Horses are like magical unicorns, and no, not because of their majestic movement and beauty. Horses have a very special digestive system that differs from most other animals on the planet. Horses are what we call “hindgut fermenters.”
The equine gastrointestinal tract is a hybrid of the typical monogastric system found in mammals (humans, pigs, dogs) and the ruminant system (cows, sheep, and goats). To understand how to properly feed the horse we must first understand how each part of the GI system functions.
1. The Mouth
The primary purpose of the horse’s mouth is to break down food particles into smaller pieces and to incorporate saliva as lubrication.
The smaller, lubricated pieces are easier to swallow, and smaller feed particles allow for a greater degree of digestion later on in the GI tract.
Food typically only spends a few minutes in the mouth being chewed, however, the time is dependent upon the type of feed. Grain takes less time to chew than forage. Once food has been properly chewed, it is swallowed to move into the esophagus.
2. The Esophagus
The esophagus is a tube composed of smooth muscle that allows for the transport of feed from the mouth to the stomach. In the horse, it is important to note that the esophagus is non-glandular, meaning it does not secrete anything.
This means that feed must be properly lubricated in the mouth to allow for proper transport of food. If there is not enough incorporation of saliva in the mouth, there is a risk of choke.
Additionally, the muscular contractions of the esophagus move only in the mouth-to-stomach direction, making this transport a one-way process.
3. The Stomach
The equine stomach is small, composing only about 10% of the digestive tract.
The stomach is responsible for mixing feed particles and beginning the digestive process through physical and chemical degradation of feed particles. The stomach’s acidic environment denatures proteins (think: unwinds their structure) which allows for greater digestion later on down the line.
Feed does not spend long in the stomach, passing through in as quickly as 15 minutes (1). Size, constant hydrochloric acid secretion, and short retention times are three reasons why it is ideal for horses to eat small meals continuously throughout the day. If horses are instead fed large meals spread widely apart, issues such as ulcers may occur.
4. The Small Intestine
The small intestine accounts for 20-30% of the digestive tract.
Here, enzymes that are produced by the horse itself--either secreted from the small intestine, liver, or the pancreas--break down digesta to the level that it can then be utilized by the animal.
Amino acids (protein), sugars (carbohydrates), and some fatty acids (fats) are absorbed into the bloodstream through the small intestine’s walls.
Digesta spends 30 minutes to 3 or 4 hours in the small intestine, with longer retention times leading to a higher degree of digestion (1). Any undigested matter will pass to the large intestine.
5. The Large Intestine
The large intestine comprises the largest portion of the digestive tract, at about 60-70%, which tells us what component of the horse’s diet is most important.
The large intestine serves as a forage microbial fermentation chamber for the horse. Here, enzymes produced by the microbiome (the bacteria that lives in the hindgut) will break down fiber that cannot be broken down by the horse’s own digestive enzymes.
This fiber will be converted to Volatile Fatty Acids (VFAs) which can then be absorbed and utilized by the horse as energy.
Additionally, the large intestine is the primary site for the absorption of water, as well as many vitamins and minerals. Digesta can remain in the large intestine undergoing fermentation for 48-72 hours. Anything that has not been digested and absorbed will be expelled as fecal matter.
A summary of equine digestive system anatomy
The horse’s digestive tract has been optimized by years of adaptation to their natural and unique lifestyle, i.e. grazing small meals continuously.
Unfortunately, this lifestyle isn’t necessarily the most convenient for those who care for our animals, so they often end up being fed two to three large meals per day. This management practice may negatively impact the health of our animals, and therefore it is important to understand how the horse’s GI tract works so that we may update our management practices to better suit their gut health.
Horses have the benefit of being able to directly absorb high-quality feed nutrients in the small intestine (like high quality sources of protein), while also being able to extract energy from lowly digestible matter such as stemmy forages in the hindgut.
By understanding this special digestive system, we can promote the health of our horses, and their guts, through proper feeding management.