CAUSES OF INACCURATE PREDICTION OF VOLATILE FATTY-ACIDS BY SIMULATION-MODELS OF RUMEN FUNCTION IN LACTATING COWS

Extant mechanistic models of rumen function are unable to predict the molar proportion of volatile fatty acids (VFA) accurately. In order to make these models useful in investigating theories on nutrient flows that go beyond the rumen, the representations adopted need to be impro ved. This theoretical study was directed at identifying what parts of a rumen model may be responsible for the inaccurate VFA prediction. Fo r five distinct parts of a model, not involving the description of the microbiological metabolism, the hypothesis was tested that their inap propriate description can be a probable cause of inaccurate VFA predic tion. These five parts were: (1) the input functions of feed ingestion ; (2) the representation of insoluble, degradable substrate of roughag e and concentrate origin; (3) the kinetics of VFA absorption from the rumen; (4) the VFA coefficients that describe the stoichiometry of the conversion of fermented substrate into VFA; and (5) the representatio n of the rumen particle dynamics. Every hypothesis was tested by modif ying the model description and simulating it to steady state. Observat ions required were derived from digestion trials with lactating dairy cows. Simulation results demonstrated that the predicted molar proport ion of rumen VFA concentrations is particularly influenced by VFA abso rption kinetics and VFA coefficients. Although the description of part icle dynamics also had a large influence with certain choices of its p arameterization, it is probably a less important cause of inaccurate p rediction when rumen feed degradation (apparent from rumen outflow) is predicted well. In conclusion, to obtain improved predictions of the molar proportions of rumen VFA, further work is required on the repres entation of VFA absorption kinetics and of VFA coefficients of ferment ation stoichiometry. (C) 1997 Academic Press Limited.