Table 1.

Summary of the rumen microbial composition of ruminants fed yeast.

Item	Diet	Dose of yeast	Response	Source
Weaner lambs	High concentrate diet	1 mL live yeast culture (1.5×109 to 2.0 × 109 CFU/mL) per kilogram live weight	Increased ciliate protozoa; increased feed intake and growth rate; increased entodinomorphs population	Tripathi and Karim (2011)
Calves	Milk replacer, starter feed, and hay	Yeast at 7.5 × 108 CFU/L before weaning and 3 × 109 CFU/kg after weaning	Increased total Lactobacilli population around weaning period	Fomenky et al. (2017)
Holstein bull calves	Milk and starter grains (no forage)	0.5% and 1% of yeast fermentation product on an as-fed basis in starter diets and milk	Increased Butyrivibrio and decreased Prevotella composition of the rumen fluid	Xiao et al. (2016)
Dry Holstein cows	F:C was 70:30	Yeast at 3.3 g/kg of diet per d (1 × 1010 CFU/d)	Increased the relative abundance of Bacteroidales, Lachnospiracea, and Flexilinea	Bach et al. (2019)
Rumen-fistulated Holstein dairy cows	F:C was 40:60	A yeast culture at 10 g/cow per d (20 × 109 CFU/cow per d)	Increased the population of Fibrobacter succinogenes during adaptation; an increased population of Megasphaera elsdenii during SARA	Malekkhahi et al. (2016)
Finnish Ayrshire cows	F:C was 50:50	Live yeast at 0.5 g/cow per d (1010 CFU/cow per d)	No effects on animal performance and rumen fermentation	Bayat et al. (2015)
Qinchuan cattle	F:C was 55:45	1 and 2 g live yeast or 20 g yeast cell wall polysaccharides/cow per d	Higher digestibility of ADF and NDF; increased the population of F. succinogenes S85, Ruminococcus albus 7 and R. flavefaciens FD-1; decreased the percentage of Streptococcus bovis JB1	Peng et al. (2020)
Holstein steers	F:C was 50:50	Live yeast at 15 g/d	Increased the population of R. albus, R. champanellensis, R. bromii, R. obeum, M. elsdenii, Desulfovibrio desulfuricans, and D. vulgaris	Ogunade et al. (2019)

F:C = forage:concentrate ratio; SARA = sub-acute rumen acidosis.