ELUCIDATION OF FACTORS ASSOCIATED WITH THE MATURITY-RELATED DECLINE IN DEGRADABILITY OF BIG BLUESTEM CELL-WALL

Big bluestem (Andropogon gerardii) forage samples were collected from three ungrazed, annually burned pastures at 38, 58, and 97 d after bur ning. Cell wall material received five treatments: chlorite delignific ation, chlorite delignification plus alkali extraction, NaOH, NaOCH3 i n methanol, or NaBH4. Untreated and treated cell. walls were analyzed for carbohydrate composition (glucose, xylose, arabinose, galactose, a nd uronic acids), acetyl bromide lignin, acid detergent lignin, alkali -labile phenolics (p-coumaric and ferulic acids), acetyl groups, and 2 4- and 72-h in vitro degradabilities of neutral monosaccharides. A num ber of compositional features, notably concentrations of xylose, core lignin as measured by acetyl bromide lignin, alkali-labile phenolics, and acetyl groups, were well related to the decline in cell wall degra dability that occurred with increasing maturity of big bluestem. p-Cou maric acid increased with increasing maturity to a greater extent than did ferulic acid. Acid detergent lignin was not well related to degra dability of the cell wall for either the untreated or chemically treat ed cell walls. Chemical treatments failed to identify any particular c ell wall component as being most inhibitory. However, all treatments i mproved in vitro degradability of the carbohydrate fraction, indicatin g that components contributing to the undegradability of big bluestem cell wall are sensitive to chemical alteration. Treatments involving a lkali were most effective for improving degradability of big bluestem cell walls.