Variability of Maize Canopy Reflectance within Field-Length Strips Fertilized with Low to High Rates of Nitrogen.

The use of optical reflectance sensors to detect N deficiency in maize for formulating site-specific fertilizer recommendations currently relies on the comparison of crop sensor measurements from the bulk field to that of a reference strip known to be N sufficient.  Establishing high N reference strips hinders the adoption of sensors due to time, resource, and environmental constraints. Our objective was to determine the incidence of chlorophyll index (CI) measurements equivalent to reference strip CI in field areas which have had N fertilizer applied at suboptimal levels. Eleven field studies were conducted throughout the state of Indiana in 2009 using a range of 6 N rates from zero to as much as 286 kg N/ha (varying by location) applied at approximately the V2 growth stage. Crop Circle sensors were used to determine CI 14 -25 days after N application. The mean of CI measurements over many 30 s-intervals (approx. 50 m of row) were calculated for each sub-optimal N rate and compared to the mean CI of the reference strip. In 5 of 11 studies starter-only N rates resulted in fewer than 2% of CI measurements greater than or equal to the reference CI. At N rates 50-70% of the eventual optimum N rate, 4 studies had fewer than 20% of CI measurements greater than or equal to the reference CI. Identifying reference-equivalent CI values in sub-optimally fertilized corn will be a significant challenge and unlikely to eliminate the need for reference strips in order to utilize crop sensors for N management.