A Dynamic Model of Fiber Digestion and Passage in the Ruminant for Evaluating Forage Quality

Abstract

A dynamic model of fiber disappearance from the digestive tract of ruminants was developed based upon acceptable and defensible concepts of fiber digestion and passage kinetics, and coefficients obtained from available literature. The model has applicability, manageability and comparability to experimental observations. Although the model can stimulate the effects of some forage and animal characteristics upon digestion, it should be realized that it is only the initial component of an overall rumen function model. Specific limitations of the present model include the aggregation of microbial interactions with fiber and fiber characteristics into the digestion rate constants and the assumption that fiber digestion is not limited by factors other than fiber characteristics. Development and use of the model suggested the need for additional information in several aspects of ruminant digestive function. More research is needed concerning particle size reduction, including: (1) particle size distribution in feed, digestive tract and feces; (2) changes in particle size in the rumen associated with mastication and remastication; and (3) description of particle size reduction as a factor influencing rate of passage. Information is also needed in describing digestion such as the effect of: (1) particle size; (2) microbial interaction; and (3) chemical, morphological or physical plant characteristics upon digestion rate. Additional research is also needed to obtain useable rate of passage coefficients under a variety of animal states and dietary situations. Although improvement of the model of fiber disappearance in ruminants will need to continue as new information is accumulated, it can be used in its present form to assess some factors that influence forage quality. Simulation of the model demonstrated that a 1.0% decrease in digestion rates results in a .6% increase in maximum digestible dry matter intake, a 1.0% decrease in turnover time results in a .9% increase in maximum digestible dry matter intake, and a 1.0% decrease in the indigestible fraction results in a 1.0% increase in maximum digestible dry matter intake. Analysis of plant and animal characteristics that influence digestion and intake suggests that maximum intake of digestible dry matter is influenced more by the proportion of fiber that is indigestible and the rate of passage than by the rate of fiber digestion. Copyright © 1979. American Society of Animal Science . Copyright 1979 by American Society of Animal Science.