Relationship of Sugarcane Biomass and Nitrogen Uptake with Canopy Reflectance At Different Nitrogen Fertilizer Rates.

Successful application of spectral remote sensing technique for precise management of nitrogen (N) fertilizer for other crops has been well-documented. It is the first and major step to understand the relationships of sugarcane biophysical attribute and canopy reflectance. Two variety x N trials were established in 2009 in St. Gabriel and Jeanerette, LA to evaluate the changes on canopy reflectance in relation to sugarcane biomass production at different N application rates and to establish the quantitative relationship spectra vegetation index with biomass. Treatments included three cane varieties L99-226, L 01-283 and HoCP 96-540, and four N rates at 0, 45, 90 and 134 kg ha^-1. Canopy reflectance readings and biomass were collected at three sampling dates: 3^rd, 4^th and 5^th week after spring N fertilization. Regression analysis was performed to determine the relationships of spectral vegetation indecies with biomass. There were no differences on biomass among varieties however separation on simple ratio (SR) and normalized difference vegetation index (NDVI) values were observed. NDVI and SR tended to increase with increasing N supply. Highest correlation of NDVI (r² = 0.42) and SR (r² = 0.37) with biomass was obtained from the data collected three weeks after N fertilization. When categorized by variety, L 99-226 and HoCP 96-540 obtained higher coefficient of determination, r²=0.45 and r²=0.43, respectively but not L 01-283 ( r²=0.20).  Further research is needed to address the influence of other biophysical attributes that commonly vary among cane varieties.