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Evaluating Constraints on Fiber Digestion by Rumen Microbes¹

Agricultural Research Service, U.S. Department of Agriculture, U.S. Dairy Forage Research Center, Madison, WI 53706

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.

KEY WORDS: • rumen fiber digestibility • indigestible fiber • digestion rate • selective retention

¹ Presented as part of the 28th Annual Ruminant Conference: Potential for Altered Productivity of the Rumen Ecosystem, given at the 71st Annual Meeting of the Federation of American Societies for Experimental Biology, Washington, DC, March 29, 1987, and supported by grants from American Cyanamid Company, Cargill Nutrena Feed Division, Eastman Kodak Company Chemical Division, Eli Lilly and Company, Merck & Co., Inc., and SmithKline Beckman Corporation.

Manuscript received 15 May 1987. Revision accepted 3 September 1987.

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