Effects of active dry yeasts on the rumen microbial ecosystem: Past, present and future

Active dry yeasts (ADYs) are increasingly used in ruminant nutrition as feed additives to improve feed efficiency and performance and, at the same time, to prevent health disorders. They are particularly useful in high-producing ruminants whose digestive microbial balance can be altered by high-dietary energy input. As ADYs can survive and remain metabolically active in the gut, they can exert probiotic effects by interacting with the autochthonous microbial species responsible for feed digestion. Up to now, the most consistent positive effects of ADYs have been reported on rumen microbial activity in young ruminants, stabilisation of rumen pH and prevention of acidosis, as well as stimulation of growth and activity of fibre-degrading bacteria. Effects of ADYs vary depending on biotic factors such as the strain of yeast and its viability, but also on abiotic factors, such as the nature of the diet or animal management. It is important to better understand the way by which yeasts can act on rumen microorganisms in order to direct selection of new generation ADYs. The objectives of this paper are to review the most important findings on effects of ADYs in the rumen, to describe identified modes of action, and to provide thoughts for further strain selection and applications in ruminant nutrition.