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Relationships of Forage Compositions With Rates of Cell Wall Digestion and Indigestibility of Cell Walls

Animal Science Research Division, USDA, Beltsville, Maryland 20705

ABSTRACT

Forage fiber digestion followed first-order reaction kinetics even though individual forages differed widely in maturity, composition, and rate of fiber digestion. Linearity of individual semilog plots of remaining digestible cell walls on time and their individually high r² (mean .978) for each of 112 different samples representing 15 species support this general model. Cell wall digestion rates were more highly correlated with soluble dry matter percentage (y = – .0299 + .00261X, r = .72, P < .001) than with lignin percentage (r = –.47, P < .001), lignin-to-cellulose (r = –.18), log lignin-to-cellulose ratio (r = –.26, P < .01), or 72 hr in vitro cell wall indigestibility (r = –.38, P < .001). Similarity of mean lignin-to-cellulose ratios in theoretically indigestible residues from legumes (1.09) and grasses (.94) suggests a similar role of lignin in limiting extent of digestion. Lignin in dry matter, lignin:cellulose, or log lignin:cellulose seem to be equally suitable predictors of cell wall indigestibility in vitro. Legumes were higher in percentage soluble dry matter and lignin and lower in percentage hemicellulose than grasses. Legume cell walls were also more lignified and less digestible, but digested faster than grass cell walls.

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