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Digestible and Indigestible Cell Wall Carbohydrates of Flaccidgrass, Tall Fescue, and Coastal Bermudagras¹

Structural carbohydrates of forage plants are possible indicators of differing ruminal fermentation characteristics. The objectives of this study were to assess the use of trifluoroacetic acid (TFA) hydrolysis in characterizing the cell wall of an immature (IM) and mature (MA) subtropical perennial grass, flaccidgrass (Pennisetum flaccidum Griseb.), and to make comparisons with a midseason's growth of IM Coastal bermudagrass (Cynodon dactylon L. Pers.) and MA Kentucky 31 tall fescue (Festuca arundinacea Schreb.). In vitro digestion for 20 and 72 h was followed by neutral detergent fiber (NDF) or acid detergent fiber (ADF) determination to estimate the disappearance of these detergent in soluble fractions. Hemicellulose (HC) was determined by difference (NDF minus ADF). vitro disappearance (IVD) of NDF, ADF, and HC (determined component sugar differences before and after 72-h fermentation) from IM flaccidgrass averaged 58% greater than MA flaccidgrass, In vitro disappearance of the fiber fractions was similar for both tall fescue and bermudagrass, and intermediate between the IM and MA flaccidgrass. The TFA hydrolysis of NDF yielded xylose, glucose, arabinose, and galactose compared with xylose, glucose, and only traces of arabinose from ADF. Xylose and glucose constituted the greatest proportion of monosaccharides hydrolyzed from all forage fiber residues. Of the individual monomers hydrolyzed from flaccidgrass NDF, xylose showed the least disappearance (652 g kg^–1 for IM and 380 g kg^–1 for MA) and glucose the most (943 g kg^–1 for IM and 853 g kg^–1 for MA). Xylose IVD was most influenced by plant maturity. Monosaccharides hydrolyzed from NDF and HC of IM flaccidgrass had greater IVD than MA tall fescue and IM bermudagrass. Mature flaccidgrass composition and IVD of the insoluble residues were similar to IM bermudagrass, indicating that flaccidgrass would be of higher quality than bermudagrass at similar maturity. Flaccidgrass maturity effects were significant (P 0.05) for xylose, glucose, and total hydrolyzed monosaccharides.

Key Words: Trifluoroacetic acid hydrolysis • In vitro disappearance • Arabinose • Xylose • Galactose • Glucose • Hemicellulose • Neutral detergent fiber • Acid detergent fiber • Pennisetum flacidum (Griseb.) • Cynodon dactylon (L.) Pers. • Festuca arundinacea (Schreb.)

² Former graduate student, Dep. of Crop Science, North Carolina State Univ. (Current address: Plant Physiology Div., DSIR, Palmerston North, New Zealand); and plant physiologist, USDA-ARS, and professor, Dep. of Crop Science and Animal Science, North Carolina State Univ., Raleigh, NC 27695-7601.

Received for publication November 12, 1985.