Evaluating constraints on fiber digestion by rumen microbes.

Factors affecting fiber digestion in ruminants were evaluated with the use of simple mathematical models. These models were constructed to define the dynamic processes involved so that constraints on fiber digestion may be elucidated. The fraction of fiber that is resistant to digestion and the rate of digestion and passage of potentially fermentable fiber were identified as constraints on fiber digestion in the rumen. Fermentation lag was shown to have no direct effect on fiber digestibility. Fiber that is resistant to fermentation by rumen microbes represents a significant fraction of forage fiber and accumulates in the rumen relative to potentially fermentable fiber. The digestibility of fiber that is potentially fermentable is a function of the rate at which the fiber is digested and its retention time in the rumen. Selective retention of potentially fermentable fiber in the rumen is necessary for the maximization of fiber digestion.