Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 11, 2002
Publication Date: N/A
Interpretive Summary: Alfalfa hay and silage are major feeds for livestock and their value exceeds $9 billion annually, fourth among crops in the United States. Fiber makes up 30 to 50% of the dry weight of alfalfa plants. The poor digestibility (<50%) of fiber, particularly in alfalfa stems, often limits livestock productivity and it increases the amount of manure that must be recycled onto cropland, hopefully without risk to the environment. We dissected alfalfa stems into two groups of tissues, poorly digested "xylem" tissues and moderately digested "non-xylem" tissues. In both groups of tissues, we found that carbohydrates composed of xylose limited the digestion of fiber. In future work, we will investigate why xylose-containing carbohydrates are so poorly digested and we will develop strategies for improving the digestion of alfalfa fiber.
Technical Abstract: Alfalfa (Medicago sativa L.) stem degradability declines during plant maturation due to the accumulation of a highly lignified ring of xylary tissues. Xylem and non-xylem tissues dissected from lower alfalfa internodes were analyzed for cell-wall constituents and degradability. Cell walls comprised 740 mg g-1 of xylem and 533 mg g-1 of non-xylem tissues with xylem tissues contributing about 60% of cell-wall mass in internodes. Xylem walls contained 28% lignin, 4% pectin, 29% hemicellulose, and 39% cellulose compared to 15% lignin, 25% pectin, 30% hemicellulose, and 30% cellulose in non-xylem walls. Fungal enzymes released 22% and 73% of sugars from xylem and non-xylem walls, respectively. In both cell-wall fractions, the extent of xylose release was 56 to 90% lower than other sugars, indicating that the degradability of glucuronoxylans in secondary walls and probably xyloglucans in primary walls were preferentially restricted by lignin. Elucidation of lignin-xylose interactions may reveal strategies fo improving fiber degradability of alfalfa.