CELL-WALL COMPOSITION AND DEGRADABILITY OF FORAGE STEMS FOLLOWING CHEMICAL AND BIOLOGICAL DELIGNIFICATION

Chemical and biological delignification methods were used to investigate the relationship between the concentration and composition of lignin and degradation of forage cell walls. Stem material from lucerne (Medicago sativa L), smooth bromegrass (Bromus inermis Leyss) and maize (Zea mays L) stalks was treated with alkaline hydrogen peroxide, potassium permanganate, sodium chlorite, sodium hydroxide. nitrobenzene. and the lignolytic fungus Phanerochaete chrysosporium. Klason lignin and esterified and etherified phenolic acids were determined. Cell wall neutral sugar and uronic acid composition and the extent of in-vitro degradability were measured. Chemical delignification generally removed lignin, but the fungal treatment resulted in the removal of more polysaccharide than lignin. The concentrations of esterified and etherified p-coumaric and ferulic acids were generally reduced in treated cell walls: chlorite treatment preferentially removing p-coumaric acid whereas nitrobenzene treatment removed more ferulic acid. Syringyl moieties were completely removed from the core lignin polymer by nitrobenzene treatment of forage stems. Alkaline hydrogen peroxide and nitrobenzene were generally the most effective delignification treatments for improving polysaccharide degradability, with the grass species responding similarly to delignification whereas lucerne was somewhat less responsive. Fungal delignification, under these experimental conditions, did not improve cell wall degradability of these forages. Multiple regression and covariate analyses indicated that the lignin components measured were not powerful predictors of cell wall degradability. Neither the concentration nor the composition of the lignin fractions was consistently correlated with degradation. This lack of effect was attributed to the more generalised disruption of the cell wall matrix structure by delignification treatments.