Estimating Crude Protein Concentration of a Grass Sward Using Spectral Measurements.

Forage mass (FM) fundamentally evaluates sward productivity, but forage quality determines nutritional value.  Optical sensing devices have potential for estimating forage quality. Instruments such as the Greenseeker^® measure normalized difference vegetation index (NDVI), but it is confounded with various canopy traits.  Further spectroscopic study is needed to indicate how CP estimates might be improved. Spectral reflectance was measured with an Analytical Spectral Devices (ASD) FieldSpec 3 portable spectrometer, followed by a Greenseeker®  RT200 sensor along with a mobile rising plate unit that estimates FM.  Spectral reflectance curves from field plots were analyzed with the corresponding N (thus CP) concentrations, resulting in coefficient of determination (R²) values of 72%. Principle-factor analysis indicated that most variation in N concentration was explained by 2, or at most 3 spectral factors, principally within the 1421-1600 nm and 2200-2400 nm spectral regions.  NDVI and FM estimates used as factors in a calibration equation resulted in a prediction coefficient of r=0.88. The spectral region(s) measured by the Greenseeker appears appropriate for detection of CP in tall fescue, but expression as NDVI must account for confounding effects on CP or any other forage characteristic.