Use of in vitro gas production models in ruminal kinetics

Physiological systems models for ruminant animals are used to predict the e xtent of ruminal carbohydrate digestion, based on rates of intake, digestio n, and passage to the lower tract. Digestion of feed carbohydrates is descr ibed in these models by a first-order rate constant. Recently, an in vitro gas production technique has been developed to determine the digestion kine tics in batch fermentation, and nonlinear mathematical models have been fit ted to the cumulative gas production data from these experiments. In this p aper, we present an analysis that converts these gas production models to a n effective first-order rate constant that can be used directly in rumen sy stems models. The analysis considers the digestion of an incremental mass o f substrate entering the rumen. The occurrence of passage is represented pr obabilistically, and integration through time gives the total mass of subst rate and total rate of digestion in the rumen. To demonstrate the analysis, several gas production models are fitted to a sample data set for corn sil age, and the effective first-order rate constants are calculated. The rate constants for digestion depend on ruminal passage rate, an interaction that arises from the nonlinearity of the gas production models. (C) 1999 Elsevi er Science Inc. All rights reserved.