CHARACTERISTICS OF PLANT-CELL WALLS AFFECTING INTAKE AND DIGESTIBILITY OF FORAGES BY RUMINANTS

Even under the intensive concentrate feeding systems of ruminant anima l production in the United States, forages continue to represent the s ingle most important feed resource. Cell-wall concentration and digest ibility limit the intake potential and energy availability of forage c rops in beef and dairy production. Identification of cell-wall charact eristics that should be targets of genetic modification is required if plant breeders and molecular biologists are to successfully improve f orages for livestock feeding, As the forage plant cell develops, pheno lic acids and lignin are deposited in the maturing cell wall in specif ic structural conformations, and in a strict developmental sequence. L ignin is the key element that limits cell-wall digestibility, but cros s-linkage of lignin and wall polysaccharides by ferulic acid bridges m ay be a prerequisite for lignin to exert its affect. Lignin compositio n and p-coumaric acid in the wall are less likely to affect digestibil ity. Voluntary intake of forages is a critical determinant of animal p erformance and cell-wall concentration is negatively related to intake of ruminants consuming high-forage diets. Cell walls affect intake by contributing to ruminal fill. A simple model of cell-wall digestion a nd passage in which ruminal fill is a function of rates of digestion a nd passage, as well as the indigestible fraction of the cell-wall indi cates that cell-wall concentration and rate of passage are the most cr itical parameters determining ruminal fill. Plant factors that affect rate of passage include those that affect particle size reduction by c hewing and those that affect particle buoyancy in the rumen. The latte r is primarily affected by 1) the ability of the particulate matter to retain gases, which is probably related to plant anatomy and rate of digestion of the plant tissue, and 2) the rate at which the gas is pro duced, which is affected by the potentially digestible fraction of the particulate matter and the rate of digestion of this fraction. Increa sing rate of digestion should increase rate of passage by diminishing the gas produced and increasing density over time. A reduction in the indigestible cell-wall fraction is beneficial because this will decrea se fill and increase digestibility. Animal production and economic ben efits from reduced cell-wall concentration and increased digestibility are significant. Because of the high cell-wall concentration and larg e digestible cell-wall fraction of grasses, reduction in cell-wall con centration would probably be of greater value than improving digestibi lity in these species. Legumes represent the opposite situation and ma y benefit more from improvements in the digestibility of their cell wa lls.