The extent of conversion of acetate-carbon to carbon dioxide in the rumen of a 40 kg wether consuming 1 kg molasses/d was estimated using isotope-tracer-dilution techniques. 2. There was a high rate of conversion of acetate to CO2 (6.0 g C/d) in the rumen. There were high concentrations in the rumen of Methanosarcina approximately 6 x 10(9)/ml which represents a significant proportion of the rumen bacterial biomass. These organisms are usually found in mud and sludge and are capable of oxidizing acetate. 3. The most likely explanation of these results was that there was an extensive secondary or sludge-type fermentation occurring in the rumen which results in volatile fatty acids being converted to CO2 and methane. In similar studies with sheep given lucerne (Medicago sativa) diets, conversion of acetate-C to CO2 within the rumen was not evident. 4. It is suggested that a major effect of the presence of secondary fermentation processes in the rumen may be to reduce availability of energy nutrients to the animal, and to alter the ratio protein:energy in the absorbed nutrients.
