Modelling production and portal appearance of volatile fatty acids in dairy cows.

Abstract

Values of stoichiometric coefficients were estimated to describe the production of volatile fatty acids (VFA) from fermented substrate in the rumen of lactating cows. Coefficient estimates were derived by regression of in vivo data of rumen digestion to a stoichiometric model with non-linear regression techniques. The model explained only part of the simulated range of VFA molar proportions. The theoretical potential of the statistical procedure was investigated by simulation studies. These studies revealed that, although the model explained only a small part of the simulated range of VFA molar proportions, coefficient estimates were fitted accurately when the ideal model is used. Simulation results closely corresponded to those obtained with regression of the in vivo data. An evaluation of the in vivo coefficient estimates with independent in vivo data showed again similar results and seemed to confirm the predictive potential of the coefficient estimates. Additionally, a model was developed which describes VFA metabolism by stomach epithelia. Inputs and outputs of the model are amounts of VFA produced in the rumen and VFA released to portal blood. The current model describes the absorption and the activation of VFA by CoAsynthetases as the first step in VFA metabolism. Activation of VFA was described by competitive inhibition between individual VFA. Parameters were derived from in vitro studies of CoA-synthetase activities in tissue samples of ovine or bovine rumen epithelium. The model is a first attempt to describe the interaction between production, absorption and metabolism of VFA, and it assists in relating VFA molar proportions or production rates in the rumen to appearance rates or concentrations of VFA in portal blood.