The digestive system: from lips to hindgut

Built for grazing

Horses are herbivores that evolved as slow, near-constant grazers. Left to themselves they forage for about 18 hours a day, because their digestive system works best with a small amount of food passing through it all the time. It can't cope with large amounts arriving at once.

The horse's gut is unusual in working in two stages. In the foregut (stomach and small intestine), food is digested by enzymes, much as in our own gut. In the hindgut (caecum and colon), fibre is fermented by microbes. Understanding both halves explains most of the "rules of feeding" that horse owners learn.

The journey at a glance

Mouth: whiskers, lips and teeth

Digestion starts before the first bite. The whiskers around the muzzle are sensory hairs with their own nerve and blood supply. They help a horse feel for its food, judge unfamiliar food and detect small objects it shouldn't eat, in an area just in front of and below the nose that it can't see. The upper lip is strong, mobile and sensitive, and moves forage towards the incisors (front teeth), which cut it. The tongue then passes it back to the cheek teeth, which grind it.

World Horse Welfare says that a horse's teeth have to grind grass and hay down to around 3 mm before it reaches the stomach. The British Horse Society says horses can chew around 30,000 times a day. See Teeth: how horses' teeth work for how the teeth cope with that much wear.

As the horse chews, its salivary glands release saliva, which lubricates the chewed food into a soft ball called a bolus. Saliva contains bicarbonate, which helps buffer stomach acid. A horse fed a natural, forage-based diet can produce 10–12 litres of saliva a day.

Oesophagus: a one-way trip

The oesophagus is a muscular tube about 1.2–1.5 metres long. Waves of muscle contraction, called peristalsis, push each bolus down to the stomach, while the epiglottis closes off the windpipe so food doesn't go down the wrong way. A mouthful that hasn't been chewed or moistened enough can get stuck, which is called choke. This is one reason regular dental checks are encouraged.

At the bottom of the oesophagus, the bolus passes through a one-way valve, the cardiac sphincter, into the stomach. This valve is so strong that horses can't vomit. So if the stomach becomes distended, it can't relieve itself and is at risk of rupturing. Always call your vet if you are concerned.

Stomach: small and always making acid

For a large animal, the horse has a small stomach: about the size of a rugby ball, holding 8–12 litres. It can't stretch to take a big meal, which is why small, frequent feeds are recommended. If a horse eats a large meal, unprocessed food can be pushed on into the small intestine and cause upset such as colic. The stomach empties when it is around two-thirds full, however long the food has been there.

The stomach has two different linings:

• The lower (glandular) part secretes acid and enzymes and is protected by a thick layer of mucus.
• The upper (squamous) part has no such mucus layer.

Horses produce hydrochloric acid continuously, even when they aren't eating. When a horse can't follow its natural feeding pattern, the risk of gastric ulcers rises.

Small intestine: fast digestion

The small intestine has three sections:

• Duodenum (about 1 m): enzymes from the pancreas and liver break food down further. Horses have no gall bladder to store bile, so the liver releases it all the time.
• Jejunum (about 19 m): the longest, most folded section, where most digestion is completed and starch, fat and protein are absorbed into the bloodstream.
• Ileum (about 1 m): absorbs more nutrients and controls the passage of food into the large intestine. By this point only fibre and water should be left.

Food can pass through the small intestine in around 45 minutes. A big meal moving quickly may not be fully digested, and what's left can cause problems further on. This is another reason trickle feeding suits horses best.

Hindgut: the fermentation vat

The large intestine, or hindgut, is about eight metres long in all: the caecum, large colon, small colon and rectum.

Caecum. The caecum holds about 35 litres and works like a huge fermentation vat. It contains millions of specialised bacteria and other microbes (the gut flora), which break down fibre by fermenting it. Fermentation gives off heat, so the hindgut acts as the horse's own central heating. The gut flora are very sensitive, so any change to the diet should be made gradually. Sudden changes can cause diarrhoea, colitis, colic or laminitis. If undigested starch reaches the caecum, the microbes ferment it quickly and produce lactic acid, which can make the gut too acidic and kill off the helpful bacteria.

Large colon. The large colon is 3.5 metres long and holds up to 90 litres, folded into a small space with tight turns. Food can get stuck at those turns, causing impaction colic, often after a sudden change in routine such as moving from full-time turnout to being stabled. A large amount of water is absorbed here.

Small colon and rectum. The small colon absorbs more water and electrolytes, and droppings form in the rectum, which is about 30 cm long.

What this means for feeding

The anatomy explains the everyday rules: feed plenty of forage, little and often, and make changes slowly. For practical advice on feeding and water, see feeding and nutrition and the daily care guide.