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Basic Digestive System

The horse has evolved as a "trickle feeder" designed to spend the majority of his time grazing. An understanding of the anatomy and function of the digestive system sheds light on how best to ensure it functions efficiently even when the horse is no longer predominantly kept in his most natural state.

 
MOUTH
  • Digestion starts with chewing - the incisor teeth at the front of the mouth bite food and tear the grass whilst the molars at the back chew and grind the food.
  • As the top jaw is wider than the bottom jaw, the molars tend to wear unevenly causing sharp edges. Regular checks are essential to ensure the horse is comfortable and chewing efficiently.
  • Chewing triggers the production of saliva which softens and moistens the food making it easier to swallow. Insufficient chewing can lead to choke.


OESOPHAGUS
  • Once the feed leaves the mouth the rest of the digestive tract is basically a muscular tube with various enlargements along the way.
  • The food is moved by contractions and relaxations of the muscles which is known as peristalsis.
  • If the movement stops, problems such as impactions (blockages) can result in colic symptoms. The rate of passage can be accelerated by fear or excitement which can cause disruptions to the microbial population in the gut.


STOMACH
  • The stomach of a 500kg horse is comparatively small, holding 7.5 to 15 litres, and is generally said to be about the size of a rugby ball.
  • The stomach is a fairly rigid structure and, rather than stretching to accommodate a large meal, it simply passes the food through more quickly so that the digestion process is less efficient.
  • The stomach is divided into two areas - the top is known as the squamous, or non-glandular, area whilst the bottom is the gastric, or glandular, region and is where the digestive secretions, such as hydrochloric acid, are produced.
  • Horses can suffer with ulcers in both regions of the stomach. In the glandular region it is thought to be non-steroidal anti-inflammatory drugs (NSAIDs), such as Bute, that break down the defence mechanisms against the acid allowing it to ulcerate the stomach tissue. In the squamous region it is thought to be a combination of a low fibre, high concentrate diet and the effects of exercise that allow the acid to come into contact with the unprotected stomach wall for prolonged periods of time.


SMALL INTESTINE
  • Consists of 3 regions known as the duodenum, jejunum and ileum.
  • Is about 50 to 70 feet (15 to 22 metres) long and 3 to 4 inches (7 to 10cm) in diameter in a 500kg horse.
  • Most of the fat, protein and about 50-70% of soluble carbohydrate is absorbed here.
  • Many of the vitamins and minerals are also absorbed here.
  • Bile drains from the liver continuously into the small intestine and aids in the breakdown of fats and oil. Unlike humans, the horse doesn't have a gall bladder.


CAECUM
  • Is about 3 to 4 feet (0.9 to 1.2 metres) long and holds 25 to 30 litres.
  • Contains bacteria and other micro-organisms that are responsible for breaking down fibre and any soluble carbohydrates that have escaped digestion in the small intestine.
  • Bacterial fibre digestion produces heat which acts as the horse's central heating system. In hot conditions horses are likely to reduce their fibre intake to regulate their body temperature.
  • In cold weather it is important to ensure that a horse has plenty of fibre to keep them warm.


LARGE INTESTINE
  • Consists of the large colon which is about 10 to 12 feet (3 to 3.6 metres) long with a diameter of 8 to 10 inches (20 to 25cm) and the small colon which is about 10 ft long (3 metres) with a diameter of 3 to 4 inches (8 to 10cm).
  • The large colon also contains micro-organisms that are responsible for digesting fibre.
  • Hind gut bacteria also produce B vitamins which are essential for metabolism and energy utilisation.   A healthy bacterial population is therefore vital to the overall health of the horse.
  • Water is absorbed in the large intestine so that all that is left is undigested material, dead cells etc that pass to the rectum.


RECTUM
  • The rectum is about 1 foot (30cm) long in the horse.
  • Horse rectum is considered a delicacy in places such as Argentina!