Evaluation of the Trimble Greenseeker to Determine Optimum Corn Sidedress N Rate.

Corn (Zea mays L.) N rate and timing trials were used to evaluate the performance of the Trimble Greenseeker ground-based sensor in a series of ten field experiments during the 2013 to 2015 crop seasons. Sites were located in North Carolina Coastal Plain and Mountain regions, and included sites differing widely in yield potential and soil texture, drainage class, and organic matter. The protocol established a range in initial crop N status due to N rates at planting ranging from 0 to 280 kg ha^-1, followed by sidedress N rates of 0, 84, or 168 kg ha^-1 at either V7 or V12. Sensor evaluation trials with a limited set of row segments found consistent differentiation among control and N-rich treatments at V5, V7, V10 and V12 with either full sun (midday) or low light (early morning or late evening), and when operated at heights above the canopy ranging from 0- to 1.5 m. Trends suggest that higher N rates led to greater NDVI values, while greater ambient light levels (due to either brighter ambient light at mid-day or increased height of sensor above canopy) led to reduced NDVI values. Sites differed in the nature of the response to total N rates and in the relative response to sidedress N. Correlations of sensor reflectance index values with N rates, crop yields, and magnitude of yield response to additional sidedress N will be discussed.