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Relationship of Chemical Analysis to In Vitro Digestibility for selected Tropical and Temperate Grasses

Richard B. Russell Agricultural Research Center Field Crops Marketing and Utilization Laboratory A.R.S., U.S.D.A., Athens, Georgia 30604

Abstract

Eighteen grass samples (four tropical species harvested after 4 and 8 weeks regrowth, and six temperate species after 4 weeks regrowth summer and fall cuttings) were analyzed for crude protein, ether extract, ash, neutral detergent fiber (NDF), acid detergent fiber (ADF), hemicellulose, holocellulose, and permanganate lignin (PML). The relationship between chemical composition and in vitro dry matter digestibility (IVDMD) was investigated by regression methods. In vitro digestibility of isolated NDF, ADF and holocellulose was determined and compared to the digestibility of whole forage. The results indicated that differences in the cell wall matrix of tropical and temperate grasses could cause discrepancies in predicting digestibility from chemical compositional data. The best regression equation for predicting the in vitro digestibility of all the grass samples contained terms for protein, hemicellulose and PML. The ADF from tropical grasses was more digestible than the ADF isolated from temperate species. Lignin was important as a predictor of digestibility and appeared in six of nine equations, particularly in the equations for temperate grasses. Protein was the best predictor of digestibility for tropical (r = .90), but not for temperate species (r = –.17). One important compositional difference was the amount of hemicellulose, which was more abundant in the tropical than in the temperate grasses (tropical, 30 to 35% vs temperate, 22 to 27%).

¹ Research Chemist, Research Animal Physiologist, Research Chemist and Research Biometrician, respectively.

² The authors acknowledge Drs. J. L. Butler, J. E. Moore, and R. C. Buckner for providing us with these grass samples and Mrs. G. M. Hall, Mr. W. R. Windham and Mr. W. W. Rahn for technical assistance.

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