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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]
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]
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