Live yeast supplementation improves rumen fibre degradation in cattle grazing tropical pastures throughout the year

The effect of live yeast (LY - Saccharomyces cerevisiae CNCM 1-1077) on in situ fibre digestibility, rumen fermentation, and rumen cellulolytic bacteria population of grazing Nellore cattle was evaluated throughout the year. Eight rumen cannulated steers were used in a 2 x 4 factorial arrangement of treatments: with or without LY, and the four seasons of the year. Yeast was given daily to provide 8 x 109 CFU per animal. Animals were kept on a rotational grazing system with mineral supplementation. Every 45 d, in situ rumen aNDF digestibility of 5 reference forages was determined after 24 and 48 h incubation. Rumen fluid was collected to measure yeast count and fermentation parameters, and rumen contents collected for quantitative PCR of cellulolytic bacteria. Although pasture composition varied greatly throughout the year, there was no LY*season interaction. Yeast supplementation increased aNDF digestibility in all seasons, but only when measured after 24 h of incubation. Among the four rumen bacteria evaluated, R. flavefaciens was the most prevalent and LY increased R. flavefaciens population in the summer and spring. However, LY decreased F Succinogenes population in autumn and spring. Yeast supplementation increased total SCFA in autumn, decreased acetate proportion and increase propionate proportion during summer. There was a tendency for greater valerate proportion when steers were fed LY. No significant effect on rumen pH, butyrate, and branched-chain fatty acids was observed. Supplementing LY can benefit fibre-degrading bacteria and increase fibre digestibility of grazing animals, independently of the season.