Review: The effects of dietary yeast and yeast-derived extracts on rumen microbiota and their function

This review outlines our current knowledge of dietary yeast supplements and their components and describes their effects on the ruminal microbiota and function. Yeast supplementation in ruminant diets has been evaluated for its impact on fiber digestibility, ruminal fermentation patterns, and nutrient use efficiency. The primary effect of yeast appears to be the stimulation in the growth of specific ruminal bacteria populations, specifically, cellulolytic and lactate-utilizing bacteria. The relationship of lactate-utilizing ruminal bacteria with dietary yeast, has been intensely researched due to their role in affecting ruminal pH parameters and ultimately animal health. Yeast supplementation has been shown to modulate ruminal pH, particularly when used in combination with high concentrate diets. This is likely due to yeast stimulating the growth of both lactate-utilizing bacteria and protozoa. Despite these recognized benefits of yeast supple-mentation in the rumen, results are variable and inconsistent across published research, likely due to the lack of consistency among yeast strain, dose, and physiological or experimental conditions. This narrative review describes yeast and its cellular components and outlines the impact of yeast on 1) ruminal bacterial diversity and protozoa numbers, volatile fatty acid profile, acetate: pro-pionate ratio, lactate accumulation, pH, CH4, NH3-N, and feed digestibility, 2) dairy production parameters including dry matter intake and milk production, and 3) ruminant health.

Automated summary

The review outlines the effects of dietary yeast supplements on ruminal microbiota and function, with yeast appearing to stimulate the growth of specific ruminal bacteria populations. Although yeast supplementation has benefits in the rumen, its effects are variable and inconsistent across studies, likely due to differences in yeast strains, doses, and experimental conditions.