How Does Hindgut Fermentation Work in the Horse?

Horses are single stomach (monogastric) herbivores that evolved to graze on fiber-rich roughage. Their unique digestive system consists of a foregut and hindgut, each with different functions for breaking down feed and absorbing nutrients. ^[1]

The stomach and small intestines, which make up the foregut, are responsible for digesting proteins, fats, and non-fibrous carbohydrates in the horse’s diet. This happens through mechanical action and chemical reactions facilitated by enzymes.

The hindgut, consisting of the cecum and colon, is where the digestion of fibrous carbohydrates takes place. Bacteria help synthesize nutrients and convert this fibre into usable energy through fermentation. ^[1][2][3][4]

The process of hindgut fermentation allows the horse to extract nutrients from plant material which are not digestible in the small intestine. ^[1][2]

Because of their specialized digestive system, fibrous forages should comprise the largest percentage of your horse’s diet. Feeding horses too much sugar and starch from grain-based feeds can disrupt digestive processes in the hindgut and cause gut problems. ^[1][2][3]

You can support your horse’s hindgut function by feeding a forage-based diet to maintain a healthy gut microbiome. ^[1] Other factors that affect bacterial populations in the hindgut include the weather, stress, and the use of medications and dewormers. ^[5][6][7]

What is Hindgut Fermentation in Horses?

Hindgut fermentation is a unique digestive process that occurs in horses and some other herbivorous animals.

The main purpose of the hindgut is to ferment the complex and structural carbohydrates found in the horse’s diet. ^[4] Structural carbohydrates include fiber components in the plant cell wall, such as cellulose and hemicellulose. Complex but fermentable carbohydrates include pectin, beta-glucan, plant sugars and fructans, largely found within the plant cell.

In horses, fiber and complex carbohydrates from fresh grass and hay are digested via fermentation by microbes residing in the hindgut. Feeding a diet rich in structural and complex carbohydrates helps to promote optimal digestion and the transit of feed through the equine gastrointestinal tract (GIT). ^[1]

Hindgut fermentation produces volatile fatty acids (VFAs) that serve as a significant energy source for horses. ^[4] Most of a horse’s energy requirements should be met through fiber fermentation in the hindgut.

Microbial fermentation also produces B-vitamins and amino acids. These B-vitamins can be absorbed and meet the horse’s requirements. Methane, carbon dioxide, and water are also produced by fermentation in the hindgut. ^[1]

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Overview of the Equine Digestive System

Two distinctive features of the horse’s digestive system is their relatively small stomach and their relatively large hindgut.

Horses have a much smaller stomach than other mammals with a similar body size. A horse’s stomach can only hold 8 – 15 liters (2 – 4 gallons) at a time, and feed quickly passes on to the small intestine. ^[23]

This means the horse’s digestive system is suited to eating smaller amounts of food more often. Feeding a few large meals a day increases the risk of gut problems and metabolic dysfunction horses. ^[29][30]

While the horse’s stomach is small, their hindgut is large. This helps them break down and ferment the fibrous foods that make up most of their diet.

Role of the Equine Foregut

The equine foregut is comprised of the mouth, esophagus, stomach, and small intestine.

The digestive activities occurring in the foregut predominantly involve mechanical and chemical actions with limited microbial involvement. Microbes found in the upper stomach and small intestine primarily feed on sugars and starch. Chemical digestion relies on biological catalysts (enzymes) to break down food.

When horses ingest and swallow food, it moves down the esophagus and into the stomach. The stomach’s role is to hold food and use acid and enzymes to start breaking it down. However, gastric acids cannot break down fiber, which makes up most of what horses eat.

After feed exits the stomach, it passes into the small intestine which is approximately 25 meters / 82 feet long in adult horses. ^[8] The digestion and absorption of protein, fat, soluble carbohydrates, starch, vitamins, and minerals primarily occurs in the small intestine. ^[3]

After transiting through the small intestine, any undigested feed moves into the hindgut along with the fiber component of the diet that is not processed in the foregut.

Role of the Equine Hindgut

The horse’s hindgut is made up of several parts. The cecum is located at the beginning of the large intestine, followed by the large colon, small colon, and finally the rectum.

Together the cecum and large colon account for ~60% of the GIT volume. ^[8] The cecum and large colon are the main sites of fermentation in the hindgut and contain numerous species of microorganisms. ^[3]

Feed, predominantly fiber, reaches the hindgut approximately three hours after being ingested and is initially fermented in the cecum. It continues to undergo fermentation in the large colon. ^[3]

The equine hindgut does not secrete digestive enzymes to digest fiber, but microorganisms residing in the hindgut do. Through the process of fermentation, these microbes convert fiber into energy and nutrients. ^[3]

After the large colon, undigested feed passes on to the small colon, where excess water is absorbed and returned to the body. Fecal balls are formed from the undigested and partially-indigestible portion of the horse’s diet. ^[3]

Fermentation Products

When the microorganisms in a horse’s hindgut ferment fiber and other complex plant carbohydrates, they produce substances such as volatile fatty acids, which are a source of energy for the horse.

The most common VFAs produced in the equine gut are acetate, propionate, and butyrate. ^[3] These fatty acids provide approximately 70% of the horse’s energy supply.

In addition to VFAs, the fermentation process also produces lactic acid, which can be used to make glucose as another energy source. While some lactic acid is normal, excessive amounts can lead to digestive issues and imbalances in the microbial populations. ^[3]

The production of VFAs and lactic acid in the hindgut results in slower energy release than the rapid breakdown of soluble carbohydrates and starch in the foregut.

Additional by-products of bacterial fermentation in the hindgut include B-vitamins (riboflavin, niacin, biotin, folate, B12, and B6) and amino acids (primarily used by microbes and not absorbed in significant quantities). ^[1][3]

Composition of Hindgut Microbiota

The equine hindgut is home to a diverse community of microorganisms including bacteria, protozoa (single-celled organisms), and fungi. ^{[1][4][9][10]}

These microorganisms work together in a symbiotic relationship with the horse, aiding in the fermentation of fibrous materials. The activities of these microorganisms allow horses to digest and extract energy and nutrients from fibrous feed in the hindgut.

The hindgut contains a more uniform range of bacteria compared to the stomach and small intestine, which host a more diverse bacterial population. ^[11][12] The diversity of bacteria present in the foregut is due to the ingestion of a high volume of forage. ^[1]

Lactic acid bacteria comprise most of the microbiota in the large intestine. ^[1] These bacteria produce the majority of volatile fatty acids (VFA) needed for energy.

Among the resident bacteria in the equine hindgut, Firmicutes, Bacteroidetes, and Verrucomicrobia are the dominant phyla. ^[1]

The equine large intestine is also home to a significant population of methane-producing archaea. These single-celled microorganisms play a role in supporting cellulose digestion in the horse’s hindgut. ^[1]

The role of protozoa such as ciliates in the equine hindgut is not fully understood, although this group of organisms may support cellulose and pectin digestion. ^[1]

How Diet Affects the Microbiota

Diet has a direct effect on the microbial composition of the equine hindgut. Feed that is not digested in the foregut moves into the hindgut and subsequently influences the microbiota (microbial population).

Because different bacterial species are needed to ferment different types of feeds, even small alterations in the diet can influence the balance of microbes. Additional factors that impact the microbiota including living environment, management practices, medications, and age. ^[5][6]

If horses consume too much starch or sugar in one meal, the small intestine can become overwhelmed and undigested sugar and starch pass into the hindgut. This triggers a chain of events that can disrupt the microbiota and interfere with proper hindgut fermentation. ^[1]

pH Balance

The hindgut must maintain a relatively stable pH level (acidity) to support the existence of the microbes needed for optimal digestion and nutrient absorption. ^[13]

Fermentation can proceed if pH is greater than 5 ^[14] Hindgut pH is normally between 6.4 and 6.7 in grazing horses ^[15] and will be somewhat lower in horses fed grain, with corn causing the greatest reductions in pH.

During the digestion process, the pH briefly drops below 6 (becoming more acidic). This change in pH level makes fibre-digesting bacteria less effective but allows other microbes to work more effectively.

However, when the hindgut becomes more acidic than it should be, it promotes the growth of pathogenic bacteria and results in a further drop in hindgut pH. ^[16][17]

Extended periods of low pH in the hindgut can lead to a condition called hindgut acidosis. ^[13][16][17]

Hindgut Acidosis

Hindgut acidosis involves the death of fiber-digesting bacteria. As the bacteria die, they release toxins that can damage the cells of the hindgut. ^[16][17]

Hindgut acidosis can result in decreased nutrient absorption and feed efficiency, inflammation in the intestinal wall, and decreased resistance to pathogens and toxins found in feed. ^[10] It also increases the risk of colic, laminitis, hindgut ulcers, endotoxemia, and metabolic acidosis. ^[16][17][18]

The most common cause of hindgut acidosis in horses is the overfeeding of hydrolyzable carbohydrates (starch and sugar). If horses are fed more of these carbohydrates than they can properly digest in their stomach and small intestine, the undigested carbohydrates spill over into the hindgut and excess lactic acid and VFA production occur in the cecum and/or colon. ^[16][17][18]

Feeding a high volume of grains such as oats, barley, or corn over a short time can lead to acidosis. The threshold for feeding starch that avoids hindgut acidosis is between 0.2 and 0.4% of bodyweight per meal ^[15]. For example, for a 500 kg horse this would be 1 to 2 kg of starch. Oats are approximately 40% starch so this would be between 2.5 and 5 kg of oats per feeding.

Hindgut acidosis can also be induced experimentally by administering extremely large (7 to 8.5 lbs) doses of pure fructan by stomach tube. Fructan is a storage carbohydrate in cool season grasses. However, the level of fructan naturally occurring in grasses is rarely (if ever) high enough to exceed the threshold for inducing severe acidosis and laminitis.

An Australian study involving racing Thoroughbreds found that 27% of horses had a hindgut pH of 6.2 or lower, indicating starch fermentation and acidosis. Horses consuming a grain-based diet with corn in their feed were most likely to have signs of hindgut acidosis. ^[24]

Note that absorption of endotoxins and health complications of increased inflammation, lactic acid build up in muscles and laminitis only occur at pH at or below 5.

Signs of Hindgut Acidosis

Horses with hindgut acidosis don’t always display visible symptoms of illness. Subclinical acidosis with pH of approximately 6 occurs in horses fed large amounts of grain. This is associated with stereotypical behaviors such as cribbing and weaving but it is unclear if this is from acidosis or insufficient forage. ^[32]

However, common signs of severe acidosis (pH below 5) include: ^[16][17]

• Laminitis/founder
• Colic
• Increased incidence of dehydration
• Reduced appetite and performance
• Poor feed efficiency
• Loose, frothy feces
• Weight loss/li>

How to Support Healthy Hindgut Fermentation

Ensure Adequate Fiber Intake:

The best way to support your horse’s hindgut function is to feed a balanced diet with adequate fiber. Insufficient fiber intake can lead to hindgut acidosis, colic, gastric ulcers, and behavioral problems.

Various sources of fiber are suitable for horses. Examples of fiber-containing feeds include pasture grass, hay, hay cubes, alfalfa or timothy pellets, beet pulp, soy hulls, oat hulls, and rice hulls.

Dietary fiber can be classified into two broad categories: soluble and insoluble. Soluble fibre refers to the non-woody portions of a plant such as sap, resin, pectin, beta-glucans or gums. Insoluble fibers refer to the structural components of a plant including lignin, cellulose, and hemicellulose.

Horses can digest most of the soluble and insoluble fiber they consume. However, lignin cannot undergo bacterial fermentation and is excreted as feces. ^[1]

Providing your horse with fermentable fiber supports a healthy bacterial population in the hindgut and helps to maintain gut health.

Feed a Forage-Based Diet:

Providing your horse with a forage-based diet will ensure they get enough fiber to maintain a healthy hindgut microbiome.

Forage as hay or pasture should comprise at least 50% of the diet and be fed at a minimum rate of 1.5% of the horse’s body weight per day.

A lack of long-stem fibre (as seen in pellet-only diets) may not directly impact gut health but does lead to hay-seeking behaviour. ^[33]

Other sources of dietary fiber such as forage cubes, pellets, or chaff, are also beneficial but have variable fermentability. These forage sources are often provided in commercial feeds in a ground or pelleted format.

Ensure Access to Free-Choice Hay:

Horses are trickle feeders and their digestive tract works best when supplied with a steady intake of small amounts of fiber.

Providing free-choice forage is the best way to mimic the horse’s natural feeding behaviors and support optimal digestive function.

Free-choice hay also ensure the horse spends little time with an empty stomach, which can reduce the risk of gastric ulcers and prevent stereotypic behaviors. ^[31]

However, free-choice access may not be appropriate for all horses. For example, horses with metabolic syndrome are also leptin resistant. ^[35] Leptin is the hormone that turns off feeding. As a result, they will overeat unless provided hay in small hole hay nets to restrict intake.

Allow Your Horse to Move:

Exercise and movement during turnout stimulate muscles in the digestive system, promote feed transit through the gut. Provide plenty of turnout time to enable your horse to move and support hindgut health.

Feed Small Meals of Concentrates:

If your horse needs concentrates or commercial feeds to meet their energy requirements, make sure to feed multiple small meals spread throughout the day (at least three to four times or more).

Feeding small meals helps to maximize the digestive capability of their stomach and small intestine, minimizing starch overload in the foregut and preventing undigested starches from reaching the hindgut.

Given typical starch levels in concentrates, meals should not exceed 0.5 lb (0.23 kg) of concentrate per 100 lb (45 kg) of body weight. For example, a 1000 lb (450 kg) horse should not be fed more than 5 lb (2.25 kg) of grain-based feeds in a single meal.

Introduce Feeding Changes Slowly:

When transitioning to a new feeding program or forage source, make changes gradually to allow your horse’s microbiota to adapt.

Abrupt feeding changes are a significant risk factor for colic and other digestive issues. ^{[25][26][27][28]} Introducing changes slowly ensures that the microbial population in the horse’s gut adjusts to the new diet composition.

Manage Pasture Access:

Carefully manage your horse’s access to grass when transitioning to pasture in the spring.

Mild hindgut upset and diarrhea are common at this time. Young grasses are low in fiber, high in protein, sugar and starch and are a significant diet change from hay.

Minimize Stress:

Reduce your horse’s exposure to stress to minimize factors that can disrupt the digestive system. Increased levels of the stress hormone cortisol can lead to changes in gut motility.

Common stressors include herd separation, long periods without access to forage, transport, and confinement.

Feed a Digestive Health Supplement:

Feeding a probiotic and prebiotic supplement can help to support the microbial populations in the hindgut and reducing the risk of digestive disorders.

Mad Barn’s Optimum Digestive Health is a comprehensive gut supplement that provides 80 billion CFUs of yeast and probiotics per serving. Optimum Digestive Health works in three key ways to support hindgut function:

• Maintains a healthy microbiome by increasing beneficial bacteria while reducing harmful bacteria.
• Improves feed efficiency by supporting the breakdown of nutrients in forage and concentrates.
• Prevents digestive upset by inhibiting the passage of starch and protein into the hindgut.

Optimum Digestive Health

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

Key ingredients in Optimum Digestive Health that support hindgut health include Bio-Mos™ yeast, Integral®A+ and Yea-Sacc 1026®.

Bio-Mos™

Bio-Mos™ is derived from a specific strain of Saccharomyces cerevisiae and brewer’s yeast. ^[19] It helps neutralize toxins in the hindgut before they can cause damage to the microbes or intestinal lining. ^[19]

Bio-Mos™ helps maintain a healthy microbial environment and optimal nutrient absorption.

Integral®A+

Integral®A+ is a prebiotic blend made from hydrolyzed yeast, beta-glucans, mannan oligosaccharide (MOS) and green algae.

Prebiotics are clinically studied to enhance digestibility, balance gut pH levels, and assist with hindgut fermentation. In horses, the addition of prebiotics to the diet has been shown to support gut health, immunity, and performance. ^[20][21]

YEA-SACC 1026®

Yea-Sacc 1026® is a strain of yeast that was specifically selected for its role in enhancing feed efficiency.

In horses, this strain of yeast helps promote nutrient digestibility and stabilizes hindgut pH, preventing digestive upset associated with stress. ^[21][22]

Consult With a Nutritionist:

Submit your horse’s diet online for a complimentary evaluation by our equine nutritionists. Our nutritionists will recommend a feeding program to help support hindgut fermentation and minimize the risk of gut problems in your horse.