Integrated Corn Plant Height and Chlorophyll Content Measurements to Estimate the in-Season Nitrogen Requirement.

Crop-based active canopy sensors (ACS) have been used to prescribe in-season nitrogen (N) rates for corn based on indirect canopy reflectance measurements of chlorophyll content. Previous studies have shown that canopy chlorophyll content is correlated with biophysical parameters such as leaf area index and plant height. The objectives of this work were to: (i) evaluate the correlation between plant height (H) measurements using an ultrasonic sensor and reflectance using an ACS; (ii) test the ability of both sensors to differentiate crop N content at several corn phenological stages and (iii) test the integration of both sensors to estimate in-season N requirement. This experiment was divided into small plots conducted with varying N rates (0, 75, 150 and 300 kg/ha) and long strips treatments in farmers fields. In the small plots, plant height, canopy reflectance (NIR and Green portions of the spectrum) and geographic position using a DGPS receiver were recorded during the phenological stages of V4, V5, V6, V8, V10 and V13. Preliminary results showed that there are correlations between plant height (H) and chlorophyll index (CI). The coefficients of correlation on small plots between H and CI were 0.07, 0.15, 0.33, 0.48, 0.74 and 0.70 for V4, 5, 6, 8, 10 and 13 growth stages respectively. For the 2008 farmer field the correlations were 0.72, 0.7, 0.52 for V7, 10 and 13. The correlations of measurements averaged by each N rate were much higher than point measurements with coefficients of correlation greater than 0.98 at V8, 10 and 13. Both sensors were able to separate different N rates applied from V8 until V13. The integration of H x CI did not increase the ability to separate different N rates applied to the long term experiment using contrasting N rates.