Winter Wheat Crop Reflectance and Nitrogen Sufficiency Index Values are Influenced by Nitrogen and Water Stress

Abstract

In‐season N applications to winter wheat (Triticum aestivum L.) may increase profits and improve N fertilizer accuracy. The objectives of this experiment were to determine the impact of N and water stress on crop reflectance and N sufficiency index (SI) values. The experiment contained five N rates, two water treatments, and four blocks. Crop reflectance was measured at the stem extension and flag leaf growth stages, sufficiency index (SI)‐NDVIwf was ratio between the underfertilized normalized difference vegetation index value {NDVI = [near infrared (NIR)‐red]/[NIR+red]} and the NDVI value from well fertilized and well watered treatments, while SI‐NDVImz was ratio between underfertilized NDVI values and NDVI values from well fertilized plots within a water stress treatment. Yield losses due to water and N stress were determined using 13C isotopic discrimination. This research shows that: (i) at the stem extension and flag leaf growth stages, water stress and N stress increased, green, red, and red‐edge reflectance and reduced NDVI values (ii) following the economic optimum nitrogen rate (EONR) produced grain with greater than 120 g kg−1 protein and <10 min stability; (iii) at stem elongation and flag leaf, N fertilizer induced yield gains had a stronger relationship with SI‐NDVImz (stem extension, r = 0.49*; flag leaf, r = 0.51**) than SI‐NDVIwf (stem extension, r = 0.29; flag leaf, r = 0.33); and (iv) SI‐NDVImz had greater fertilizer recommendation accuracy than SI‐NDVIwf. These findings suggest that in wheat production, SI should be referenced to well fertilized areas within a management zone.