Predicting Yield and Response to Nitrogen Sidedress in Spring Wheat Using Active Canopy Sensors.

Active canopy sensors offer accurate measurements of crop growth status that have been used in real time to estimate nitrogen refertilization requirements. A major challenge of the application of active sensor readings however is the normalization of readings to compensate for differences in phenological crop growth stage. The objective of this work was to evaluate the capacity of active canopy measurements to determine yield and nitrogen refertilization requirements. For this purpose a set of 8 nitrogen response field experiments was conducted at contrasting sites in Uruguay where canopy readings were acquired with Greenseeker handheld and Cropscanner, and complementary information (leaf area, biomass, and nitrogen absorption) was obtained.  Sensor readings correlated well with biophysical measurements and were good predictors of relative yield for treatments with and without fertilization after canopy readings. Absolute yield however was not predicted accurately even after considering measurements obtained at similar phenological growth stages, and an assessment of yield potential for the site was necessary in order to estimate refertilization requirements. The combination of sensor readings and crop growth model simulations may prove useful to estimate yield potential and normalize readings for growth stage, improving absolute nitrogen requirement estimates across sites. Even when absolute requirements seem difficult to estimate in this location, sensors provide an objective and accurate evaluation of crop status across the field at the time of measurement.