Corn Hybrid Growth Stage Influence on Crop Reflectance Sensing

Active-light crop canopy sensing for corn (Zea mays L.) N fertilizer rate decisions typically include measurements of N-sufficient plants as a reference. When producers use multiple hybrids in one field, the question is raised of whether an N-sufficient reference is needed for each hybrid. The objective of this research was to assess the impact of sufficiently N-fertilized, similar-maturing corn hybrids on crop-reflectance measurements. Eleven similar-maturing hybrids were selected from three sites in 2008 and eight hybrids from two sites in 2009. When the corn was about 10 cm tall and on 3 to 5 d intervals canopy reflectance, leaf chlorophyll, and plant height measurements were obtained. Results were classed into two growth periods based on crop height: 20 to 70 cm and 71 to 120 cm. In three of the four growing periods assessed corn hybrid had no significant effect on reflectance. In 2008, reflectance for corn 71 to 120 cm tall was affected by hybrid; however the effect was minor leading to an average N rate recommendation difference of 10 kg ha−1. More pronounced differences occurred among hybrids for leaf chlorophyll measurements, resulting in differences in N fertilizer recommendations of 55 kg ha−1. Reflectance differences among similar maturing hybrids would have minimal impact on N fertilizer recommendations. Models were also developed to represent typical upper and lower values for various vegetative indices as a function of corn height. These models can help guard against using questionable data when assessing N-sufficient corn.