MODE OF ACTION OF THE YEAST SACCHAROMYCES-CEREVISIAE AS A FEED ADDITIVE FOR RUMINANTS

Two suggested modes of action of yeast in stimulating rumen fermentati on were investigated. The first, that yeast respiratory activity prote cts anaerobic rumen bacteria from damage by O-2, was tested using diff erent strains of yeast that had previously been shown to have differin g abilities to increase the viable count of rumen bacteria. Saccharomy ces cerevisiae NCYC 240, NCYC 1026, and the commercial product Yea-Sac c(R), added to rumen fluid in vitro at 1.3 mg/ml, increased the rate o f O-2 disappearance by between 46 and 89%. The same three preparations also stimulated bacterial numbers in an in vitro fermenter (Rusitec). S. cerevisiae NCYC 694 and NCYC 1088, which had no influence on the v iable count in Rusitec, also had no effect on O-2 uptake. Respiration- deficient (RD) mutants of S. cerevisiae NCYC 240 and NCYC 1026 were en riched by repeated culturing in the presence of ethidium bromide. S. c erevisiae NCYC 240 and NCYC 1026 stimulated the total and cellulolytic bacterial populations in Rusitec, while the corresponding RD mutants did not. Rigorous precautions to exclude air from Rusitec resulted in S. cerevisiae NCYC 240 no longer stimulating total bacterial numbers, although it still increased numbers of cellulolytic bacteria. The seco nd hypothesis, that yeast provides malic and other dicarboxylic acids which stimulate the growth of some rumen bacteria, was examined by com paring the effects of yeast and malic acid on rumen fermentation in sh eep. Three mature sheep were given 0.85 kg barley/d plus 0.55 kg chopp ed ryegrass hay/d either unsupplemented, or supplemented with 4 g S. c erevisiae NCYC 240/d or 100 mg L-malic acid/d either mixed with the di et or in aqueous solution infused continuously into the rumen. Yeast i ncreased the total viable count of bacteria (P < 0.05) whereas malic a cid did not, and no other effect of the treatments reached statistical significance. It was concluded, therefore, that the stimulation of ru men bacteria by S. cerevisiae is at least partly dependent on its resp iratory activity, and is not mediated by malic acid.