Precision Nitrogen Injection with Integrated Optical Sensing and Variable Rate Technologies.

Interest in variable-rate N applications based on optical sensing of crop canopy and yield maps has grown in the Mid-South during the past several years. The objective of this study was to evaluate the effects of variable-rate N management systems on cotton based on optical sensing of crop canopy and yield maps in comparison with the current uniform-rate N application systems. Eight field trials were conducted on cotton on private farms in Tennessee during 2012-2014. Four sensor-based variable-rate N management systems via side-dress of urea and ammonium nitrate (UAN) during the early growing season were evaluated in comparison with the local farmer’s uniform-rate N application systems in large strip plots under a randomized complete block design with three replicates. Each strip plot was divided into various split-plots which received different N application rates under the variable-rate N management systems but received the uniform N rate under the uniform-rate N application systems. The in-season variable-rate N management systems produced lint yield higher at one location-year, similar at six location years, but lower at one location-year than the uniform-rate N application systems. On average, the variable-rate N management systems had significantly lower seasonal N fertilizer consumption, but higher N use efficiency than the uniform-rate N application systems. The variable-rate N management systems reduced seasonal N fertilizer consumption by 9.3 to 14.8%, but increased N use efficiency by 9.2 to 11.0% compared with the uniform-rate N application systems. No significant effect of variable-rate N management systems was observed on post-harvest residual N levels in the top 60 cm of soil. The benefits of incorporating yield maps of previous years into the optical sensor-based variable-rate N management systems were generally not noticeable. In summary, in-season sensor-based variable-rate N management systems use less N fertilizer to produce comparable cotton yield relative to the local farmer’s uniform-rate N application systems.