Canopy Reflextance At Multiple Wavelengths and Indices for Early Season Detection of Cotton Nitrogen Status

The Normalized Difference Vegetation Index (NDVI) is utilized by many commercially available crop sensors, but often fails to correlate strongly to early season cotton (Gossypium hirsutum L.) N status.  The objectives of this study were to examine sensitivities of published indices calculated from passive canopy reflectance to changes in cotton biomass and N status, and to identify indices most compatible to current N management practices.  Field trials were conducted from 2008-2010 at the Plant Science Research Farm, Mississippi State, MS. Fertilizer N rates of 0, 40, 80, and 120 lb N/acre were applied to establish wide growth differences.  Plant height, leaf N, and proximal canopy reflectance were measured at various developmental stages each growing season.  Results suggest difficulty in determining cotton N status using NDVI may be due to the index’s stronger sensitivity to plant height  than to leaf N or greenness.  The index which was most sensitive to leaf N and least sensitive to plant height was the Canopy Chlorophyll Content Index (CCCI).  This index was sensitive to N status early in the growing season and held a relatively consistent range of values throughout the application window.