Corn Reflectance and Yield Response Spatial Variability along a Hillslope.

The success of site-specific N recommendations for corn (Zea mays L.) will depend on a better understanding of the causal impacts on the variability of economic optimum N rate (EONR). The objective of this study was to evaluate the relationships between crop reflectance at the six-leaf (V6) growth stage, soil water content during the growing season, and EONR for grain yield. In each of three years, 10 locations were selected along a 300-m toposequence (10-m relief) in central Pennsylvania. At each location, two reps of six N treatments were applied at planting. Soil water content (0-90 cm) was recorded weekly at each location between mid June and September. Ear leaf N content was determined in two years and V6 leaf N content determined in one year. Grain yield was determined at harvest each year. An EONR was determined for all 10 locations (each year) based on a quadratic-plateau response to N fertilizer. The EONR ranged from 47 to 188 kg N ha^-1 in 2005, 147 to 280 kg N ha^-1 in 2006, and 106 to 280 kg N ha^-1 in 2007. We will present the results from this study and consider implications for variable rate N management for corn.