Modeling of rumen water kinetics and effects of rumen pH changes.

Mechanistic elements for determining water kinetics and effects of pH on VFA production and cellulose hydrolysis were incorporated into a model of rumen digestion and metabolism. Elements necessary for water kinetics were estimates of salivary flow during eating, resting, and ruminating, time course and amount of drinking water, osmotic flux across the rumen wall based on changes in rumen fluid osmolality, and liquid turnover rate. Osmotic flux equations predicted a significant net absorption of water from rumens of dairy cattle. Water kinetic equations predicted experimental changes in rumen volume during twice daily feeding, and produced marker dilution data qualitatively resembling published data. Increased marker dilution during feeding was due to increased rumen volume. An empirical equation predicting rumen pH from VFA concentration was validated against literature data. Changes in molar percentages of acetate and propionate as pH declined below 6.2 were produced by decreasing cellulose hydrolysis and fermentation and by altering stoichiometric coefficients for fermentation of soluble sugars and starch. Insufficient data prevented an adequate challenge of equations predicting decreases in cellulose hydrolysis and changes in stoichiometric coefficients as pH decreased.