The relative roles of the ruminal fluid and epithelium in the aetiology of ruminal acidosis

Acute lactic acidosis (ALA) and subclinical ruminal acidosis (SARA) are economically-important pathologies associated with poor adaptation of the ruminal ecosystem to diets high in soluble carbohydrates. Both pathologies result from an imbalance in the rate of production of fermentation acids relative to removal of these acids by absorption, flow of acids post-ruminally, and buffering of the acids by bicarbonate from the saliva and blood. In ALA, lactic acid accumulates, driving the pH of the ruminal fluid below 5.0, while subclinical acidosis is characterised by the accumulation of short-chain volatile fatty acids and a ruminal pH of 5.0-5.5. These reductions in ruminal fluid pH reflect changes in the profile and activity of the microbial populations, the buffering capacity of the ruminal fluid, and the absorptive capacity of the ruminal epithelium. The relative importance of the ruminal fluid (microbial and chemical properties) and the ruminal epithelium (absorptive capacity) is unknown. A novel ruminal transfaunation model was used to separate these two components of the pathologies. Four groups of sheep were formed by combining two ruminal fluid adaptions (adapted to high-energy density or low-energy density diets) and two ruminal epithelium adaptations (adapted to high-energy density and low-energy density diets). The four combinations of fluid and epithelium were then challenged with an intra-ruminal infusion of glucose to simulate conditions conducive to the development of acidosis. All groups developed acute acidosis in response to the glucose challenge and there were no differences between groups in the pattern of pH decline over a 24-h period. There was no difference between groups in ability to regulate rumen fluid pH, suggesting the major factor contributing to development of acidosis is rapid microbial fermentation of soluble carbohydrates irrespective of adaption of the epithelium or microbial populations. Modifications to the challenge protocol to more closely reflect normal dietary challenges would be a useful modification in future studies using this technique.