SUMMARY
Aim of study: To determine the dose response of Saccharomyces cerevisiae on rumen fermentation of concentrates, and to compare it with the effect of malate salts.Material and methods: S. cerevisiae (0.7, 1.4 and 2.1 mg/g) and malic acid salts (4, 8 or 12 mg/g) were added to barley and compared with barley alone (CTL), in three 24 h in vitro incubation series, using rumen inocula from beef cattle receiving a high concentrate diet. Yeasts were pre-activated by aerobic incubation for 24 h at 30ºC. Incubation pH was recorded at 8 and 24 h and volatile fatty acids (VFA) and lactate at 8 h were analysed.Main results: Gas produced with S. cerevisiae was higher than malate (p<0.001). Yeast addition linearly (p<0.01) and quadratically (p<0.05 at 4 h and from 10 to 18 h) increased gas production, but no dose response to malate levels was observed. Dry matter disappearance at 24 h was not affected by S. cerevisiae but increased linearly with malate. Microbial mass linearly increased with the level of yeast (p<0.01) and malate (p=0.09). Adding yeasts did not affect 8 h total VFA concentration compared with CTL, but linear valerate (p<0.01) and butyrate (p=0.092) increases, and a decrease of acetate (p=0.064) were detected. Malate salts linearly increased (p<0.05) total VFA concentration but did not affect VFA proportions.Research highlights: Addition of active yeasts linearly increased barley fermentation and microbial synthesis, whereas the effect of malate salts was of minor magnitude.