Two suggested modes of action of yeast in stimulating rumen fermentation were investigated. The first,
that yeast respiratory activity protects anaerobic rumen bacteria from damage by 0,, was tested using
different strains of yeast that had previously been shown to have differing abilities to increase the viable
count of rumen bacteria. Succhurumyces cerevisiue NCYC 240, NCYC 1026, and the commercial
product Yea-Sac@, added to rumen fluid in vitro at 1-3 mg/ml, increased the rate of 0, disappearance
by between 46 and 89 YO. The same three preparations also stimulated bacterial numbers in an in vitru
fermenter (Rusitec). S. cerevisiae NCYC 694 and NCYC 1088, which had no influence on the viable
count in Rusitec, also had no effect on 0, uptake. Respirationdeficient (RD) mutants of S. cerevisiue
NCYC 240 and NCYC 1026 were enriched by repeated culturing in the presence of ethidium bromide.
S. cerevisiue 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. cevevisiue NCYC 240 no longer stimulating total bacterial numbers, although it still
increased numbers of cellulolytic bacteria. The second hypothesis, that yeast provides malic and other
dicarboxylic acids which stimulate the growtb of some rumen bacteria, was examined by comparing the
effects of yeast and malic acid on rumen fermentation in sheep. Three mature sheep were given 085 kg
barley/d plus 0.55 kg chopped ryegrass hay/d either unsupplemented, or supplemented with 4 g
S. cerevisiue NCYC 240/d or 100 mg L-malic acid/d either mixed with the diet or in aqueous solution
infused continuously into the rumen. Yeast increased the total viable count of bacteria (P < 0.05)
whereas malic acid did not, and no other effect of the treatments reached statistical significance. It was
concluded, therefore, that the stimulation of rumen bacteria by S. cerevisiue is at least partly dependent
on its respiratory activity, and is not mediated by malic acid.