Secondary fermentation in the rumen of a sheep given a diet based on molasses

Abstract

I. The extent of conversion of acetate-carbon to carbon dioxide in the rumen of a 40 kg wether consuming I kg molasses/d was estimated using isotope-tracer-dilution techniques. 2. There was a high rate of conversion of acetate to COz (6.0 g C/d) in the rumen. There were high concentrations in the rumen of Methanosarcina approximately 6 x 1o8/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 COz and methane. In similar studies with sheep given lucerne (Medicago sativa) diets, conversion of acetate-C to COz 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. The major source of energy nutrients to ruminant animals is the volatile fatty acids (VFA) produced during fermentation in the rumen. It is generally assumed that most of the VFA produced are absorbed and provide energy to the animal and that the utilization of VFA in the rumen is negligible, but this may not always be true. There are good relationships between the entry rates and concentrations of each VFA in the rumen fluid of sheep for a wide range of diets based on cereals and dry herbage (see Leng, 1974). However, in studies with sheep in which molasses was the major dietary component, the concentration of acetate in the rumen fluid was far lower, for any given acetate entry rate, than in animals given roughage-based diets (J. B. Rowe, unpublished observations). This finding led us to examine the utilization of acetate in the rumen of molasses-fed sheep. In this study, estimates of the interconversions between rumen acetate-carbon and blood and rumen carbon dioxide were obtained by a method of isotope-tracer analysis developed by Nolan et al. (1976). The results indicated that there was appreciable conversion of acetate-C to C 0 2 within the rumen of sheep given molasses-based diets whereas there was negligible conversion in earlier studies with sheep given lucerne (Medicago sativa) diets. E X P E R I M E N T A L Animals and diet A mature Merino x Border Leicester wether (3 years old) weighing approximately 40 kg and prepared with a rumen cannula was used. The animal was given free access each day to I kg molasses mixture containing (g/kg): 910 molasses, 30 urea, 45 water, 7.5 rock phosphate, 2.5 sodium chloride, 5 trace element mixture (Susta-Vet-R; Bristol Veterinary Products). In