Yeast are single-cell organisms that include over 1,500 unique species found naturally in soil, plants, fruit, and on the skin and in the intestinal tract of mammals.
Live yeast are used as probiotic supplements for horses, primarily to improve fibre digestion in the hindgut, bind toxins and support healthy intestinal tissue.
Other signs of impaired gut function include laminitis, poor coat and hoof quality, and frequent allergies.
The most common species of yeast used in animal feed are Saccharomyces cerevisiae, the same active culture used in baking and brewing. They are made up of several components that are known to benefit gut health including mannan-oligosaccharides, nucleotides and beta-glucans.
Yeast themselves are also high in digestible proteins, vitamins (vitamin B6, thiamin, biotin, riboflavin, nicotinic acid and pantothenic acid), magnesium and zinc. When they are digested in the hindgut these nutrients can be absorbed to support the health of the horse. 
Mad Barn’s gut health supplements Visceral+ and Optimum Digestive Health contain Yea-Sacc 1026â„¢, a proprietary live yeast culture based on Saccharomyces cerevisiae strain 1026. This strain has been shown to normalize pH in the gut and support the growth of fibre-digesting microbes which increases energy availability to the horse.
Benefits of Yeast for Horses
The best defense against disrupted hindgut function is a well-balanced diet that meets the horse’s energy, protein and vitamin/mineral needs while supporting a healthy microbial environment in the hindgut (cecum and colon).
Beneficial microbes in the hindgut digest structural carbohydrates (fibre) in forages to produce nutrients that the horse can absorb.
These include volatile fatty acids (acetate, propionate, and butyrate) used for energy by the horse, amino acids and B-vitamins that support numerous enzymatic processes in the horse.
Feeding your horse live active yeast cultures can support gut health by seeding the gut with beneficial yeast strains, aiding in fermentation function, and out-competing harmful pathogenic microorganisms.
Below are the top 8 science-backed benefits of yeast for horses:
1) Minimize negative effects of high-starch diets
One of the main causes of hindgut dysfunction in horses is a high-grain diet. When starch in the diet exceeds the capacity for digestion in the small intestine, starch can reach the hindgut.
Starch fermentation in the hindgut favours microbes that produce lactic acid, causing the hindgut to become excessively acidic. Too much lactic acid lowers the pH of the hindgut, resulting in a condition known as hindgut acidosis.
Hindgut acidosis can make the horse susceptible to hindgut ulcers, inflammation, colic, laminitis and other health concerns.
Adding yeast cultures (Yea-Sacc 1026â„¢) to the diet of horses fed a high-starch diet helped minimize hindgut disturbances by stimulating the growth of microbes that use lactic acid. These microbes increase pH in the hindgut, meaning they reduce the level of acidity to bring the hindgut environment back into balance.  
These shifts in lactic acid levels support fibre digestion in the hindgut by making the environment more favourable to microbes that break down cellulose and hemicellulose in forages.
In a recent review, 11 out of 14 studies in which horses were fed live Saccharomyces cerevisiae yeast showed improvements in fibre digestibility.  This may support nutrient assimilation, helping your horse extract and absorb more nutrients from their forage.
2) Improve Protein Digestion
Dietary proteins are predominantly broken down and absorbed in the small intestine. Dietary protein that reaches the hindgut can be broken down into amino acids by some microbes.
Live yeast culture supplementation in yearlings not only increased fibre digestibility, but also increased microbial production of amino acids. 
Whether these microbial amino acids can be absorbed in the equine hindgut in sufficient quantities to affect amino acid balance of the horse is unclear. Based on minimal expression of amino acid transporters that move amino acids into the blood, there is likely little amino acid absorption occurring in the hindgut of horses. 
However, these microbial amino acids can be used by other microbes to make proteins that support their function and proliferation in the hindgut.
3) Improve phosphorus and calcium availability
Most phosphorus in the horse’s diet is bound within phytate which can only be digested by microbes to make this mineral available for absorption.
Fibre digesting microbes in the hindgut produce the enzyme phytase which breaks down phytate. Horses supplemented with yeast cultures had higher digestibility of phosphorus and calcium which releases these minerals for absorption in the gut. 
Young growing animals have a high demand for calcium and phosphorus to support healthy bone development. Foals given yeast from 8 to 24 months of age were more likely to have closed distal radius growth plates at 24 months than those not given supplemental yeast. 
This suggests yeast can support healthy bone formation in growing animals and might help decrease the risk of developmental orthopedic disorders such as osteochondritis dissecans.
Improving mineral availability for the horse can also decrease phosphorus output in feces which helps reduce the negative environmental impacts of animal waste.
4) Improve nutritional value of low-quality hay
Low-quality hays are often unavoidable. Adding the right supplements can help balance the diet to ensure your horse is still meeting their nutritional requirements.
The digestibility of low-quality hay can be improved by adding yeast to the diet. Horses consuming low-quality Bermuda grass hay supplemented with yeast had higher dry matter, crude protein, and fibre digestibility compared to horses with no added yeast. 
It is likely that horses consuming low-quality hays require more than just yeast to help balance the diet. A hay analysis and diet evaluation by an equine nutritionist is always recommended to ensure your horse is meeting all their nutrient requirements.
5) Improve athletic performance
The improved digestive function and nutrient availability can help meet higher energy demands of performance horses.
In one study, previously untrained horses gradually increased exercise to 35 minutes of treadmill running over a period of six weeks. Half of the horses received yeast culture supplementation and the other half were not supplemented to serve as a control group.
Horses consuming yeast cultures had lower lactate levels at the end of 35 minutes of exercise and lower heart rates, suggesting improved athletic fitness. 
More research is required to understand the effects of yeast supplementation on athletic performance in trained horses.
6) Support nutritional demands of pregnant mares
Mares in the stages of late gestation and early lactation have higher demands for energy, protein, and minerals.
Mares supplemented with yeast culture from 4 weeks before foaling to 8 weeks after had increased fibre and protein digestibility and increased availability of calcium and phosphorus. 
A simple dietary change of added yeast significantly improved nutritional value of the pregnant mares’ diet, improving nutrient availability for the growing foal.
7) Better growth in suckling foals
Mares supplemented with yeast culture from 4 weeks before foaling to 8 weeks of lactation produced more milk. The milk they produced also had higher energy, sugar, fat, protein, and total amino acid content. 
The foals consuming this milk had greater intake of these nutrients, especially the essential amino acids leucine, lysine and valine.
The foals nursing on yeast-supplemented mares had faster body weight gain and growth in the withers at 6 weeks of age, compared to foals nursing on non-supplemented mares. In fact, yeast supplementation of mares resulted in a 24% greater conversion of mare feed to foal body mass. 
Supplementing the pregnant and lactating mare’s diet helps meet her nutrient needs and those of her foal to support healthy growth in the early postnatal period.
You can submit your horse’s diet for complementary evaluation and customized recommendations by our equine nutritionists.
Reasons to Include Yeast in the Diet
Supporting gut health by adding yeast and other probiotics to the equine diet can benefit all horses by improving feed digestibility and promoting the function of the immune system.
Poor gut health can contribute to several conditions and diseases including:
- Suboptimal growth in young animals due to poor nutrient absorption
- Poor hoof, coat and skin quality related to impaired nutrient absorption
- Allergies due to a hypersensitive immune system
- Inflammation due to impairments in gut barrier function that allows absorption of bacterial products
- Equine metabolic syndrome can arise from inflammation and altered nutrient absorption due to an unhealthy gut
- Laminitis risk increases in equine metabolic syndrome and under chronic systemic inflammation
- Submaximal exercise performance due to energy deficit, poor stress tolerance, impaired nutrient absorption
Horses that are most likely to benefit from yeast and other probiotics in the diet are those that show signs of hindgut digestive upset including colic, diarrhea, compaction and constipation, and free fecal water syndrome.
Athletic horses, growing animals, hard keepers and senior horses could all benefit from improved nutrient digestibility and absorption to support their extra demand for nutrients.
Similarly, horses consuming low quality forages or those on high-concentrate diets could also benefit from yeast supplementation to support fibre-digesting microbes in the hindgut.
To find whether supplements containing yeast cultures would be appropriate for your horse, you can submit your horse’s diet for complementary evaluation by our equine nutritionists.
How Does Yeast Work for Horses?
Research across several livestock species, including horses, has shown that Saccharomyces cerevisiae elicit health benefits through a variety of mechanisms including: 
- Inhibits pathogenic microbes: Components of the cell wall of yeast, including mannan-oligosaccharides (MOS), can bind proteins on the surface of pathogenic microbes such as E. coli and Streptococcus to prevent them from producing toxins.
- Bind toxins: Pathogenic microbes in the gut secrete compounds that are toxic to other microbes or to cells of the intestinal wall. Similarly, mycotoxins (molds found in feed) can damage the intestine and interfere with digestibility. Components of the yeast cell wall such as beta-glucans can bind these and minimize their negative effects on the microbial environment and animal health.
- Changes intestinal microbiota: By inhibiting pathogenic microbes and minimizing dramatic shifts in pH, yeast supports the function of beneficial, fibre-digesting microbes. This improves feed digestibility helping horses gain more nutritional value from their diet.
- Stimulates natural defenses in the intestine:Supplementation of Saccharomyces cerevisiae activates the immune system to help prevent disease and improve immunity in animals.
- Removal of oxygen: The cellulolytic, fibre-digesting microbes in the hindgut are strict anaerobes, meaning oxygen is lethal to these microbes. Yeast removes oxygen from the environment, helping these microbes survive and improving fibre digestion. This effect is likely more relevant to ruminants who can have higher levels of oxygen in the rumen, compared to the equine hindgut which already has very low oxygen levels.
Types of Yeast for Horses
There are many species and strains of yeast that can be used in animal feed. The most common species are Saccharomyces cerevisiae and Saccharomyces boulardii.
These forms of nutritional yeast are typically grown in a laboratory environment and lyophilised (freeze-dried) to ensure stability in the final product.
These organisms are viable in the hindgut where they can digest nutrients, bind toxins and support a healthy gut even though they do not actually proliferate (divide and increase in numbers) in the hindgut. Therefore, they must be continuously provided in the diet to keep up adequate numbers in the hindgut.
This species of yeast survives in the gastrointestinal tract of horses and is viable in the hindgut. Research has shown a slight benefit in decreasing the severity and duration of acute enterocolitis when horses are in hospital. 
Horses with diarrhea associated with antibiotic use had no difference in improvement of symptoms when given S. boulardii or placebo. Even though this species was viable in the hindgut it was not able to improve symptoms of antimicrobial-associated diarrhea. 
This is the most common species used in animal feed and human supplements. There are several strains of Saccharomyces cerevisiae that different companies use.
Although there are no studies that directly compare different strains in horses, whether or not this species improves the hindgut environment might depend on the strain.
For example, up to 30 grams per day of strain CNCM I-1077 had no effect on nutrient digestibility in mature, healthy horses.  In contrast, 10 grams per day of strain 1026 (Yea-Sacc 1026â„¢) improved fibre digestibility, VFA production and induced favourable shifts in the microbial environment.  
How to Feed Yeast and Safety
Dried yeast cultures can be added on their own in doses ranging from 5 to 20 grams per horse per day as seen in the studies reported on here. Typical doses included in animal feeds provide between 108 (100 million) to 1010 (10 billion) colony forming units (cfu) per gram.
Adding yeast in the equine diet is very safe and no adverse effects have been reported. 
There is often concern about potential negative impacts on the gastric environment when feeding prebiotics and probiotics. The horse’s stomach and small intestine contain microbes to begin the digestive process.
These can be influenced by the diet, for better or worse. For example, high-starch diets can induce starch fermentation in the stomach which contributes to gastric ulcers.
Adding yeast to high-starch diets numerically increased pH level in the stomach and decreased some microbial populations. This suggests a protective effect against the development of gastric ulcers in high-grain diets although more research is needed to directly assess that potential benefit. 
Mad Barn’s Visceral+ has been clinically studied in horses with ulcers. It is formulated with natural ingredients including prebiotics and probiotics that work with the body’s natural healing mechanisms and nourish the microbiome. It can be used during and following Gastrogard treatment, on its own in horses with ulcers or as a preventative measure.
Mad Barn’s Optimum Digestive Health is highly palatable pelleted formula designed to support hindgut health. It works in five key ways:
- Stabilizes the gut microbiome to reduce risk of dysbiosis and colic
- Contains digestive enzymes that improve nutrient digestibility
- Contains probiotics and other factors that support fibre fermentation
- Contains toxin binders like Bio-Mos that bind pathogenic microbes and toxins to neutralize them and help eliminate them from the body
- Is a source of natural immune modulators
Optimum Digestive Health is an all-in-one digestive health supplement that contains natural ingredients to optimize the gut microbiome and reduce the risk of digestive dysfunction and colic.
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.
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