A total of 21 rumen-cannulated wether sheep and six rumen-cannulated Ayrshire cows were used in a series of experiments to investigate fermentation in the rumen in animals given silage diets and the nominal metabolism of silage lactic acid. Dietary experiments with sheep (three experiments) and cows (two experiments) given diets consisting of formic acid–preserved perennial ryegrass (Lolium perenne) silages, in some cases with supplements of rolled barley, were conducted to investigate the ruminal concentrations of lactic acid and of volatile fatty acids (VFA) after feeding. Ingestion of silage was followed by a short-lived peak in lactic acid concentration in the rumen and by a subsequent increase in the concentration of propionate or propionate and butyrate. Butyrate production was more evident in cattle than in sheep. Taken as an average over a 6 h period of sampling the proportions (m-mol/mol total VFA) of acetate, propionate and butyrate in the rumen in animals given silage alone were typically 590–600, 210–250 and 70–150 respectively. When silage was supplemented with barley, in both sheep and cattle, the proportion of propionate was reduced and the proportion of butyrate was increased. In one experiment with sheep these changes in fermentation pattern were shown to be accompanied by an increase in the number of rumen protozoa from 2·78×105/ml to 18·66·105/ml (P <0·01). However, in a subsequent experiment it was shown that defaunation led to an increase in the proportion of butyrate in the rumen.

Twelve estimates were also made with sheep of the metabolism of lactic acid infused into the rumen in a single dose of 20–50 g. These experiments showed that both (L+) and DL-lactio acid were metabolized equally rapidly with a half-life of approximately 25 min. In faunated animals the major products of fermentation were propionate with a small amount of butyrate. In defaunated animals lactate metabolism was slowed and the major product of fermentation was butyrate.

In vitro studies of lactate fermentation were carried out using strained liquor and isolated bacterial and protozoal fractions from the rumen of sheep receiving silage diets. The rates of lactate disappearance (μg/mg protein/h) were 50 and 22·2 for the rumen liquor and bacterial fraction, respectively, which were significantly (P <0·05, n = 5) less than the rate of 294·7 for the protozoal fraction. Rumen liquor, bacterial and protozoal fractions yielded a similar mixture of fermentation end-products, propionate accounting for approximately 60% of the VFA produced.

The results are discussed in relation to the metabolism of silage lactic acid in the rumen, the production of VFA and the yield of ATP for microbial cell synthesis. It is concluded that the pathways of lactate fermentation are regulated by the rumen microbial population and that protozoa may have an important role in the regulation of lactate fermentation.