Alfalfa stem tissues: Cell wall deposition, composition, and degradability

Declining cell wall degradability of alfalfa (Medicago sativa L.) stems with maturation limits the nutritional value of alfalfa for ruminants. This study characterized changes in cell wall concentration, composition, and degradability by rumen microbes resulting from alfalfa stem tissue proliferation and development during maturation. The seventh internode from the shoot base of three alfalfa clones was sampled after 12, 17, 21, 31, and 87 d of regrowth in 1996 and 21 and 31 d in 1997. Cross sections were examined by light microscopy for tissue development, and after 48-h in vitro degradation. Cell wall concentration and composition of the internodes were determined by the Uppsala dietary fiber method, and cell wall degradability by rumen microbes was measured after 12 and 96 h. All stem tissues were pectin-rich and nonlignified at the two youngest maturities in 1996, except for primary xylem vessels which had lignified and thickened walls, and the internode was actively elongating. Primary xylem was the only tissue not degraded from immature stems. The 21-d-old internodes had completed elongation and begun secondary xylem proliferation. Secondary xylem lignified immediately, and lignification of primary phloem and pith parenchyma began when elongation ended. As tissues lignified, their cell walls became undegradable. Maturation increased stem proportion consisting of undegradable secondary xylem, and cell wall polysaccharide composition shifted from predominantly pectin toward cellulose. Degradability of pectin remained high regardless of maturity stage, but cellulose and hemicellulose degradabilities declined as secondary xylem proliferated. Degradability of alfalfa stems would be improved if the amount of lignified secondary xylem was reduce