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 (R2 = 0.69) and GS‐30 N uptake (R2 = 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