The horse’s digestive system

When chewing on hay or any other paddock vegetation, the horse chews with long jaw sweeps

Horses are trickle feeders and non-ruminant herbivores, meaning that they are designed to graze for most of the day on vegetation, and process their feed through one stomach chamber. The equine digestive system is unique in the way in which it processes feed. Feed is first processed enzymatically in the foregut (stomach and small intestine) and then ferments in the hindgut (large intestine).

We look at how a horse’s feed is broken down and digested through the horse’s system.

Mouth

Horses get food into their systems by grasping it with their lips and incisor teeth. The premolars and molars are then responsible for chewing and grinding down the feed. Chewing stimulates the production of saliva, which is made up of water, bicarbonate and amylase – which assist respectively in lubricating swallowed food, the buffering of the acid in the stomach and, to some extent, the digestion of carbohydrates. However, the main purpose of the saliva is to help move the food through the oesophagus easily and to buffer the stomach acid, as the actual levels of the enzyme amylase in the saliva are low in horses compared to in other species. Horses can produce between 20 and 80ℓ of saliva per day depending on how much of their day they spend feeding, and how dry the food they ingest is.

The rate at which a horse chews is referred to as a ‘jaw sweep’. Horses typically take 60,000 jaw sweeps per day if grazing constantly. The type of grass, time spent feeding and the size of the horse will all influence the rate at which a horse consumes his grass. The average 500kg horse takes 40 minutes and 3,400 jaw sweeps to consume 1kg of hay.

Influence on the teeth

When chewing on hay or any other paddock vegetation, the horse chews with long jaw sweeps. When feeding on grains and concentrates, much shorter jaw sweeps are required and so the outer edges of the teeth are not naturally filed down, therefore resulting in the development of sharp edges. This is why horses who live out and constantly graze rarely develop sharp edges on their teeth, and domesticated horses have to have regular floating from a dentist in order to maintain their teeth.

Oesophagus

The horse’s digestive system

The oesophagus is a muscular tube (up to 1.5m long in an adult horse) that is responsible for passing food from the mouth to the stomach chamber. Horses have a very strong, muscular sphincter at the bottom of the oesophagus (which is why they can’t vomit), and the oesophagus is long, meaning that if something solid were to get stuck in the horse’s oesophagus, they wouldn’t be able to ‘cough’ it back up as people and some other mammals can do. It’s therefore important to maintain a horse’s teeth on a regular basis so that they are able to properly grind down their food into small particles before swallowing. It’s also important to break down big solid foods, such as carrots, into smaller chunks, so that they don’t end up choking on large pieces.

In the stomach

A horse’s stomach has a capacity for 8 to 15ℓ in volume – quite small if you consider the size of an adult horse. This isn’t a problem in nature, however, as horses are trickle feeders, and therefore are designed to digest small amounts of food all day round. But with domestication, we now expect the horse’s digestive system to process two to three big meals of grain per day, with little roughage in between. This creates issues as the horse’s stomach is found to work best when it is ¾ full. The feed that reaches the stomach is broken down with pepsin, an enzyme responsible for digesting proteins, and hydrochloric acid, which breaks down the more solid foods into smaller pieces.

The stomach itself is divided into three areas: the saccus caecus, fundic and pyloric regions.

The saccus caecus region is the first region food enters from the oesophagus. As the food enters, hydrochloric acid and pepsin start to break down the proteins. In addition, the carbohydrate component of the feed continues to break down to release soluble sugars for absorption and the process of bacterial fermentation, which produces lactic acid and gas, begins. However, as the food starts to mix with the hydrochloric acid, the pH level drops and fermentation slows down. Fermentation has to stop because if it continues unchecked, the stomach will quickly fill with gas, which can’t easily be released through the oesophagus. If pressure builds up, the horse could colic or the stomach could rupture.

The feed then moves to the fundic region of the stomach. pH levels drop to around 5.4 and fermentation processes stop completely. Pepsin and stomach acids start to further digest and break down fats and proteins. Finally, the food moves to the pyloric region of the stomach, where the stomach joins the small intestine. At this point of digestion, pH levels are around 2.6 (very acidic), so no fermentation occurs. Protein digestive activity in this area of the stomach is 15 to 20 times higher than that found in the fundic region, as the enzymes work best at this pH.

The full article appears in the March issue (131) of HQ > Shop now