| Abstract: | Alfalfa (Medicago sativa L) leaves and stems were incubated in the rumen of an alfalfa-fed steer for 0, 12, 24 and 48 h. Residues were analysed for monosaccharides remaining in chemically isolated fractions. Pectic substances were rapidly and extensively degraded. Xyloglucans also were quite degradable, whereas glucuronoxylans were very resistant to fermentation. Cellulose disappearance was intermediate between that of xylan and xyloglucan. For all non-pectic polysaccharides, leaf material was more degradable than stem, and this difference between leaf and stem was greater for cellulose than for most hemicellulosic fractions. Acetyl groups associated with the oxalate-insoluble fractions were degraded similarly to xylans. Alkali-labile phenolic acid content was lower in stems than in leaves, but contributed only 4 g kg?1 of leaf DM. Less than 10% of alkali-labile phenolics were associated with hemicelluloses. Structures which limit alfalfa xylan fermentation may include acetyl groups and linkages of uronic acid side chains to phenolic material. |
