RUMINAL DIGESTION AND GLYCOSYL LINKAGE PATTERNS OF CELL-WALL COMPONENTS FROM LEAF AND STEM FRACTIONS OF ALFALFA, ORCHARDGRASS, AND WHEAT-STRAW

Samples of alfalfa, orchardgrass, and wheat straw were hand-separated into leaf and stem fractions that were subjected to in situ ruminal fe rmentation for various lengths of time to assess the rate and extent o f degradation of cell wall neutral monosaccharides, uronic acids, acet yl groups, and hydroxycinnamic acids. A second objective was to measur e the glycosyl linkage patterns of leaf and stem fractions of substrat es before and after ruminal fermentation. Samples were fermented for 0 , 6, 12, 24, 48, 96, and 192 h in each of two ruminally cannulated ste ers. In situ disappearance data were fitted to a first-order exponenti al equation to estimate the following substrate parameters: insoluble, potentially digestible fraction (f(d)), indigestible fraction (f(i)), and fractional rate constant of degradation of the potentially digest ible fraction (k). Leaves contained larger concentrations of crude pro tein and smaller concentrations of cell wall components than did stem fractions. Estimates of f(i) were 7.3, 39.2, 22.1, 49.3, 27.7, and 36. 3 for dry matter disappearances for alfalfa leaf, alfalfa stem, orchar dgrass leaf, orchardgrass stem, wheat straw leaf, and wheat straw stem , respectively. Averaged across substrates, estimates of fi for arabin ose, galactose, glucose, xylose, uronic acids, acetyl groups, and p-co umaric acid were 16.5, 11.4, 14.8, 31.2, 12.8, 25.3, and 22.6% in leaf fractions and 29.5, 19.9, 37.5, 56.2, 35.0, 52.4, and 44.6% in stem f ractions. Rates of digestion of all monomeric components except galact ose and xylose were greater (P < .05) for alfalfa than for orchardgras s or wheat straw. Differences in digestibility of cell wall components from leaf and stem fractions were greater in alfalfa and orchardgrass than in wheat straw. Glycosyl linkage analysis indicated that xylans in leaf and stem fractions of alfalfa, orchardgrass leaf, and wheat st raw stem that resisted degradation had a lower degree of substitution with acid-labile constituents (i.e., other monosaccharides) than was f ound in original substrates. Different rates and extents of digestion of leaf and stem fractions of forages explain part, but not all, of th e observed differences in digestibilities of cell wall monomers by rum inants.