Hindgut acidosis is a condition in which the hindgut of the horse becomes excessively acidic. It is usually caused by too much starch in the horse’s diet, resulting in increased production of lactic acid in the lower intestinal tract.

When lactic acid levels rise, the result is a lower pH environment in the hindgut and disturbances to the microbial population.

This can also result in inflammation in the intestinal wall and decreased resistance to pathogens and toxins found in feed.

Hindgut acidosis can have meaningful consequences for your horse’s overall well-being, including decreased nutrient absorption and feed efficiency, increased risk of hindgut ulcers, and increased risk of digestive or immune complications.

Prevention and treatment of hindgut acidosis start with dietary management. Ensuring that your horse’s feeding program is designed to minimize starch overload is an important first step.

Eliminate or reduce feeding grain and concentrates. Give your horse access to high-quality forages at least 12 hours per day.

You may also want to consider feeding digestive support supplements such as probiotics, yeast, or digestive enzymes.

Horse Digestive Tract - Hindgut & Foregut

Proper Hindgut Function in Horses

The horse’s gastrointestinal tract can be broken down into two main sections: the foregut and the hindgut. [1]

The foregut is composed of the mouth, esophagus, stomach, and small intestine. It makes up approximately 30% of the overall digestive tract.

Digestion begins in the foregut. The small intestine contains enzymes that start to break down feed.

The small intestine absorbs simple sugars, starches, fatty acids, amino acids as well as vitamins and minerals. However, it does not have the ability to break down fibre or other complex carbohydrates which make up a large portion of a horse’s diet. [1][2]

Fibre, fructans, complex plant sugars, pectin, and beta-glucan are broken down in the hindgut through microbial fermentation. In human nutrition, all of the carbohydrates that cannot be digested in the small intestine are collectively called “fibre” and we will use that terminology in this article.

The hindgut consists of the cecum and colon and makes up approximately 60% of the digestive tract.

Healthy hindgut function is critical for a horse to absorb and utilize the nutrients found in the diet to meet daily requirements. [1][2][3]

Hindgut Fermentation

Horses are hindgut fermenters because fibre digestion by microbes occurs in the hindgut (cecum and colon).

The hindgut contains a complex ecosystem of bacteria, protozoa, yeast, and other microorganisms that break down fibre in the diet through fermentation. [3]

Hindgut fermentation breaks down fibre and some of the protein in the diet to produce:

  • Energy: Mostly in the form of volatile fatty acids (acetate, butyrate and propionate) that can be absorbed and used by the horse. Lactate can also be absorbed and used as an energy source.
  • Amino acids: The building blocks of proteins, these are mostly used within the microbial environment and not absorbed in significant quantities.
  • B-vitamins: Water soluble b-vitamins such as biotin and thiamin. It is unclear how much of these vitamins produced by the hindgut can be absorbed.
Hindgut fermentation is completely reliant on the delicate microbiome to maintain proper digestion. The hindgut is the main site of fibre digestion to supply a significant portion of the horse’s energy needs
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What is Hindgut Acidosis?

The hindgut needs to maintain a relatively stable range of pH to support optimal digestion and nutrient absorption.

It is normal for the pH to change depending on the diet. However, if the hindgut environment becomes too acidic or too basic, it can interfere with the proper functioning of the beneficial bacterial species that populate the gut.

The ideal pH level in the hindgut for proper fibre fermentation is between 6.5 – 7.0. [6][7][8][9]

During the fermentation process, the pH drops below 6 (becoming more acidic) for a short period of time. This change in pH level makes fibre-digesting bacteria less effective but allows other microbes to work more effectively. [6][7][8]

However, long periods of low pH in the hindgut can be detrimental. Low pH is associated with excessive sugar or starch in the equine diets.

In extreme cases of “horse broke into the feed room” or experimental fructan overload, this causes fibre digesting bacteria to die off and allows Streptococci to proliferate causing the pH to drop below 6.

These microbes release toxins that can damage the junctions between cells of the hindgut and cause local and systemic inflammatory responses and laminitis. [6][7][8]

Hindgut Function in Horses

 

Hindgut acidosis is a condition of sustained low pH in the cecum which is detrimental to fibre-digesting microbes and contributes to serious health problems such as systemic inflammation, diarrhea, endotoxemia with fever, and potentially laminitis.

 

Signs of Acidosis in Horses

Horses with clinical hindgut acidosis are obviously ill with colic, fever, diarrhea and potentially laminitis. This occurs within the framework of horses getting unlimited access to grain or experimentally induced acidosis with large doses of corn starch or chicory root fructan.

Horses receiving grain meals which are too large or have too much poorly digestible starch may show signs such as bloating and soft manure.

Hindgut acidosis has been blamed for causing stereotypic behaviors, however this is likely not the case.

One study compared behaviour following all concentrate feeding versus forage feeding. [4] As expected cecal pH was lower in grain-fed horses and these horses also performed more wood-chewing and coprophagy. However, infusions of sodium bicarbonate to correct the pH had no significant effect on behaviour.

Stereotypic behavior with high grain and low forage feeding may have more to do with a frustrated urge to forage, rather than hindgut acidosis.

Causes of Hindgut Acidosis

Hindgut acidosis is caused by management factors, particularly related to diet and feeding schedules.

Wild horses spend up to 18 hours per day consuming small, frequent meals that consist mainly of forages high in fibre.

In contrast, domesticated horses often have an intermittent feeding schedule, usually consisting of two-to-four meals per day containing high-starch cereal grains. Horses might also have limited free access to forages which could impair their digestive health.

High starch or high sugar diets

When horses over-consume a high starch concentrate diet or are experimentally administered very high amounts of chicory root fructan, this can lead to the development of hindgut acidosis.

Starch should be digested in the stomach and small intestine with the majority of sugars absorbed in the small intestine.

However, when diets are high in poorly digestible starch such as unprocessed corn or barley, the foregut can become overwhelmed and unable to digest the starches within the small intestine. [6][7][8][9]

This results in the starch entering the hindgut where it is fermented. As starch fermentation occurs, the production of lactic acid and volatile fatty acid increases, which decreases pH in the hindgut.

If this reaches a critical level of acidosis, fibre-fermenting bacterial numbers drop and proliferating Streptococci release endotoxins. Endotoxins are toxins in the cell wall of bacteria that are released when the bacteria start to break apart.

With exposure to a sufficiently acidic hindgut environment, it can lead to damages to the cell wall of the cecum and colon making them more permeable.

Damage to the cell wall allows endotoxins to be absorbed into the blood which can result in a series of health problems such as a systemic inflammatory response and laminitis. [6][7][8][9]

Starch overload in the hindgut is the most common cause of hindgut acidosis.

 

Prevalence of Hindgut Acidosis

A 2006 study of Australian racing thoroughbreds analyzed the rates of hindgut acidosis among horses on high-starch diets. [10]

The researchers surveyed 72 trainers who had a total of approximately 690 horses in training. Among this group, horses were fed an average of 7.3 kg of grain concentrates per day, with 82% of trainers giving their horses grain at least twice a day.

The researchers obtained fecal samples from the horses to determine pH levels and the presence of indigestible starches.

Fecal analysis showed that approximately 27% of the horses had a pH level lower than 6.2 in the hindgut. [10]

In comparison, fecal pH in horses on a forage only diet is typically in the neighborhood of 6.4.

However, it should be noted there is no correlation between fecal pH and the pH in the cecum. [5] Therefore, the true prevalence of hindgut acidosis has not been established.

Management and Prevention of Hindgut Acidosis

1) Examine the potential risk factors of your horse

The first step to addressing better hindgut health in your horse is to examine current risk factors in their environment.

Feed storage bins, feed carts and treats should be locked away out of the potential reach of a loose horse.

2) Diet and feeding schedule

Your horse’s feeding program is essential to maintaining good gut health. The diet is directly linked to what microorganisms are present within the digestive tract. [10]

When thinking about your horses’ diet regimen a few important aspects to ensure are:

  • Any changes to the diet are done gradually to allow the microbial populations in the digestive tract to adapt
  • Horses should maintain eating throughout the day to ensure the digestion process is occurring at all times. Several small meals is preferable to two large ones, especially with concentrates.
  • Constant free access to fresh clean water
  • Maintain a consistent feeding schedule
  • Avoid unprocessed corn or barley

It is normal for the hindgut pH to be lower in horses being fed concentrates because they are fermented differently from forages. As long as the horse continues to have normal manure, there is no problem.

Mild grain intolerance will manifest first as soft manure and possibly bloating. This should be managed by:

  • Reducing meal size
  • Increasing meal frequency
  • Feeding oats or processed corn and barley with better starch digestibility
  • Replacing calories from concentrates with lower starch options such as soybean hulls, beet pulp or wheat bran

You can submit your horse’s diet for a complimentary evaluation online by our university-trained equine nutritionists. They can recommend a feeding program to help minimize the risk of hindgut acidosis in your horse and ensure optimal digestive function.

The best way to prevent hindgut acidosis is to ensure your horse is being fed a balanced diet. Any changes to the diet composition or feeding schedule should be introduced gradually.

 

3) Probiotics and prebiotics

Probiotics and prebiotics are dietary supplements that promote a healthy microbial ecosystem within the gastrointestinal tract. [11]

Probiotic supplements supply the gastrointestinal tract with live beneficial microbes. Prebiotics are growth substrates that stimulate the growth of probiotic bacteria. [11]

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

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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  • 20 billion CFUs per serving
  • Pure probiotic with no fillers
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Optimum Digestive Health

Mad Barn’s Optimum Digestive Health pellets (ODH) is a comprehensive gut health supplement formulated to help restore and maintain balance in the gut microbiome.

ODH is a pelleted equine supplement targeting hindgut health. It is designed to support fibre fermentation, inhibit toxin absorption and reduce levels of harmful bacteria in the hindgut.

Each serving of ODH supplies 20 billion CFUs of probiotics, in addition to prebiotics, yeast, toxin binders, and digestive enzymes that support digestive function and a healthy microbiome.

Optimum Digestive Health

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

Some ingredients found in ODH include:

Bio-Mos™

Bio-Mos is a prebiotic derived from a specific strain of yeast. It neutralizes toxins in the hindgut before they can cause damage to the microbes or intestinal lining.

This helps maintain a healthy microbial environment and optimal nutrient absorption. [15]

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 proper gut pH levels, and assist with hind-gut fermentation.

In horses, the addition of prebiotics has been shown to enhance gut health, immunity, and performance. [11]

Yea-Sacc 1026®

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

In horses, this strain of yeast helps promote nutrient digestibility and stabilizes hindgut pH, preventing digestive upset associated with stress. [13][14]

Optimum Digestive Health (ODH) is formulated to support a healthy hindgut by stabilizing the microbiome, providing digestive enzymes, and supporting the immune system. Horses prone to digestive upset of any kind could benefit from ODH to improve nutrient digestion and absorption.

Examining the diet and your horse’s daily routine for factors that indicate or could create gut health issues is a vital first step to address digestive upset. Submit your horse’s diet online and one of our equine nutritionists can help you make appropriate adjustments for your horse.

Summary

  • Hindgut acidosis is a condition caused primarily by starch overload in the digestive system which results in a more acidic hindgut environment.
  • It is normal for the hindgut pH to be lower in horses being fed concentrates because they are fermented differently from forages. If your horse is fed concentrates, monitor their fecal consistency. If the manure is normal, there is no problem.
  • Soft manure, and possibly bloating, are the first signs of mild grain intolerance. Reduce the meal size, increase meal frequency, and replace some concentrates with lower starch options.
  • Horses with serious hindgut acidosis of cecal pH below 5.0 will be obviously very sick. These horses may experience diarrhea and laminitis.

Following the strategies in this article will help you prevent hindgut acidosis from occurring.

The right feeding program will help ensure your horse’s gut health is in the best condition it can be to aid in better performance inside the ring.

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References

  1. Dicks, L.M.T. et al. The equine gastro-intestinal tract: An overview of the microbiota, disease and treatment. Livestock Sci. 2014.
  2. Frape, D. Equine Nutrition and Feedi, 4thed. Wiley-Blackwell. 2010.
  3. Thirumalaisamy, G. et al. Hindgut fermentation in horses. Indian Farmer. 2016.
  4. Willard, J.G. et al. Effect of Diet on Cecal Ph and Feeding Behavior of Horses. J Anim Sci. 1977. View Summary
  5. Naesset, J.A. and Austbo, D. Using faecal pH to predict gut health in horses. IN: The impact of nutrition on the health and welfare of horses. Wageningen Academic. 2010.
  6. Rowe, J.B. et al. Controlling acidosis in the equine hindgut. Advances in Animal Nutrition in Australia. 1995.
  7. Destrez, A. 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
  8. Davies, J. et al. Development of electrochemical DNA biosensor for equine hindgut acidosis detection. Sensors. 2021. View Summary
  9. Pagan, J.D. et al. Feeding protected sodium bicarbonate attenuates hindgut acidosis in horses fed a high grain ration. AAProceedings.2007.
  10. Richards, N. et al. The effect of current grain feeding practices on hindgut starch fermentation and acidosis in the Australian racing Thoroughbred. Aust Vet J. 2006. View Summary
  11. Kritas, S.K. Probiotics and Prebiotics for the Health of Pigs and Horses. Probiotics and prebiotics in animal Health and food safety (pp 109-126). 2017.
  12. Spring, P. et al.Areviewof 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.
  13. Jouany, J-P. et al. Effect of live yeast culture supplementation on hindgut microbial communities and their polysaccharides and glycoside hydrolase activity in horses fed a high-fiber or high-starch diet. J Anim Sci. 2009.
  14. Grimm, P. et al. Effect of yeast supplementation on hindgut microbiota and digestibility of horses subjected to an abrupt change of hays. Livestock Sci. 2016.