Are Pre- and Probiotics Effective in Horses?

Tanja M. Hess, MV answers this reader question.

I know there are products on the market for people who experience trouble with their digestive systems or as prevention for gastrointestinal issues. Do these supplements work the same for horses? —Name withheld by request

Tanja M. Hess, MV (DVM), Msm, PhD: Digestion of forage in the horse’s gastrointestinal tract is mainly done by microbial bacteria. If the microbial balance is disrupted—for example, by illness or incorrect feeding—it has become common practice to supplement pre-and probiotics to restore the microbial balance in the horse’s hindgut or simply to maintain it. Prebiotics are dietary supplements that stimulate nonpathogenic intestinal bacteria by feeding them and helping them grow and colonize in the horse’s hindgut. Prebiotics may also improve the energy metabolism of the horse. 

Probiotics are microbes. They include bacteria or yeast cultures. They are directly fed to the horse and don’t have to be produced in the hindgut like prebiotics. Probiotics supposedly help to reduce the growth of pathogenic, or disease-causing, bacteria in the gastrointestinal tract, keeping the microbial balance in check. They are often administered to horses with poor appetite, horses who have been treated with antibiotics or who have had episodes of colic. Stressed horses, such as highly competitive dressage horses, might benefit as well. Yeast supplements have also been shown to improve weight gain in growing horses. 

To date, studies aren’t conclusive on whether pre- and probiotics are effective in horses. First of all, a horse’s gastro-intestinal tract is much larger than that of a human. It also has many more bacteria in it and it is difficult as well as extremely expensive to test whether the microbes successfully colonize the anatomic regions of the gut for which they are intended. Also, we don’t know how many microbes or fungi are needed to effectively address a certain problem. For example, in a study testing the effectiveness of oligofructose—a form of dietary fiber found in vegetables and other plants—in yearlings, it was found that it did increase volatile fatty acids in fecal matter. However, if more were absorbed by the gut, we don’t know. Also, currently there is no agency that controls or regulates the quality of supplements, as we have no ability to determine colonization. When you buy probiotics, the label usually tells you the amount of bacteria or fungi the supplement supposedly has. 

However, sometimes the product contains far fewer bacteria than stated on the label. Factors such as storage or manner of production, both of which may influence the life span of the microbes, influence the amount of colonies in the product.

If you decide to feed pre- or probiotics, you should do so for at least one week to see an effect as passage time takes about three days. It is safe to feed them long-term unless they are contaminated, which happens rarely.

Tanja M. Hess, MV (DVM), Msm, PhD is an associate professor at the College of Agricultural Sciences, Equine Science, at Colorado State University, in Fort Collins, Colorado. From 2003 to 2005 she has received the Pratt Fellow Honors.






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