Forage Lignins and Their Effects on Fiber Digestibility

The lignin fraction of forage cell walls is negatively associated with fiber digestion by ruminants. Lignins can be classified as core lignins, highly condensed polymeric matrices; and non-core lignins, low molecular weight phenolic monomers. Core lignins are covalently linked to hemicellulose in forage cell walls, with at least some of these linkage being alkali-labile in grasses. Among the non-core lignins, p-coumaric acid is primarily associated with the core lignin fraction of cell walls whereas ferulic acid is generally linked to the hemicellulose fraction. The majority of non-core lignin moities are esterified to other cell wall components, but ether linkages also seem to exist. Legumes and grasses differ in lignin concentration and composition. Physiological maturity of forages influences lignin content. Genetics and environmental influences have been observed to alter both concentration and composition of forage lignins. Lignification seems to depress digestion of grass cell walls more strongly than in legumes. Composition of core lignin may influence fermentation of forages more than does simply concentration. Non-core lignins also are negatively correlated with fiber fermentation, but this may just reflect the positive correlation between core and non-core lignin concentrations. Covalent linkage of lignin to carbohydrate has been shown to be important in model cell wall systems for inhibiting fermentation. Much remains to be learned about lignin structure and the mechanisms by which lignins interfere with ruminal digestion of forages. Contribution from the USDA-ARS in cooperation with the Minnesota Agric. Exp. Stn. Scientific Journal Series Paper no. 15 724.