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The objective of this study was to evaluate the effects of supplementing ruminally protected and non-protected active dried yeast (ADY; AB Vista, UK) on fecal bacterial diversity of finishing beef steers. Seventy-five Angus steers (initial BW 448 kg) were randomly assigned to one of five treatments: control, antibiotics (ANT, 330 mg monensin + 110 mg tylosin/d), ADY (1.5 g/d), encapsulated ADY (EDY; 3 g/d), and mixture of ADY and EDY (MDY). The ADY (1.7×1010 cfu/g) was encapsulated with equal amount of ADY and capsule material. Steers were fed a diet containing 10% barley silage and 90% barley concentrate (DM basis), ANT, ADY, EDY and MDY were top-dressed at feeding. Fecal samples were rectally collected from 10 steers of each treatment on d 56 to determine fecal bacterial diversity using high throughput MiSeq sequencing. Alpha diversity indices were not different among treatments. Principal coordinate analysis revealed that samples were not clustered by treatments. Bacteroidetes and Firmicutes were dominant phyla in the fecal bacterial community for all treatments, with a tendency (P < 0.10) of ANT, EDY and MDY to have greater relative abundance of Bacteroidetes (averaged 51.3 vs 46.8%) but lesser Firmicutes (averaged 39.5 vs 47.9%) than control steers. About 50 to 61% of sequences remained unclassified and identified 32 genus, with Prevotella ranging from 15.8 to 25.5% as the dominant genus in all treatments. Notably, steers with ANT, EDY, MDY vs. control had greater (P < 0.05) relative abundance of Prevotella (averaged 23.5 vs 16.8%) but lesser (P < 0.03) Oscillospira (averaged 1.7 vs 2.8%). No differences between ADY and control were observed for aforementioned variables. The results indicate that supplementing ruminally protected or non-protected ADY have limited effect on diversity of fecal bacteria, whereas feeding protected ADY and ANT to finishing beef steers altered dominant fecal bacteria at phylum and genus level.

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