Determinants of Grazed Forage Intake and Digestibility

Abstract

The inability consistently to predict voluntary intake of forage by ruminants reflects an incomplete quantitative understanding of the dynamic processes. Evidence is that voluntary intake of less digestible forages is limited by a) volume of the reticulorumen, b) volume occupied therein by forage residues undergoing digestion, and c) rates of chemical and physical processes which determine turnover of this volume. Two simplified expressions of progressively increasing detail are presented, and typical data illustrate consequences of varying selected digestive processes upon digestibility and voluntary intake of coastal bermuda hay. The expressions provide explanations consistent with observations on interactions between intake, rates of digestion and passage, and extent of digestion of varied entities. However, the assumption that rate of chemical digestion has a fixed and constant relation to rates of physical degradation is an over-simplification and emphasizes the critical need for quantitative information on factors determining rate of fragmentation of forage tissues. Elucidation of determinants of intake of grazed forage must consider component attributes of the forage-animal-grazing system complex and express such attributes in quantitative expressions to predict the net result of these dynamic processes.