Modelling rumen volatile fatty acids and its evaluation on net portal fluxes in ruminants

The production of volatile fatty acids (VFA) is still poorly predicted by rumen models despite its major contribution to ruminant energy supply. We aimed at building an empirical model describing production rates (PR) of individual VFA in the rumen. Three distinct bibliographic data bases were used. They gathered published in vivo measurements on ruminal VFA-PR, digestive fluxes and molar proportion of VFA in the rumen, and net portal fluxes of VFA. These data bases cover a wide range of intake levels and dietary composition. A meta-analysis was performed, using within-experiment models. Models were derived, describing total VFA-PR and individual VFA molar proportions in the rumen from variation in dry matter intake level and dietary composition. The VFA-PR was explained by intake of rumen fermentable organic matter (RfOM), calculated from detailed knowledge of the chemical composition of diets according to INRA Feed Tables, with the effect of other factors remaining non significant. The VFA molar proportions were explained by dry matter intake, digestible organic matter, digestible neutral detergent fibre and rumen starch digestibility. Based on these results, we proposed an empirical model to predict individual VFA-PR from simple predictors which represent the nature of digested substrates, as well as their site and extent of digestion. The model can be used under a wide range of levels of feed intake and dietary composition. Evaluation of predicted VFA-PR indicated close relationships with observed net portal appearance of VFA. The relationships were consistent with quantitative knowledge on VFA metabolism by the digestive tissues.