Quantitative approaches for using color infrared photography for assessing in-season nitrogen status in winter wheat

Due to the timing and rates of N applications in wheat (Triticum aestivum L.), the potential exists for high N loading to the environment. Plant tissue tests offer growers the ability to determine in-season. N status, and to optimize N applications and N use efficiency. However, sampling and N analysis can be costly, difficult, and time consuming. Remote sensing may offer a solution to these problems. The objectives of this study were to determine (i) if remote sensing could be used to estimate in-season N status, (ii) if within-field calibration would improve the ability of remote sensing to estimate crop N status, and (iii) if optimum N rates could be estimated using remote sensing. Research was conducted in 1999 to 2001 at eight sites. Two sites had randomized complete block designs with variety, seeding rate, and N rate as treatments. Six sites had a single seeding rate and wheat variety. Biomass was found to influence spectral measurements of in-season N status. A strong relationship between the normalized difference vegetation index (NDVI) and growth stage (GS)-30 whole-plant N concentration (R-2 = 0.69) and GS-30 N uptake (R-2 = 0.61) was found. Within-field calibration did not improve the estimation of in-season N status by NDVI. While it was possible to use NDVI to estimate GS-30 N uptake, predicted N fertilizer rates based on N, uptake were highly unreliable. However, NDVI reliably predicted GS-30 N fertilizer rates based on whole-plant N concentration for wheat that had mean GS-30 biomass values >1000 kg ha(-1).