Do Pre- and Probiotics Help Horses?

Q: I see a lot of supplements these days that contain pro- and prebiotics. Are they necessary, and will they benefit my horse?–via e-mail

A: You’re right, it’s common to find ingredients such as Lactobacillus acidophilus, Mannan-oligosaccharides (MOS), and yeast in a range of products, some not even aimed at gastrointestinal (GI) health. And when you look at products designed specifically to support GI health, the options are endless. The premise behind these ingredients is that they help support a healthy GI environment. The question: Do they really work?


The U.S. Food and Drug Administration and the American Association of Feed Control Officials define probiotics (sometimes referred to as “direct fed microbials”) as a source of live, naturally occurring microorganisms. Probiotics are fed with the goal of enhancing or restoring the beneficial bacteria in the GI tract and are frequently measured in colony forming units (CFUs).

Most bacteria in the equine GI tract are in the hindgut (cecum and large colon); that means any live bacteria consumed as a probiotic must survive the acidic stomach before proliferating in the hindgut. A limited number of studies have looked at various bacteria’s success at surviving the stomach.

One strain that has been studied is Lactobacillus rhamnosus. Studies in humans have shown this microorganism helps control overpopulations of negative bacteria in the female genitourinary tract. In horses, researchers have studied it in foals, showing it can survive passage through the GI tract but only when fed in very large quantities; the researchers didn’t evaluate whether the same is true in mature horses. Other studies looking at different Lactobacillus strains showed a decreased incidence of foal diarrhea, suggesting the bacteria were benefiting the hindgut environment. Another study, however, revealed that Lactobacillus pentosus worsened severity of foal diarrhea.

Much of the work looking at the use of probiotics has been in horses with some form of diarrhea. As with the studies in foals, the results have sometimes been beneficial, but effective doses were often far larger than what commercial supplements commonly contain. Researchers have done less work in healthy horses with no clinical signs of GI distress. So, the questions of whether these ingredients benefit healthy horse, and, if so, at what amount are largely unanswered.

Saccharomyces cerevisiae yeast and Aspergillus oryzae are other common ingredients shown to have probiotic effects. Study results have shown that feeding yeast to horses on high-starch, low-forage diets can decrease lactate (a volatile fatty acid produced when simple carbohydrates are fermented) production in the hindgut. Research investigating whether feeding yeast to mature horses improves nutrient absorption yielded conflicting results, but there’s some indication that it might help improve digestibility of some complex carbohydrate fractions.

Some researchers have also looked at feeding yeast to horses with GI distress. Feeding 10 x109 live Saccharomyces boulardii every 12 hours for 14 days to horses with enterocolitis (inflammation of the small intestine and colon) reduced severity of their clinical signs compared to the horses given a placebo. Because most research conducted has used live yeast cultures fed to healthy horses, the National Research Council says further research is needed on the use of nonfermentative yeast cultures in horses.


Prebiotics are nondigestible food ingredients (such as oligosaccharides) that promote beneficial microorganisms in the GI tract. These complex carbohydrates are easily fermented by the microorganisms in the horse’s hindgut.

Ingredient labels often list MOS or FOS (fructooligosaccharides), which are naturally occurring short- to medium-chain carbohydrates that, when hydrolyzed in the hindgut, become a potential energy source for beneficial bacteria. By encouraging the growth of desired bacteria, it might be possible for them outcompete undesirable pathogenic bacteria.

Research in other species suggests that feeding MOS might prevent pathogenic bacteria from adhering to the GI tract’s lining. Some forms of MOS might also help prevent mycotoxin absorption in horses.

Contrary to popular belief, there is little evidence in horses that feeding FOS or MOS helps stimulate the immune system. However, they might help reduce intestinal disease, and this area warrants further study.

Factors to Consider

So where does this leave us as far as whether these ingredients in your supplements benefit your horse? It depends on a couple of factors:

  • Your horse and whether he’s currently healthy or showing signs of GI distress. Research shows that some ingredients might benefit horses with GI distress but aren’t so impactful if your horse is healthy.
  • The dose provided by the product, which often is far below those studied and shown to have some positive impact. For example, in a study of Thoroughbred and Quarter Horse broodmares fed either 0 g or 10 g of MOS, foals born to the supplemented mares had fewer cases of diarrhea that required treatment. Similarly, researchers found that pregnant mares consuming 20 g of live yeast a day had improved digestibility of dietary energy, protein, and fiber. While optimal probiotic doses have not been determined, researchers typically recommended starting with 1 x 109 to 1 x 1010 CFU per 50 kilograms (110 pounds) of body weight—substantially higher than most commercial probiotic recommendations. Do not assume that FOS, MOS, and yeast provided in milligram quantities and bacteria provided in millions of CFU will be as effective as when supplied in amounts that more closely follow current research findings.

Take-Home Message

If your horse is healthy, in good weight, eating a consistent diet low in starch, and producing normal manure, there’s little reason to add these ingredients to his ration. However, they might be beneficial for healthy horses consuming high-starch, low-forage diets or experiencing diet changes due to travel. Feeding these ingredients directly might act as an insurance policy, because research suggests that some pre- and probiotic ingredients are more effective when fed consistently rather than for short periods of time.