The horse's digestive system explained by MB Equine Services

Anatomy of the Horse’s Digestive Tract, Part 1: From Mouth to Stomach

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The horse’s digestive system labelled and explained, with facts, figures and other important information that is relevant to their health.

In this three part series we dissect the equine gastrointestinal (GI) tract to gain a better understanding of the digestive process and learn how we should keep our horses, and what feeding management we should follow to maintain optimal (digestive) health in our horses.

The horse evolved primarily as a grazing herbivore, eating a diet based on fibre, mainly grass and some browse (shrubs and other foliage). Horse owners are mostly aware of this, and we all try to offer our horses as much access to pasture as we can. Nevertheless, and with the best intentions, many owners manage their horses in a similar way to livestock, when in fact the purpose of keeping horses is generally very different.

Horses require a different housing system and feeding management than livestock. Horses may be similar to cattle in that they are large herbivores that have adaptated to grassland life, but they developed different feeding strategies and digestive physiology to cattle.

Horses evolved to consume large quantities of poor quality forages (low energy and high fibre), whereas cattle do better on medium or high quality forages that are relatively low in fibre.

By managing horses on pastures with high quality grasses that contain high levels of sugars and protein, we challenge our horses at a physiological level, and expose them to the risk of developing digestive and metabolic disorders such as insulin resistance, equine metabolic syndrome, hindgut acidosis, laminitis, tying up, developmental orthopaedic disease, colic and other related diseases.

An added issue is that grass/plant based-only diets are usually not sufficient to maintain the performance horse, and as a result, cereals, cereal by-products, protein meals and oils have been added to the majority of equine diets. These have many benefits but can also potentially cause a lot of health problems.

THE GASTROINTESTINAL TRACT

The GI tract of an adult horse (~500kg) is about 30 meters long and has a total volume of approximately 180 litres (see figure 1). The entire tract can be divided into two functional parts; the foregut and the hindgut (see fig 2). In this first part we will discuss the mouth to the stomach of the horse. In the second part we will continue with the small intestine and in the last part of the series we will describe the large intestine.

THE MOUTH

The digestive process starts when food enters the mouth. The lips and teeth of the horse are adapted for grazing and browsing. The upper lip is strong, mobile and sensitive and is used to place forage between the teeth. Horses have upper and lower incisors which enable them to shear off the grass/foliage close to the ground.

The tongue pushes the food to the molars and premolars where it is chewed to smaller particles. The jaw of the horse makes lateral and vertical movements facilitating mechanical grinding. When horses chew the feed in the mouth, saliva is produced to assist with the breakdown, swallowing and buffering of stomach acids. Under normal circumstances (when grazing and chewing fibrous material) horses will produce up to 10-20 litres of saliva a day. Saliva contains high levels of bicarbonate and other alkaline buffers to neutralise the acid in the stomach, as well as to lubricate the food.

When horses eat roughage they have to make more chewing movements than when they consume concentrates. Horses on average make between 3000-3500 chewing movements when they eat 1 kg of long hay, whereas 1kg of concentrate only requires 800-1200 chew movements (ponies have to work a bit harder).

Chewing and saliva production play important roles in the digestion of foods. Horses rely much more on their teeth than we do, and also more than cattle, sheep and goats, because they can’t regurgitate and re-chew food, so they need more time to chew. Ruminants don’t chew a lot and swallow the grass or hay first, which is fermented by bacteria in the rumen to disrupt the fibre. This process makes it easier to grind the foods to smaller particles during the second “chewing the cud”.

Horses that are fed high levels of concentrates and not enough fibre/roughage spend less time chewing and may even eat too fast, swallowing large parts of their ration at a time, which results in very little saliva production. Over time these high concentrate –fast food- diets with low fibre can cause teeth problems and can lead to digestive dysfunction such as colic and stomach ulcers (see text box).

THE OESOPHAGUS

The oesophagus is the passageway from the mouth to the stomach. It contains rings of muscles that relax and contract to move the food down towards the stomach, a process known as peristalsis.

THE STOMACH

The stomach is a sack-like expansion of the digestive system, between the oesophagus and the small intestine. Horses are monogastrics (they have one stomach) as opposed to the compartmentalized stomach of ruminants. A horse’s stomach is similar to that of humans and pigs.

In comparison to the horse’s body size the stomach is a small organ; its capacity is about 7.5 to 15 liters, 8% of the total volume of the GI tract of an adult horse (see fig 1 & 2). In the suckling foal, the stomach capacity represents a larger proportion of the total digestive tract. The smaller stomach limits the amount of feed the horse can eat at one time.

The horse stomach can be divided into two main (mucosa) regions, the (upper) non-glandular region and the (lower) glandular region. The lining of the stomach is divided by a ridge or fold of the mucosa called the margo plicatus.

The non-glandular region takes up the top third of the horse’s stomach. This region is covered by a thin tissue called epithelium similar to the oesophagus, and is very sensitive; it does not produce any acids and also does not have protective factors against them. This is why the majority of ulcers occur in this region.

The glandular region, covers the remaining (lower) two-thirds of the stomach and contains glands that secrete hydrochloric acid (HCL), pepsin, bicarbonate and mucus. This region has much tougher tissue than the non-glandular region and produces mucus to protect the stomach wall. Although solid particles are broken down by hydrochloric acid (HCl) and the enzyme pepsin in the stomach, relatively little digestion occurs in the stomach. In contrast with ruminants, horses cannot vomit or regurgitate material from the stomach. All feed ingested will normally pass on to the small intestine, which we will discuss next month.

In a natural environment…

  • Horses eat on a continuous basis spending between 16-20 hours per day grazing and browsing.
  • Their diet is rich in fibre and contains very small amounts of simple sugars. Fibrous foods stimulate chewing and saliva production which can buffer acids in the stomach.
  • Horses naturally eat slowly and chewing is prolonged, so the stomach receives only small portions of food at a time, yet remains partially filled all the time.

In the domestic horse it is therefore important…

  • To allow your horse to eat small amounts of feed on a continuous basis, and preferably feed high fibrous foods.
  • To mix the concentrate diet with a fibre source such as low in non-structural carbohydrate (NSC) chaff or super fibers.
  • To offer roughage before feeding of the concentrate diet to slow down the passage rate and facilitate fiber digestion.
  • To divide the concentrate ration into multiple meals when large quantities are fed.

FACTS ABOUT THE STOMACH:

  • Food in the stomach is held for a relatively short time.
  • High fibre foods tend to stay in the stomach for longer periods than liquid meals (watery or non-fibrous foods including grains and mashes).
  • Gastric emptying of liquid meals occurs approximately 30 minutes after ingestion, whereas complete gastric emptying of hay occurs in 24hrs. The actual emptying time of the stomach also depends on the size of the meal.
  • A large meal will empty faster than a small meal.
  • Feeding a large meal limits the mixing action of the stomach and may prevent feed from coming into sufficient contact with gastric secretions in the stomach. This can have a negative effect on feed digestion.
  • When food is swallowed it enters the stomach via the oesophagus. The cardia sphincter, a powerful muscular valve, guards the entrance to the stomach. This valve function is very effective and prevents the horse from vomiting. In the rare occasion that it occurs, such as when the stomach is ruptured, the food will rush out the nostrils.

Read Part 2: The small intestine here.

Read Part 3: The large intestine here.

FURTHER READING:

  • Davies, Z. 2009. Introduction to horse nutrition. Wiley-Blackwell, 1st edition, United Kingdom
  • Frape, D. 2010. Equine Nutrition & Feeding. Wiley-Blackwell; 4th edition; United Kingdom.
  • Lewis, L.D. 1996. Feeding and care of the horse. 2nd edition, Lippincott Williams & Wilkins, USA.
  • More articles on nutrition and health at MB Equine Services
Mariette van den Berg, PhD, BAppSc (Hons), RAnNutr

Mariette van den Berg has a PhD in Equine Nutrition and Foraging Behaviour, is a RAnNutr equine nutritionist, a Certified Permaculture Designer and a dressage rider. She is the founder of MB Equine Services.

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