Wednesday, November 15, 2006
266-10

Estimation of forage nitrogen concentration and in vitro dry matter digestibility of grass pastures using plant canopy hyperspectral reflectance.

Patrick Starks1, Duli Zhao1, Michael Brown1, and Samuel Coleman2. (1) USDA-ARS, Grazinglands Research Laboratory, 7207 W Cheyenne St., El Reno, OK 73036, (2) USDA-ARS, Subtropical Agricultural Research Station, Brooksville, FL 34601

Timely assessment of forage nitrogen (N) concentration and in vitro dry matter digestibility (IVDMD) during the growing season can help livestock managers make decisions for adjusting stocking rate and managing pastures. Nondestructive measurements of pasture canopy hyperspectral reflectance may provide a rapid and inexpensive means for estimating these two nutritive variables. An experiment was conducted in the 2002 and 2003 growing seasons at the USDA-ARS, Grazinglands Research Laboratory to determine relationships between forage N and IVDMD and canopy reflectance of warm season grass pastures. Canopy reflectance from eight bermudagrass pastures was measured using a portable spectroradiometer.  The reflectance data were used to develop and validate algorithms for estimation of forage N concentration and IVDMD. Forage N concentration linearly correlated (r = 0.82, P < 0.0001) with a ratio of reflectance at 705 and 1685-nm wavebands (R705/R1685) and IVDMD correlated with R705/R535 (r = 0.71, P < 0.0001).  Multiple regression, band-depth analysis with stepwise regression, and partial least squares regression techniques enhanced the relationships between N and IVDMD and canopy reflectance values (0.81 < |r| < 0.90, P < 0.0001).  However, validation of these  equations indicated only a slight improved model efficiency for predicting the two forage nutritive values. Our results suggest that forage N concentration and IVDMD of bermudagrass  pastures can be rapidly and nondestructively estimated during the growing season using canopy  reflectance in a few narrow wavebands.