Probiotics and Yeast Cultures

Q. My personal experience has been very positive in using probiotics and yeast in the equine diet. My question is about the best formulation of the products. Does using only yeast culture with no bacteria diminish the effects? Which bacteria are beneficial, and which are just "cosmetics" that aren't necessary? What is the minimum number of these organisms that must be present in order to be effective?


A. Since there are so little hard data on any of these products, especially in horses, I can't address these very specific questions. The activated yeast products do confer slight benefits in digestion in some studies. As far as the bacteria/probiotics go, no one really knows other than testimonials, as far as I know.

Single-species inoculants (products with only one bacterial or yeast species), however, do not make good sense in that the population of the equine hindgut is very complex. It has multiple bacterial and protozoal species that are all assumed to serve specific functions in the digestive process, but which have been very poorly characterized. There is some cattle and human literature that indicates benefits from probiotics, but both species are quite different in their digestive processes than the horse!

I would use such products with caution and carefully follow the manufacturer's directions. If benefits aren't seen in a reasonable period of time (a week or two), or any sort of adverse effect appears (such as diarrhea, colic, or poor appetite), I'd discontinue their use immediately.

About the Author

Sarah Ralston, VMD, PhD, Dipl. ACVN

Sarah L. Ralston, VMD, PhD, Dipl. ACVN, is a professor in the Department of Animal Sciences at Rutgers' School of Environmental and Biological Sciences, specializing in equine nutrition. Her research has focused on the effects of diet on metabolism, behavior, and the development of orthopedic disease in young horses, and she has additional interests in nutritional modulation of stress, metabonomics (the study of metabolic responses to drugs, environmental changes, and diseases), and pasture management. Previous research highlights were the pioneering work she did in nutrition for geriatric horses and post-surgical colics while at Colorado State University in the 1980s and the discovery of the correlation of hyperinsulinemia with development of osteochondrosis in young Standardbreds.

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