It’s large and it’s complicated, but a horse’s digestive system, much like a person’s, works like a dream if it’s maintained in a healthy state. A good diet is key to digestive health, but it also helps hugely if you understand how this complex system works and what can trigger problems — some bigger and potentially more deadly than others. Horses are hind gut fermenters, meaning their digestive system is very different from ours, a dog’s or even a cow’s. Rabbits, elephants, and rhinoceros are some of the other animals that have this type of digestive system, but there are few of them.
“A horse’s lips and sensitive muzzle mark the start of their digestive system. Their lips, tongue and teeth are designed to grasp food, which they do effectively,” explains equine vet Sara Fleck from Bishopton Equine Vets, an XLVets Equine member practice. “Horses can move their jaws from side to side, back and forth, and they also have very moveable tongues and lips — to the point where they could just about form words.”
Once food is in your horse’s mouth, their teeth break it down by crushing and grinding, which releases nutrients from inside the plant cells. “The food becomes a soft pulp and is made into a bolus [ball] by the tongue,” says Sara. “When a horse swallows, the trachea is blocked by the epiglottis to make sure food goes into the oesophagus and not the windpipe.”
The oesophagus is a muscular tube about 1.5m long that transports food from the horse’s mouth into the stomach by rhythmic contractions known as peristalsis. The bolus then passes through the cardiac sphincter and into the lumen of the stomach. The sphincter acts as a one-way valve, which explains why horses can’t vomit.
Horse’s digestive system: inside the mouth
“A horse’s mouth is an important and sometimes overlooked part of the digestive system,” states Dr Laura Wilson, a vet and technical advisor at Dodson & Horrell. “It is in their mouth that mechanical digestion begins via chewing, and saliva is secreted.”
Dental health is vital to a horse’s overall digestive health, and it is therefore important to have your horse’s teeth checked regularly.
“Food is broken down into smaller pieces in the mouth to improve its digestibility and absorption of vital nutrients,” explains Laura “Forage such as hay, haylage or even grass requires more chewing than concentrate feeds. Increased time spent chewing is beneficial for the horse as it promotes healthy wearing of the teeth and leads to increased saliva production.”
Did you know that adult horses may secrete up to 35-40 litres of saliva per day? Saliva plays a very important role in lubricating the food ingested and buffering stomach acid, which is important for the prevention and management of gastric ulcers.
Horse’s digestive system: inside the stomach
Once food has been swallowed, it passes down the horse’s oesophagus into their stomach. A horse’s stomach contains gastric juices, pepsin and hydrochloric acid, which break down food into semi-digested liquid (chime). The stomach is made up of two types of tissue and the lower section is lined with glandular mucosa, which secretes acid to help digestion.
A horse’s stomach can be divided into the following two parts, separated by a line called the margo plicatus:
- The lower part of the stomach has a glandular lining, and it is here that the acid is produced in response to a complex pathway of stimulatory and inhibitory signals.
- The upper part of the stomach has a squamous lining. It is this part of the stomach that is sensitive to contact with the acid from the lower part which can, in some cases, lead to gastric ulceration.
“The lower area also produces mucus and has protective mechanisms to ensure it’s not damaged. The upper section is lined with squamous mucosa, which doesn’t have this protection so is therefore vulnerable if it comes into contact with stomach acid,” explains Sara.
The stomach empties when two thirds full, which means continuous foraging or several small feeds are preferable to large ones. A horse’s stomach is a muscular organ that’s about the size of a rugby ball (between nine and 15 litres), which is very small.
“This is because horses are designed to trickle feed, eating little and often, and the majority of digestion occurs in the hindgut,” explains Sara.
Horse’s digestive system: inside the small intestine
Once processed by the stomach, semi-digested food passes through a valve (the pyloric sphincter) into the small intestine. This is around 25 metres long in a 500kg adult horse.
“Horses don’t have multi-compartmented stomachs like cattle, sheep and goats do,” explains Sara. “Instead, they have a simple stomach similar to a human’s. Preliminary digestion occurs first in the foregut (mouth, stomach, small intestine), before fermentation takes place in the hindgut (caecum). Horses are therefore known as hindgut fermenters.”
The small intestine consists of the duodenum, jejunum and ileum, with the duodenum around 1m long. Enzymes secreted from the pancreas and liver break down food into basic nutrients here. Bile is also secreted direct from the liver. Some of the protein, fat and non-structural carbohydrates from food will be digested and absorbed in the small intestine. It is also an important site of mineral absorption, with horses absorbing most, although not all, of their daily requirements prior to the food entering the large intestine.
“While most mineral absorption occurs in the small intestine, research in more recent years has shown that minerals are also absorbed in the large intestine,” states Laura.
The jejunum and caecum
The jejunum is roughly 19m long and the chemical breakdown of food is finished in this part of the small intestine, with nutrients absorbed into the bloodstream to be used by the body or stored in the liver.
“The ileum, around 1m long, is the final part of the small intestine. It continues the absorption of nutrients and controls the passage of partially digested food into the caecum,” explains Sara. “The junction between the small intestine and the caecum is called the illeo-caecal junction and is a prime spot for tapeworms.”
The caecum itself is 1.5m to 2m long and holds up to 30 litres of fibrous food and fluid.
“Contraction of the caecal muscles results in mixing of the food with microbes that digest tough plant cells via fermentation. This fibre breakdown produces substances called volatile fatty acids, which can then be absorbed and used by your horse for energy,” explains Sara.
The large intestine
Otherwise known as the horse’s hind gut, the large intestine is what makes a horse’s digestive system unique. As hind gut fermenters, horses rely upon a ‘factory’ of micro-organisms that live within the large intestine to be able to release the energy from a forage-based diet.
“A regular supply of fibre into the micro-organism factory is key to keeping it healthy and functioning well,” advises Laura. “Regular fibre intake maintains the complex balance in the large intestine, reducing fluctuations in pH and preserving the microbes at optimum levels.”
The large intestine, which is almost 8m in length, consists of the large colon, transverse colon and small colon, and is where most of the water absorption takes place and the remaining food matter is turned into faeces.
“The amount and type of food ingested has a large impact on the amount of water held in the large intestine and the absorption of water and electrolytes,” adds Dr Laura. “This is important for maintaining gut motility and keeping your horse hydrated (two key factors for general health and the prevention of colic).
The diameter of the large intestine varies from five to 50cm. The pelvic flexure, only 8cm, is where the large colon ‘turns’ to change direction and this is a common site for impactions. The small colon is where faeces form into balls and are evacuated.
“While most mineral absorption occurs in the small intestine, research in more recent years has shown that minerals are also absorbed in the large intestine.”