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.
Proper Hindgut Function in Horses
The horse’s gastrointestinal tract can be broken down into two main sections: the foregut and the hindgut. 
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.
Fibre fermentation occurs in the hindgut. The hindgut consists of the cecum and colon and makes up approximately 60% of the digestive tract.
Horses are hindgut fermenters because fibre digestion by microbes occurs in the hindgut (cecum and colon).
The hindgut contains a complex ecosystem of bacteria, yeast, and other microorganisms that break down fibre in the diet through fermentation. 
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.
- 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.
What is Hindgut Acidosis?
The hindgut needs to maintain a relatively stable pH balance to support optimal digestion and nutrient absorption.
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.
During the digestion 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. 
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.
Signs of Acidosis in Horses
- Decreased appetite or loss of appetite
- Weight loss/anorexia
- Laminitis & founder
- Decreased performance
- Mild signs of colic (abdominal discomfort, laying down, restlessness)
- Lack of energy
- Development of new stereotypic behaviour such as cribbing, weaving, pacing, pawing/digging
- Hindgut ulcers
- Intermittent diarrhea
- Joint inflammation
- Soreness or lameness after exercise with prolonged recovery time
- Reduced growth rate
- Reduced reproductive function
Causes of Hindgut Acidosis
Hindgut acidosis is typically 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, spending as little as 10% of the day feeding.
This usually consists of two-to-four meals per day containing high-starch processed cereal grains. Horses might also have limited free access to forages which can impair their digestive health.
High starch or high sugar diets
When horses over-consume a high starch concentrate diet or pasture grass diet that is high in fructans (sugars found in grass), 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.
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 occurs repeatedly, the good fibre-fermenting bacteria will die off and release endotoxins. Endotoxins are toxins in the cell wall of bacteria that are released when the bacteria start to break apart.
With long-term exposure to an acidic hindgut environment, it can lead to damages to the cell wall of the cecum and colon causing poor nutrient absorption which can result in numerous secondary nutritional deficiencies.
Although an imbalanced feeding program is the major cause of hindgut acidosis, stress can play a role in this condition.
Stress can cause a range of digestive issues in horses, including colic, ulcers, diarrhea, and more. Common stressors include: 
- Abrupt dietary changes
- Poor diet choices
- Changes to the feeding routine
- Separation anxiety
- Changes to the herd structure
- Travel and competition
- Changes to exercise regime or too much high-intensity exercise
Prevalence of Hindgut Acidosis
A 2006 study of Australian racing thoroughbreds analyzed the rates of hindgut acidosis among horses on high-starch diets. 
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 gut 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. This is a sign of starch fermentation in the hindgut and is indicative of acidosis. 
The researchers further pointed out that corn was a major contributor to the incidence of excessive acidity in the hindgut. They hypothesize that this is because of its poor pre-caecal starch digestibility.
Given these findings, the researchers caution against feeding practices that can contribute to hindgut starch fermentation, such as feeding large quantities of grain with poor digestibility in the small intestine.
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 diet, routine and health history.
Think about your horse’s history in regard to gut health and behavioural changes. A few questions to review include:
- Is your horses prone to gastric ulcers?
- Has your horse shown signs of colic or has colicked in the past?
- Is your horse known to react to dietary changes or be sensitive to changes in diet/routine?
- Has your horse developed any stereotypic behaviour recently such as weaving, pacing, cribbing, etc.?
- Is your horse overly stressed?
Reviewing the history of your horse’s gut health will help your veterinarian diagnose if your horse could be at risk of hindgut acidosis.
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 .
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
- Constant free access to fresh clean water
- Maintain a feeding schedule
- Allow for herd environments to reduce stress
- Minimize environmental stress
You can submit your horse’s diet for a complimentary evaluation online by our university-trained equine nutritionists. They can recommended a feeding program to help minimize the risk of hindgut acidosis in your horse and ensure optimal digestive function.
3) Probiotics and prebiotics
Probiotic supplements supply the gastrointestinal tract with live beneficial microbes. Prebiotics are growth substrates that stimulate the growth of probiotic bacteria. 
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 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 absoption 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.
Some ingredients found in ODH include:
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. 
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. 
Yea-Sacc 1026® is a strain of yeast that was specifically selected for the role it plays in feed efficiency.
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.
Hindgut acidosis is a condition caused primarily by starch overload in the digestive system which results in a more acidic hindgut environment.
- Loss of appetite
- Weight loss
- Decreased energy levels
- Hindgut ulcers
- Prolonged soreness after exercise
As most horse owners know, horses have a very delicate digestive system and even the slightest change in diet can cause a digestive imbalance.
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.
Is Your Horse's Diet Missing Anything?
Identify gaps in your horse's nutrition program to optimize their well-being.
- Dicks, L.M.T. et al. The equine gastro-intestinal tract: An overview of the microbiota, disease and treatment. Livestock Sci. 2014.
- Frape, D. Equine Nutrition and Feeding, 4th ed. Wiley-Blackwell. 2010.
- Thirumalaisamy, G. et al. Hindgut fermentation in horses. Indian Farmer. 2016.
- Costa, M.C. et al. Comparison of the fecal microbiota of healthy horses and horses with colitis by high throughput sequencing of the V3-V5 region of the 16S rRNA gene. PLoS One. 2012.
- Lindroth, K. Free faecal liquid in horses. Acta Universitatis Agriculturae Sueciae. 2020.
- Kienzle, E. et al. Field Study on Risk Factors for Free Fecal Water in Pleasure Horses. J Equine Vet Sci. 2016.
- Rowe, J.B. et al. Controlling acidosis in the equine hindgut. Advances in Animal Nutrition in Australia. 1995.
- 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.
- Davies, J. et al. Development of electrochemical DNA biosensor for equine hindgut acidosis detection. Sensors. 2021.
- Pagan, J.D. et al. Feeding protected sodium bicarbonate attenuates hindgut acidosis in horses fed a high grain ration. AAEP Proceedings. 2007.
- 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.
- Willard, J.G. et al. Effect of diet on cecal pH and feeding behavior of horses. J Anim Sci. 1977.
- Cipriano-Salazar, M. et al. The dietary components and feeding management as options to offset Digestive disturbance in horses. J Equine Vet Sci. 2019.
- 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.
- 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.
- 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.
- 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.