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