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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 reduced.

Growth temp. effects on anatomy and digestibility of leaf and stem of tropical and temperate forage species (Cynodon dactylon, Panicum maximum var. trichoglume, P. laxum, Lolium perenne and Medicago sativa), representing a range of anatomical types, was evaluated. Plants were grown in controlled-environment greenhouses at day/night temp. of 32/26 or 22/16 degrees C. Tissues of defined development stage were sampled for analysis of proportion of cell types, cell wall thickness, staining properties and digestion, using transverse sections on microscope slides immersed in rumen fluid. Leaf and stem harvested from whole plant tops was analysed for OM digestibility (OMD), cell wall digestibility (DCW), lignin, cell wall and insoluble ash. Temp. had little consistent effect on the proportion of different cell types, or the thickness of cell walls, cell wall content or insoluble ash. The digestion of the walls of sclerenchyma, bundle sheath and xylem cells was lower in tissues grown at the higher temp., especially for leaf. High temp. decreased OMD by an average of 6.6 percentage units for leaf and 12.4 percentage units for stem in the grasses, and by 2.6 percentage units for both tissues in M. sativa. The decrease in OMD and DCW with high temp. (excluding stem of M. sativa) was correlated with increase in lignin concn. High temp. appeared to increase the intensity of lignification of the existing lignified cells rather than increase the proportion of cells becoming lignified. Staining intensity was an insensitive indicator of this change in lignification. (Abstract retrieved from CAB Abstracts by CABI’s permission)

Two diferent chemical methods, sequential extraction with alkali and sodium chlorite and treatment with 4-methylmorpholine N-oxide ( M MN

No abstract