An assessment of the stoichiometry of carbohydrate fermentation in the rumen of sheep
Measurements of the disappearance of total carbohydrate (TC), anterior to the duodenum, and the appearance of the major end products of TC fermentation in the rumen were made in sheep given varying levels of intake of pelleted dried grass (DG) and dried grass plus 50% barley (DGB) diets. TC disappearance was obtained as the difference between the TC intake and the flow of TC at the duodenum, measured in sheep fitted with simple, duodenal cannulae. Duodenal flow rates were calculated using the dual phase digesta markers ¹⁰³Ru-phenanthroline and ⁵¹Cr-EDTA. Individual VFA (acetate, propionate and butyrate) production rates were obtained using continuous infusion isotope kinetic techniques and methane production rates by whole-animal respiration calorimetry. Both the TC disappearance and the end-product appearance rates were linearly related to TC intake on each diet. TC disappearance increased with TC intake of both the DG and the DGB diets. No significant difference was demonstrated between the relationship of the two diets (P<0.01). In contrast, there was a major difference between the relationship obtained for VFA, methane and CO₂ production rates relative to intake on the DG and the DGB diets. When the DG diet was fed, the VFA and methane production rates increased with intake, but the rates declined with increasing intakes of the DGB diet. The betweeen-diet differences in the relationship of total VFA production rate versus TC intake was significantly different (P<0.02) and this was mainly associated with a significant (P<0.05) drop in propionate production rate as the TC intake of the DGB diet rose. C0₂ production rates determined by isotope dilution were confounded by an unidentified, probably physiological, source of variation. There was no discernible change in C0₂ production rate as intakes of TC were increased on either diet; mean values for C0₂ production rates on each diet were obtained from the whole population of values. Finally, when carbon balances were calculated using all the derived relationships, it was concluded that whereas the carbon associated with the TC disappearance in sheep given the DG diet could be accounted for by the carbon present in the main products of fermentation, there was no such agreement when the DGB diet was fed. These phenomena were discussed in relation to known parameters of rumen fermentation.