PLANT LIMITATIONS TO FIBER DIGESTION AND UTILIZATION

Energy availability from forages is limited by fiber concentration bec ause fiber is slowly and incompletely digested, whereas cell solubles are almost completely digested. Thus, the proportion of fiber to cell solubles is a major determinant of energy availability in forages. Gra sses normally have more fiber than legumes, especially in leaves. Gras s fiber is more digestible than that of legumes, but that of legumes d igests at a faster rate. Ruminants digest 40-50% of legume fiber and 6 0-70% of grass fiber. Some fiber cannot be digested no matter how long it remains in the rumen. Lignin is thought to interfere with microbia l degradation of fiber polysaccharides by acting as a physical barrier and by being cross-linked to polysaccharides by ferulate bridges. In addition to the effects of lignin, physical and structural barriers ma y limit fiber digestibility. Because the middle lamella and primary wa ll of thick-walled cells are so highly lignified, many cells can be di gested only from the interior of the cell. For many cells, access to c ell interiors is limited because of large particle sizes. Forage diges tibility could be improved by reducing the amount of lignified cells o r by developing improved cultivars so that lignified cells are more di gestible.