential of improving in-season soft red winter wheat (SRWW) variable rate N recommendations based on crop canopy reflectance. Small-plot N rate response calibration studies guided development of the Virginia Wheat Algorithm (VWA) for grain yield prediction and variable rate N fertilizer rate determination for SRWW. Large plot, replicated validation studies conducted for 15 site-years included an N-rich strip installed at growth stage (GS) 25 and various treatments at GS 30; four or five fixed-rate treatments applied to evaluate site N response, a variable rate based on the VWA applied using a GreenSeeker® RT 200 system and a “standard” fixed rate based on GS 30 wheat tissue N concentration. All sites responded positively to GS 30 N application. When data from one site were excluded, rates were 8 and 3 kg ha−1 below the economically optimal N rate (EONR) for the VWA and standard methods, respectively. Based on these data, the GreenSeeker® RT 200 system employing the VWA was equivalent to the current standard method and offers real-time rate prescriptions with less labor and less delay than the current tissue N concentration sufficiency standard.