Probiotics are live microbes (bacteria, yeast, fungi and protozoa) that help horses maintain a healthy gut by protecting against “bad” microbes or pathogens. Maintaining a healthy microbiome promotes optimal digestive function and absorption of nutrients and helps support the immune system.

Horses that have digestive problems such as bloating, mild discomfort, loose stools (diarrhea), chronically dry stools (constipation) or horses that are intolerant to grain in the diet are good candidates for probiotic supplementation.

If you are concerned that your horse is losing weight or is unable to gain weight, you may want to consider probiotics to help balance their gut microbes and improve nutrient absorption.

There are a number of different strains of probiotic bacteria that are fed to horses to support gut health, help balance gut microflora and support digestion. Five popular strains are Lactobacillus acidophilus, Lactobacillus fermentum, Lactobacillus casei, Lactobacillus plantarum, and Enterococcus faecium.

These strains of probiotic bacteria survive the acidic environment of the horse’s stomach and are not affected by bile or digestive enzymes in the intestine. This means a large proportion of the probiotics consumed will reach the hindgut where they proliferate.

Probiotic supplements are designated as Generally Regarded As Safe (GRAS) by the US Food and Drug Administration. No adverse effects have been found in adult horses whose diets are supplemented with probiotics. They can safely be fed to healthy horses on a preventative basis or to horses that are experiencing digestive issues.

Mad Barn’s Optimum Probiotic is a pure blend of 5 strains of probiotics with no additional fillers. Each serving provides a guaranteed minimum of 20 billion viable CFUs (Colony Forming Units) of beneficial bacteria.

Optimum Probiotic

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Benefits of Probiotics for Horses

Probiotics, sometimes called ‘direct fed microbials’ are defined by the World Health Organization as “live organisms which when administered in adequate amounts confer a health benefit on the host.”

The most common strains of probiotics used to promote human and animal health are of the genus Lactobacillus, Bifidobacterium and Enterococcus. These have been evaluated in horses and have shown some beneficial effects in restoring a healthy gut and preventing dysbiosis.

Upwards of 70% of the horse’s immune system is supported by the gut. A healthy gastrointestinal tract can support the overall performance and well-being of your horse and prevent a number possible secondary health problems.

Below are the top 6 science-backed benefits of probiotics:

  1. Probiotics have been shown to help horses recover from digestive issues like acute enterocolitis (inflammation of the intestine). Probiotics stimulate the production of mucous within the intestine which forms a protective barrier and minimizes inflammation.
  2. Horses that are on antibiotics or dewormers can be given probiotics before, during and after treatment to help their gut re-establish the correct microbial populations. [1][2]
  3. Probiotics can lead to a healthier immune system by supporting a healthy gut and preventing “bad” or pathogenic bacteria from triggering inflammatory responses.[3]
  4. The improved immune status probiotics can confer may help horses meet the challenges of stressors such as transportation and competition.
  5. High grain diets can cause disruptions to the gut microbiome and lead to conditions like ulcers and laminitis. Probiotics may help counterbalance the effects of high-grain diets on the gut microbiome.
  6. Probiotics may help horses that have difficulty gaining weight (“hard keepers“) by improving digestion of feeds and absorption of key nutrients.

Adding probiotics to your horse’s diet is a cost-effective and safe way to help balance the gut microbiome and support digestive function.

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How do Probiotics Work?

The microbial population within the horse’s gut is very diverse. There are many “good” bacterial organisms that help to support immune function and aid in the digestion of food.

There are also some “bad” bacterial strains naturally present such as E. coli, C. difficile or Salmonella. These are considered bad because they produce toxins that are harmful to good bacteria and to the cells that line the gut.

Pathogenic microbes are also harmful themselves, because they can cause inflammatory responses when they attach to the lining of the gut.

Small amounts of pathogenic bacteria might not cause any significant health issues when the population of good bacteria vastly outnumber them. However, if conditions are favourable to the pathogenic microbes, they may take over the gut environment and there can be significant health consequences to the animal.

Feeding your horse probiotics can shift the balance back in favour of good bacteria, helping to maintain a healthy gut microflora, supporting optimal digestion and protecting a healthy gut lining.

Probiotics can improve gut and animal health in four main ways: [15]

1. Lowering the pH of the Gut

They produce substances that alter the local environment in the gut making it less hospitable to pathogenic bacteria. Such substances include lactic acid and acetic acid which lower the pH of the gut.

They also produce specific enzymes and antibiotics that directly kill the unwanted bacteria.

2. Combating Pathogenic Bacteria

Probiotics can inhibit the growth of pathogens through the principle of competitive exclusion. They bind to specific sites on the gut wall and block pathogenic bacteria from binding those sites.

When probiotics bind to the intestinal lining, they initiate beneficial pathways that help the horse’s cells produce protective mucous and create tight junctions with each other.

Probiotics also support healthy cell maturation and division. This is important because gut cells typically only live around 3 days. Cells are constantly being replaced through maturation of stem cells and cell division.

3. Stimulating Immune Cells

Probiotics can influence the animal’s immune system by stimulating immune cells that are nearby to produce antibodies and other beneficial compounds. This blocks the growth of pathogenic bacteria and supports the horse’s natural defenses against infection.

Probiotics have anti-inflammatory effects that prevent misguided immune responses, protecting the gut from inflammatory conditions like enteritis.

4. Protecting Against Toxins

Probiotics can inhibit or inactivate toxins that are released by pathogenic bacteria. They work by creating substances that bind toxins before they can negatively affect other cells.

Signs of an Imbalanced Equine Microbiome

You might suspect an imbalanced microbiome if you observe the following signs in your horse:

  • Bloating, abdominal discomfort: Shifts in the microbiome might cause gas buildup in the gut which can be observed as bloating and discomfort. These horses might show changes in their behaviour and mood, including a reluctance to being handled.
  • Colic and colitis: Horses with altered gut microbiomes are more likely to develop colic or colitis. [4]
  • Diarrhea or constipation: Changes in the microbiome can cause too much or not enough fluids to be pulled into the intestine leading to diarrhea or constipation. This could also be related to changes in gut motility.
  • Weight loss or poor weight gain: Affected horses might experience poor digestion of feed related to an imbalance of microbes. This may also be attributed to gastric ulcers or inflammatory bowel diseases.
  • Periodontal disease: Poor condition of teeth in horses has been associated with imbalances in the microbiome.
  • Laminitis: Dietary factors such as lush grass and high-grain diets are likely to change the microbiome and cause intestinal inflammation which can lead to laminitis.
  • Ulcers: Horses with gastric ulcers are likely to be on high-grain diets that can also cause hindgut issues. If your horse has Equine Gastric Ulcer Syndrome (EGUS) look out for signs of hindgut problems such as diarrhea, bloating or hypermotility.
  • Systemic inflammation: Fever, difficulty breathing, and rapid heart rate can all indicate infections in the body that might be due to intestinal inflammation caused by overgrowth of pathogenic bacteria.
  • Changes in behaviour: Horses with gut disturbances might be more agitated, reactive, less settled and hypervigilant of their environment. [5]

If you see any of these signs in your horse, they could indicate disrupted gut function. There are many potential causes of gut dysfunction in horses, but a common contributing factor is the composition of your horse’s diet.

You can submit your horse’s diet for analysis online and one of our nutritionists will be happy to provide a complementary evaluation.

The Equine Microbiome

The hindgut of the horse, which includes the cecum and colon (large intestine), is populated by an enormous number of microorganisms. The horse relies on these microbes in order to absorb nutrients and energy from high-fibre feed.

Horses are hindgut fermenters, which means that the large intestine and cecum is where the fermentation of ingested fiber primarily occurs. Billions of bacterial organisms help to digest plant fibres and starches, breaking them down into simpler molecules that can be absorbed and utilized in the horse’s body.

These microorganisms digest structural carbohydates like cellulose and hemicellulose and convert them into compounds called volatile fatty acids (VFA’s) which are absorbed through the intestinal wall.

While the hindgut does contain digestive enzymes which help to break down foods, the horse would not be able to obtain enough energy from the foods they eat without the presence of these intestinal microbes.

The volatile fatty acids (acetate, butyrate and propionate), also generally called short chain fatty acids, that are produced by microbes in the hindgut can provide as high as 42% of the horse’s energy requirement.[6]

Gut microbes also produce several B Vitamins like folate (Vitamin B9) and Vitamin K which are absorbed and used by the horse.

In analyses of the gut microbiomes of individual horses, results show that there is a lot of variability in the microbial profile between different horses. This can lead to significant differences in the condition and well-being of the horse.

To determine what a “normal” gut microbiome looks like, one study took fecal samples from seven horses every two weeks over the course of a year. The horses were on the same grass paddock year-round with no concentrate feed or supplements.

Even in this natural setting, minor shifts in the gut microbiome were continually occurring but no clinical signs of dysbiosis were observed. [7]

In general, the most abundant phylums of bacteria in the horse’s gut are: [8]

  • Firmicutes – 20-59% of the microbial population including Lactobacillus and Streptococcus
  • Bacteroidetes – 2-65% of the microbial population including Prevotella

The less abundant species include proteobacteria, actinobacteria and fibrobacteres. The equine microbiome also includes yeast species such as Saccharomyces boulardii and fungi which also contribute to digestion of feed and provide nutrients to the horse.

Dysbiosis in Horses

Dysbiosis is a general term for significant imbalance in the intestinal microbiota which has negative impacts on the health and function of the digestive tract.

There are several factors that can lead to, or are associated with, dysbiosis:

Changes to the Diet

The most common cause of a dramatic shift in the horse’s microbiome is a change in the diet.

Transitioning from a forage-based diet to a high-grain diet that contains rapidly fermentable starches and sugars causes significant disruptions to hindgut bacterial populations.

In one study, adding barley which is high in starch to the diet of ponies increased lactobacilli and streptococci in the cecum and colon within just 29 hours. This occurred whether barley accounted for 30% or 50% of the diet. [9]

The shift in microbial populations in this study was enough to change the ratio of volatile fatty acids produced by the microbes in the hindgut. Changes in volatile fatty acid composition can affect energy metabolism in the horse.


Administering antibiotics including penicillin, cephalosporins or fluoroquinolones can cause imbalances in the equine intestinal microbiome. Antibiotic use can cause colitis or inflammation of the intestinal mucosa which can lead to further issues like diarrhea.

Following a regimen of antibiotics, it takes at least 25 days for the microbial composition to return to the pre-antibiotic profile. This can provide opportunities for pathogenic bacteria such as Salmonella and C. difficile to take over. [10]

Adding a probiotic to the diet when horses are on antibiotics might help prevent dysbiosis during and after antibiotic treatment.


Anything that causes stress for horses can impact the microbiome.

A study that mimicked travel to a weekend horse show in an unfamiliar environment showed changes to the diversity of the microbiome and shifts in the microbial profile. [11]

Another study analyzed several factors that could cause stress to determine whether they alter the microbiome. The study showed that just eight hours of transportation was sufficient to change some classes of microbes. [12]

The researchers also showed that fasting for twelve hours resulted in alterations in the microbial profile. Fasting for long periods may occur during transportation to and from competitions.

Horses that undergo surgery with anaesthesia can also experience alterations in the gut microbiome.

Health and performance outcomes were not assessed in these studies. It remains to be seen whether microbial changes due to transportation and fasting have direct effects on performance in competition.


Deworming with anthelmintic drugs to kill parasites in the gut can cause significant changes to the gut environment including shifts in the microbial populations. [2]

Adding a probiotic to the dietary regimen before and after deworming could help to prevent gut problems that are often associated with administration of dewormers.


Research shows that horses that are obese or overweight have different microbial profiles compared to lean or normal horses.

In obese horses, the fecal abundance of bacteroidetes is lower and firmicutes is higher compared to non-obese horses. [6]

Across the weight categories of lean, normal, and obese there were specific bacteria that were only found in each category. [6]

More research needs to be conducted to understand those differences. It is not known whether the difference in bacterial population is a cause or a result of being overweight and whether probiotic supplementation can be used to address body condition.

Probiotics for Foals

Foals (<1 year of age) have an immature digestive system and are in the process of establishing their gut microflora.

Some studies show benefits of probiotic supplementation [13] while others show negative effects of probiotics [14] for the treatment of diarrhea in foals.

Consult a veterinarian if you are considering probiotics for foals.

Probiotic Supplements for Horses

Mad Barn carries three feed supplements for horses that contain probiotic bacteria or direct fed microbials: Optimum Probiotic, Optimum Digestive Health Pellets, and Visceral+.

Optimum Probiotic

Our Optimum Probiotic is a pure 5-strain blend of beneficial bacteria in a concentrated powder formula. It provides 20 billion viable CFUs per serving at a cost of as little as $0.27 per day.

This supplement is formulated for regular maintenance of hindgut health with a small serving size of just 1 gram per day.

Optimum Probiotic

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  • 20 billion CFUs per serving
  • Pure probiotic with no fillers
  • Blend of 5 beneficial strains
  • Only $10 for 1 month

Optimum Digestive Health

For horses that need additional gut support, Mad Barn’s Optimum Digestive Health Pellets provides a blend of probiotics, prebiotics, enzymes and nucleotides to support hindgut health.

Each serving size includes 80 billion colony forming units (CFUs) of beneficial bacteria and yeast.

Optimum Digestive Health is also designed to support the immune system.

Optimum Digestive Health

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


Visceral+ is a pelleted supplement that supports gastric and hindgut health in horses.

Visceral+ also supports the immune system in horses.


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  • Our best-selling supplement
  • Maintain stomach & hindgut health
  • Supports the immune system
  • 100% safe & natural

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  1. Pyles, MB et al. Effect of probiotics on antibiotic-induced changes in fecal bacteria of horses. J Equine Vet Sci. 2017.
  2. Kunz, Isabelle et al. Equine Fecal Microbiota Changes Associated With Anthelmintic Administration. J Equine Vet Sci. 2019. View Summary
  3. Furr, Martin. Orally Administered Pediococcus acidilactici and Saccharomyces boulardii–Based Probiotics Alter Select Equine Immune Function Parameters. Equine Vet J. 2014.
  4. Weese, JS et al. Changes in the Faecal Microbiota of Mares Precede the Development of Post Partum Colic. Equine Vet J. 2015. View Summary
  5. Destrez, Alexandra et al. Changes of the Hindgut Microbiota Due to High-Starch Diet Can Be Associated With Behavioral Stress Response in Horses. Physiol Behav. 2015. View Summary
  6. Biddle, Amy et al. Microbiome and Blood Analyte Differences Point to Community and Metabolic Signatures in Lean and Obese Horses. Front Vet Sci. 2018. View Summary
  7. Salem, Shebl et al. Variation in faecal microbiota in a group of horses managed at pasture over a 12-month period. Scientific Reports. 2018. View Summary
  8. Julliand, V. and Grimm, P. HORSE SPECIES SYMPOSIUM: The Microbiome of the Horse Hindgut: History and Current Knowledge . J Anim Sci. 2016. View Summary
  9. de Fombelle, A. et al. Feeding and microbial disorders in horses: 1-effects of an abrupt incorporation of two levels of barley in a hay diet on microbial profile and activities. J Equine Vet Sci. 2001.
  10. Kauter, Anne et al. The gut microbiome of horses: current research on equine enteral microbiota and future perspectives. Animal Microbiome. 2019. View Summary
  11. Perry, Erin et al. Effect of Road Transport on the Equine Cecal Microbiota. J Equine Vet Sci. 2018. View Summary
  12. Almeida, Maria et al. Intense Exercise and Aerobic Conditioning Associated with Chromium or L-Carnitine Supplementation Modified the Fecal Microbiota of Fillies. PLoS One. 2016. View Summary
  13. Swarthout, H et al. Effect of probiotic administration on diarrhea incidence, severity, and the fecal microbiome in neonatal foals. J Equine Vet Sci. 2017.
  14. Stroebel, Christina et al. Effects of Oral Supplementation of Probiotic Strains of Lactobacillus Rhamnosus and Enterococcus Faecium on Diarrhoea Events of Foals in Their First Weeks of Life. J Anim Physiol Anim Nutr. 2018. View Summary
  15. Schoster, A et al. Probiotic Use in Horses – What is the Evidence for Their Clinical Efficacy?. J Vet Intern Med. 2014. View Summary