Prebiotics are types of fibre that are given to horses as a food source for the beneficial microbes in the hindgut. Prebiotics support fibre digestibility, gut health, and nutrient assimilation in the hindgut.

Horses with disrupted digestive function, observed as bloating, discomfort, diarrhea, constipation, free fecal water, or intolerance to grain might benefit from prebiotics in the diet.

For horses that struggle with undesirable weight loss or hard keepers that need to gain weight, consider prebiotics to support the gut microbiome and increase nutrient absorption.

Prebiotics are soluble plant fibres that can not be digested by enzymes in the stomach or small intestine. This differs from probiotics which are living microorganisms.

Prebiotics reach the hindgut where they are broken down by specific microbes and function as substrates to stimulate the growth of probiotics. This helps the beneficial microbes proliferate and synthesize volatile fatty acids and vitamins that support a healthy gut and are beneficial to the horse.

The most common prebiotics are oligosaccharides which are long chains of sugar molecules that serve as an energy source for beneficial microbes. Fructo-oligosaccharides (FOS) are derived from chicory root. FOS primarily supports microbes such as Bifidobacteria.

Mannan-oligosaccharides (MOS) derived from yeast are added to equine diets as toxin binders. They work by binding to pathogenic microbes and toxins to limit their harmful effects. This helps support a healthy gut microbial environment and minimize inflammation of the intestinal wall.

Mad Barn’s Optimum Digestive Health (ODH) is a complete gut health supplement that contains Bio-Mos, a propriety prebiotic derived from yeast that has been shown to improve gut function in horses. ODH also supplies probiotics, digestive enzymes, and yeast to support hindgut health and feed efficiency.

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Benefits of Prebiotics for Horses

A healthy gut is critical for supporting overall health and well-being of horses. The hindgut is home to trillions of micro-organisms that are necessary for digesting dietary fibre to meet the energy needs of the horse.

The hindgut is also the main site of synthesis for the water-soluble B-vitamins which have diverse roles in equine physiology.

Prebiotics nourish the beneficial microbes or probiotics, to help maintain a healthy gut environment and support efficient digestion and nutrient absorption.

Below are the top 6 science-backed benefits of prebiotics for horses:

1) Improved Tolerance to High-Grain Diets

High-grain diets are typically fed to performance horses to maximize energy supplied by the diet. However, rapid changes in the diet from high-forage to high-grain can negatively impact the hindgut environment.

Starch overload in the hindgut increases lactate production which lowers the pH, making the environment less favourable for fermentation of fibre and other complex plant carbohydrates. This lowers the pH of the hindgut, and in extreme cases can contribute to hindgut acidosis, colic and laminitis in the horse.

Adding short-chain FOS to a high-barley diet in mature geldings minimized shifts in microbial populations in the hindgut, suggesting prebiotic supplementation can be beneficial in reducing hindgut disruptions related to starch overload. [1]

2) Improve Digestion in the Hindgut

Fibre fermentation in the hindgut creates volatile fatty acids (VFAs) that are absorbed by the horse and used as an energy source. Shifts in the microbial populations can change the production of these and potentially leave horses in an energy deficit.

Horses given the prebiotic FOS for 10 days had increased levels of VFAs (acetate, butyrate, propionate) and lactate measured in feces. [2]

By providing prebiotics to nourish the microbes, they were able to produce more VFAs as an energy source for the horse. Horses with high energy demands, including growing and heavily exercised horses would likely benefit from prebiotics to increase VFA production by the hindgut.

3) Improve Insulin Sensitivity

Insulin resistance is present in 22% or more of horses, depending on breed, and contributes to laminitis in these horses.

Obese horses (body condition score of 8 out of 9) supplemented with short-chain FOS for six weeks had improved insulin sensitivity. This occurred even with no change in body weight or body condition score by the end of the study. [3]

More research is needed to understand how prebiotics support insulin sensitivity in obese horses. It may be related to improved fermentation of simple sugars and starches in the stomach and small intestine so that the glucose loads from the meals were reduced (65% of the calories in the diets used came from grain). Alternatively, there may be hormonal effects from the increased production of volatile fatty acids.

Whatever the mechanism, improving insulin sensitivity is critical to support metabolic health in obese horses and to decrease the risk of laminitis.

Non-obese horses also benefited from prebiotic supplementation using Jerusalem artichoke meal which contains FOS. These horses had improved glucose removal from the blood after a meal, suggesting improved insulin sensitivity. [4]

In lean horses, improving insulin sensitivity can help prevent weight gain and support a healthy metabolism, particularly in the liver and muscle tissue.

4) Improve Digestibility in Senior Horses

Senior horses often experience weight loss or struggle to maintain their weight. Improving digestive function and nutrient assimilation might help support weight management in these horses.

On control diets without prebiotics, senior horses (>23 years of age) had lower dry matter, energy, and fibre digestibility than mature horses (average 7.5 years of age). However, when horses were given short-chain FOS for 25 days, these differences were no longer evident. [5]

This suggests that prebiotics can help senior horses maintain feed efficiency and extract more nutrients from their diet which might support weight gain in these horses.

5) Decreased Diarrhea in Foals

Foals are born without any antibodies and are susceptible to pathogens in the environment that can cause diarrhea and other infections. They must get antibodies from the mare’s antibody-rich milk called colostrum.

Mares fed Bio-MOS during the last two months of pregnancy and the first month of lactation produced colostrum with higher levels of antibodies. [6]

Foals consuming colostrum from mares fed Bio-Mos had higher levels of these antibodies in their blood and decreased incidence of diarrhea.

Although the study only looked at a small number of foals, out of six foals in the control group five got diarrhea that needed treatment. Of mares fed Bio-Mos, all five foals in the study did not get diarrhea. [7]

6) Protect against Sand Colic

Consuming sand and dirt can cause diarrhea, weight loss and colic in horses. In one study, clinically normal horses were given a supplement containing a mixture of prebiotics, probiotics and psyllium to evaluate whether this could prevent sand colic.

After just 4 days, higher levels of sand were measured in the feces and this continued until the end of the study at 31 days. The increased fecal sand output suggests this supplement can support gut health to protect against sand colic. [8]

However, in a study of horses that already had significant levels of sand in the colon, supplementation with a similar product did not affect sand accumulation in the hindgut. [9]

More research is needed to determine whether other factors in a gut health supplement could support sand clearance in horses and protect against sand colic.

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Which Horses Could Benefit from Prebiotics

Supporting digestive health is crucial for the health and well-being of all horses. Up to 70% of the immune system is supported by the gut. Changes in the gut that irritate the intestinal lining or alter absorption patterns can trigger immune responses that affect the whole body.

One example is loss of barrier integrity, in which bacterial products such as lipopolysaccharide (LPS) enter the circulation. The ensuing systemic inflammatory response can contribute to laminitis, fever, and diarrhea.

Gut disturbances have been studied in relation to several conditions and diseases in horses including: [10]


Age, diet, gut pathogens, antibiotics, and stress are the main factors associated with shifts in the microbial environment that may contribute to the onset of these illnesses.

Senior Horses

The hindgut microbiome is relatively stable from 9 months of age to adulthood. Older horses (over 19 years of age) have lower diversity of microbes in the hindgut. [11]

Senior horses can benefit from prebiotic supplementation to support the beneficial microbes and improve nutrient digestibility and increase nutrient absorption. This is especially important for older horses that struggle to maintain a healthy weight or for those with Cushing’s/PPID that need to gain weight.

Performance Horses

Performance horses undergoing heavy work and exposed to stressors during travel and competition are susceptible to hindgut disturbances.

Their diets might also consist of high levels of grain to meet their energy needs which can lead to starch overload and dysbiosis. These horses could benefit from prebiotics to help mitigate the negative effects of stress and diet changes on microbial populations and nutrient digestibility.

Characteristics of Equine Prebiotic Supplements

Any nutrient can be considered a prebiotic if it meets the following criteria: [12]

  1. Is resistant to digestion in the upper digestive tract and reaches the hindgut intact
  2. Is fermentable by hindgut microbiota
  3. Has beneficial effects on the horse’s health
  4. Stimulates growth of select microbes
  5. Is stable through feed processing

How do Prebiotics Work?

Prebiotics are naturally occurring in feed or can be added to improve the nutritional and health value of the diet. The most common type is dietary fibres that can not be digested in the upper gastrointestinal tract. Instead, they reach the hindgut intact and are digested by microbes.

Specific microbes break the chemical bonds between sugar molecules in the prebiotic and use those sugars for energy. This allows beneficial microbes such as Bifidobacteria to flourish and out-compete detrimental microbes such as pathogenic Clostridia.

Beneficial microbes support a healthy gut environment and overall well-being of the horse in several key ways: [13]

  • Produce volatile fatty acids (VFAs): Short-chain fatty acids (primarily propionate, butyrate, and acetate) that the horse can absorb and use for energy. In total, the VFAs produced by the hindgut can supply up to 42% of the horse’s energy requirements. [14]
  • Lower the gut pH: These VFAs also lower the pH of the gut which can limit the growth of pathogenic microbes.
  • Produce antibiotics: The beneficial microbes produce antibiotic substances that can limit the growth of pathogenic microbes.
  • Enhance natural defenses of the gut: Microbial products such as butyrate can be used by intestinal cells to build the mucous barrier which protects cells of the intestinal wall from inflammatory products and the acidic environment of the gut.
  • Modify immune cells of the gut: Beneficial microbes can communicate with immune cells nearby to help protect the horse from infection.

Common Prebiotics for Horses

Fructo-oligosaccharides (FOS)

FOS are found in many plants including wheat and barley. FOS are also found at high levels in chicory root and Jerusalem artichoke. For commercial purposes, FOS is typically extracted from chicory root or synthesized from sucrose by fungi such as Aspergillus niger.

FOS is a dietary fibre consisting of chains of glucose and fructose molecules. Sequences that are shorter than 10 molecules are considered short-chain FOS (scFOS).

Inulin is a common prebiotic derived from chicory root. It is a long-chain FOS containing up to 60 sugar molecules.

These prebiotics are typically added to equine diets to nourish beneficial bacteria, such as those of the genus Bifidobacteria or Lactobacilli.

By supporting the colonization of these bacteria, FOS increase the production of short-chain fatty acids and other acids that lower the pH in the intestinal environment. Lowering the pH helps inhibit the growth of pathogenic bacteria that can disrupt gut health. [15]

Mannan-oligosaccharides (MOS)

Mannan-oligosaccharides (MOS) is a toxin binder that is derived from the cell walls of yeast. These are not strictly prebiotic, because they are not consumed by microbes in the gut. Instead, they promote gut health by other mechanisms including binding to pathogenic microbes and supporting gut tissue.

MOS is commonly used in animal agriculture as an alternative to antibiotics because it can attach to lectins on the surface of pathogenic microbes such as E. coli, Campylobacter and Salmonella and reduce their colonization in the gut.

Although MOS do not directly feed beneficial microbes, decreasing the pathogenic load can support proliferation of beneficial microbes such as Lactobacilli.

MOS is positively charged and can bind bacterial components such as lipopolysaccharide (LPS) that are negatively charged. This prevents LPS from being absorbed and triggering inflammatory responses in the body. [16]

MOS supplementation in other animals has been shown to alter local immune cells in the intestine, helping them respond quickly to viral infections while dampening the immune response to allergens. This helps the animal respond appropriately to infections while minimizing chronic inflammation associated with a hypersensitive immune system. [17]


Bio-Mos is a propriety product developed by Alltech that is derived from the outer cell wall of a specific strain of yeast called Saccharomyces cerevisiae. It is used in a variety of livestock feeds to support gut integrity and animal performance.

This form of MOS can bind mycotoxins, which are molds found in feed that can negatively affect the horse’s health. Bio-Mos decreases their toxic effect and helps eliminate them from the gastrointestinal tract. [18]

In horses, Bio-Mos has been shown to support gastrointestinal integrity and stability, efficient transfer of nutrients from mare to foal, and performance. [19]

How to Feed Prebiotics to Horses

Prebiotics can be fed alone as a top dress. This can have benefits for digestive and metabolic health as reviewed in this article.

However, they are likely more effective when included in a complete gut health supplement that contains prebiotics, probiotics, and other factors that support the gastrointestinal tract.

Probiotics are live microbes and prebiotics are the foods they need to thrive. Combining the two can have additive effects compared to prebiotics alone. [20]

Mad Barn offers two gut health supplements that contain prebiotics (Bio-Mos), probiotics, yeast, digestive enzymes and other nutritive factors that support gut function:

  1. Visceral+ supports gastric and hindgut health as well as immune function.


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    • Our best-selling supplement
    • Maintain stomach & hindgut health
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  2. Optimum Digestive Health is a pelleted formula designed to support hindgut health
  3. Optimum Digestive Health

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    • Prebiotics, probiotics & enzymes
    • Support hindgut development
    • Combats harmful toxins in feed
    • Complete GI tract coverage

To learn more about these products or find out what is best for your horse, submit your horse’s diet for a complementary evaluation by our equine nutritionists.

Adverse Effects of Prebiotic Use

Prebiotics are a natural part of the equine diet and generally considered safe. Because microbes exist throughout the gastrointestinal tract, not just the hindgut, prebiotics might also influence microbial populations in the stomach and small intestine.

In one study, mature horses were given Jerusalem artichoke meal which is abundant in short- and long-chain FOS. The researchers observed changes in the abundance and diversity of microbes in the hindgut, as well as the stomach and small intestine. [21]

This suggests that digestion of prebiotics in this supplement begins in the foregut. This can increase VFA production in the stomach, decrease gastric pH and potentially damage the gastric mucosa. [22]

These studies highlight the need to choose prebiotics and probiotics that survive the acidic stomach environment without being digested there and reach the hindgut intact.

In several species, Bio-Mos and the probiotics found in Mad Barn supplements have been shown to survive the gastric environment and target the hindgut. [23] Therefore, these are unlikely to negatively shift bacterial species in the foregut.

Nonetheless, our complete gut health products also contain compounds that nourish and protect the gastric mucosa to support your horse’s foregut.

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  1. Respondek, F. et al. Effects of dietary short-chain fructooligosaccharides on the intestinal microflora of horses subjected to a sudden change in diet. J Anim Sci. 2008. View Summary
  2. Berg, EL. et al. Fructooligosaccharide supplementation in the yearling horse: Effects on fecal pH, microbial content, and volatile fatty acid concentrations. J Anim Sci. 2005. View Summary
  3. Respondek, F. et al. Dietary supplementation with short-chain fructo-oligosaccharides improves insulin sensitivity in obese horses. J Anim Sci. 2011. View Summary
  4. Glatter, M. et al. Glycaemic and insulinaemic responses of adult healthy warm-blooded mares following feeding with Jerusalem artichoke meal. J Anim Physiol Anim Nutr. 2017. View Summary
  5. Heaton, C.P. et al. Are prebiotics beneficial for digestion in mature and senior horses?. J Equine Vet Sci. 2019.
  6. Spearman, K.R. Effect of mannan oligosaccharide (MOS) supplementation on the immune system of mares and their foals. University of Florida. 2004.
  7. Ott, E.A. Influence of Bio-Mos®, a mannan oligosaccharide supplement, on the immune system of the mare and neonatal foal. Engormix. 2007.
  8. Landes, A.D. et al. Fecal Sand Clearance Is Enhanced with a Product Combining Probiotics, Prebiotics, and Psyllium in Clinically Normal Horses. J Equine Vet Sci. 2008.
  9. Hassel, D.M. et al. Evaluation of Fecal Sand Clearance in Horses With Naturally Acquired Colonic Sand Accumulation With a Product Containing Probiotics, Prebiotics, and Psyllium. J Equine Vet Sci. 2020. View Summary
  10. Kauter, A. et al. The gut microbiome of horses: current research on equine enteral microbiota and future perspectives. Anim Microbiome. 2019. View Summary
  11. Dougal, K. et al. Characterisation of the faecal bacterial community in adult and elderly horses fed a high fibre, high oil or high starch diet using 454 pyrosequencing. PLoS One. 2014. View Summary
  12. Markowiak, P. and Slizewska, K. Effects of Probiotics, Prebiotics, and Synbiotics on Human Health. Nutrients. 2017.
  13. Lindinger, M.I. Integrated Nutraceutical – Nutritional Approaches to Address Equine Leaky Gut Syndrome. Vet Sci Res. 2019.
  14. Biddle, A.S. et al. Microbiome and Blood Analyte Differences Point to Community and Metabolic Signatures in Lean and Obese Horses. Front Vet Sci. 2018. View Summary
  15. Gurbuz, E. et al. Effects of supplemental fructo-oligosaccharide and mannanoligosaccharide on nutrient digestibilities, volatile fatty acid concentrations, and immune function in horses. Turk J Vet Anim Sci. 2010.
  16. Diaz, T.G. et al. Inclusion of live yeast and mannan-oligosaccharides in high grain-based diets for sheep: Ruminal parameters, inflammatory response and rumen morphology. PLoS One. 2018.
  17. Halas, V. and Nochta, I. Mannan Oligosaccharides in Nursery Pig Nutrition and Their Potential Mode of Action. Animals. 2012.
  18. Kogan, G. and Kocher, A. Role of yeast cell wall polysaccharides in pig nutrition and health protection. Livestock Sci. 2007.
  19. Alltech, Inc. Bio-Mos. Alltech. 2012.
  20. Coverdale, J.A. HORSE SPECIES SYMPOSIUM: Can the microbiome of the horse be altered to improve digestion?. J Anim Sci. 2016. View Summary
  21. Glatter, M. et al. Modification of the equine gastrointestinal microbiota by Jerusalem artichoke meal supplementation. PLoS One. 2019. View Summary
  22. Cehak, A. et al. Does prebiotic feeding affect equine gastric health? A study on the effects of prebiotic-induced gastric butyric acid production on mucosal integrity of the equine stomach. Res Vet Sci. 2019. View Summary
  23. Spring, P. et al. A review of 733 published trials on Bio-Mos®, a mannan oligosaccharide, and Actigen®, a second generation mannose rich fraction, on farm and companion animals. J Appl Anim Nutr. 2015.