| Summary: | The effect of dry yeast (DY) (<i>Saccharomyces cerevisiae</i>) supplementation in a high-concentrate diet was evaluated for rumen fermentation, blood parameters, microbial populations, and growth performance in fattening steers. Sixteen crossbred steers (Charolais x American Brahman) at 375 ± 25 kg live weight were divided into four groups that received DY supplementation at 0, 5, 10, and 15 g/hd/d using a completely randomized block design. Basal diets were fed as a total mixed ration (roughage to concentrate ratio of 30:70). Results showed that supplementation with DY improved dry matter (DM) intake and digestibility of organic matter (OM), neutral detergent fiber (NDF), and acid detergent fiber (ADF) (<i>p</i> < 0.05), but DM and crude protein (CP) were similar among treatments (<i>p</i> > 0.05). Ruminal pH (>6.0) of fattening steer remained stable (<i>p</i> > 0.05), and pH was maintained at or above 6.0 with DY. The concentration of propionic acid (C<sub>3</sub>) increased (<i>p</i> < 0.05) with 10 and 15 g/hd/d DY supplementation, while acetic acid (C<sub>2</sub>) and butyric acid (C<sub>4</sub>) decreased. Methane (CH<sub>4</sub>) production in the rumen decreased as DY increased (<i>p</i> < 0.05). <i>Fibrobacter succinogenes</i> and <i>Ruminococcus flavefaciens</i> populations increased (<i>p</i> < 0.05), whereas protozoal and methanogen populations decreased with DY addition at 10 and 15 g/hd/d, while <i>Ruminococcus albus</i> did not change (<i>p</i> > 0.05) among the treatments. Adding DY at 10 and 15 g/hd/d improved growth performance. Thus, the addition of DY to fattening steers with a high concentrate diet improved feed intake, nutrient digestibility, rumen ecology, and growth performance, while mitigating ruminal methane production.
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