REVIEWSForage Quality for Ruminants: Plant and Animal Considerations1

Forage quality is a function of nutrient concentration, intake, nutrient availability, and partitioning of metabolized products within animals. Of these factors, intake potential is the major determinant of production by animals fed forage-based diets; however, it is one of the most difficult aspects of forage quality to determine or predict because variation among animals has a large influence on intake. Physical fill limits intake of forages with high cell wall concentrations when fed to animals with high energy demand. Hence, grasses, with their higher cell wall concentration, typically have lower intake than legumes. Energy availability of forage is also limited by cell wall concentration because cell contents are almost completely digested, whereas forage cell walls are slowly digested. Thus, the proportion of cell walls to cell contents is a major determinant of energy availability in feeds. Protein digestion by ruminants is complex. When crude protein concentration in herbage drops below 7% of dry matter, ruminal fermentation of forages may be limited and protein requirements of animals may not be met. Additionally, inefficient use of protein in high quality forages may limit performance of high producing animals. Usually, only about 25% of the forage protein escapes degradation from the rumen. Efficiency would be improved if a larger portion of forage protein passed from the rumen undegraded so that it can be degraded in the intestines where absorption is more efficient. Another important plant factor influencing forage quality is herbage maturity. Systems are now available for determining maturity of both legumes and grasses that will become more important as aids for predicting forage quality before forages are harvested or grazed. Forage quality is also influenced by the environment in which forages are grown and by soil fertility and these cause year-to-year, seasonal, and geographical variation in forage quality even when herbage is harvested at the same stage of maturity.