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Title: Estimation of nitrogen concentration and in vitro dry matter digestibility of herbage of warm-season grass pastures from canopy hyperspectral reflectance measurements

Author
item Starks, Patrick
item ZHAO, DULI - UNIVERSITY OF FLORIDA
item Brown, Michael

Submitted to: Grass and Forage Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/21/2007
Publication Date: 1/15/2008
Citation: Starks, P.J., Zhao, D., Brown, M.A. 2008. Estimation of nitrogen concentration and in vitro dry matter digestibility of herbage of warm-season grass pastures from canopy hyperspectral reflectance measurements. Grass and Forage Science. 63:168-178.

Interpretive Summary: Real-time 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. Traditional laboratory analysis of forage N and IVDMD are time consuming and costly. Nondestructive, remote sensing measurements of pasture canopy reflectance may provide a rapid and inexpensive means of estimating these two nutritive variables. Canopy reflectance was measured using a portable remote sensing instrument in eight warm season grass pastures at the USDA-ARS Grazinglands Research Laboratory during the 2002 and 2003 growing seasons. The canopy reflectance data were used to develop and validate prediction equations for estimating forage N concentration and IVDMD. Forage N concentration was found to linearly correlate with the ratio of reflectance in the 705 and 1685-nm wavebands (R705/R1685), and IVDMD was found to correlate with R705/R535. Compared to the simple 2-waveband reflectance ratios, prediction equations based upon multiple regression, band-depth analysis with stepwise regression, and partial least square regression increased the correlation between N and IVDMD and canopy reflectance values. Validation of these prediction equations indicated that multiple regression only slightly improved model accuracy compared to simple reflectance ratios. The results of this study suggest that forage N concentration and IVDMD of warm season grass pastures can be rapidly and nondestructively estimated during the growing season using canopy reflectance in a few narrow wavebands.

Technical Abstract: Real-time 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. Traditional laboratory analysis of forage N and IVDMD are time consuming and costly. Nondestructive measurements of pasture canopy hyperspectral reflectance may provide a rapid and inexpensive means of estimating these two nutritive variables. Canopy reflectance was measured using a portable spectroradiometer in eight warm season grass pastures at the USDA-ARS Grazinglands Research Laboratory during the 2002 and 2003 growing seasons. The canopy reflectance data were used to develop and validate prediction equations for estimating forage N concentration and IVDMD. Forage N concentration linearly correlated with the ratio of reflectance in the 705 and 1685-nm wavebands (R705/R1685) and IVDMD correlated with R705/R535. Compared to simple 2-waveband reflectance ratios, prediction equations based upon multiple regression, band-depth analysis with stepwise regression, and partial least square regression enhanced the correlation between N and IVDMD and canopy reflectance values. Validation of the prediction equations indicated that multiple regression only slightly improved model accuracy compared to simple reflectance ratios. Our results suggest that forage N concentration and IVDMD of warm season grass pastures can be rapidly and nondestructively estimated during the growing season using canopy reflectance in a few narrow wavebands.