Equine Digestion

How has changing the horse’s natural diet and environment affected his digestion?

Przewalski's Horse - in a more natural environment.Horses evolved to eat large volumes of low quality forage, spending up to 16 hours a day grazing. The modern horse is often stabled for most of the day and goes for long periods without food. His feed usually consists of small amounts of high quality feed plus a limited amount of forage. Even the grass (and hay) he eats is unnatural in that it consists of a limited number of species and is likely to be much more nutritious than the uncultivated grass. Understanding how the horse’s digestive system works is essential in order to avoid the digestive problems which can result from this change in his natural diet.

Mouth

The mouth is an important part of the digestive system. The teeth need to be in good condition in order to chew food properly. Chewing prepares food for digestion by breaking it down into small pieces and lubricating it with saliva. Although horses do browse trees and bushes the natural position of the horses head when it is eating is close to the ground. Forcing the horse to eat with his head up by use of hay nets and hay racks causes unnatural wear of the teeth and is one of the reasons that regular teeth rasping is necessary. Another reason is that grass has a high silica content and the constant grinding wears the teeth more evenly than a softer diet.

Glands in the mouth produce saliva which wets and lubricates the food to help it move down the oesophagus to the stomach. Saliva is only produced when the horse chews. Feeding very dry food, particularly if it does not require much chewing (e.g. pony nuts) can result in choke – a lump of food stuck in the oesophagus. Saliva also prevents acid building up in the stomach as it contains bicarbonate. If the horse regularly goes for long periods without food the build up of stomach acid can result in ulcers. This is a common problem in racehorses as they are usually given high energy, low forage diets.

Stomach

Food enters the stomach through the cardiac sphincter. This small ring of muscle prevents food passing back up the oesophagus. It can withstand great pressure so it is rare for food to be regurgitated. This means that anything toxic must pass through the entire digestive system.

The stomach produces strong enzymes which begin to digest the soluble carbohydrates (simple sugars and starch). The stomach is particularly important in the digestion of hard feeds as they consist mainly of soluble carbohydrates. Protein and fat digestion also begins in the stomach.

Domesticated horse - stabled for much of the day. The stomach is small so giving a feed which is larger than the stomach can hold is not only wasteful but can be harmful as the food is passed on to the small intestine without the soluble carbohydrates being properly digested. Digestion is optimal when the food is less than two thirds full. For a 500kg horse this would be a feed of about 2kg. A very large intake of food can block the mechanism which allows food to trickle through to the small intestine. The stomach then becomes distended and, as the food begins to ferment, gas is produces which aggravates the situation and can rupture the stomach.

It is often thought that allowing the horse to drink with a feed will fill the stomach and push the food through the stomach too quickly. However as both the inlet and outlet are in the upper part of the stomach water tends to wash over the top of the feed and does not cause a problem.

Small intestine

Digestion of soluble carbohydrates, protein and fat is completed in the small intestine and the resulting components (glucose, amino acids and triglycerides) are absorbed into the bloodstream. Most vitamins and minerals are also absorbed here. The remainder, consisting mainly of fibre and water, is passed on to the large intestine.

Large intestine

The first part of the large intestine is the caecum. Food takes about 3 hours to reach it and usually remains there for 36 – 48 hours. The caecum is a large fermentation vat where fibre is broken down into fatty acids by enzymes. produced by. The horse cannot produce these enzymes himself and is dependent on a large microbial population (bacteria, yeasts and funghi) to produce them. The microbes also synthesise essential vitamins and are constantly changing depending on the type of food received.

Stress, wormers and antibiotics can have an adverse effect on the microbes and can lead to colic and digestive upsets. A probiotic can be fed to compensate. Changes in diet should introduced gradually (over about 7 days) to give the microbes time to adapt. A sudden influx of soluble carbohydrates which were not handled by the stomach (e.g. spring grass or a large amount of starch) can result in metabolic disorders such as laminitis.

After passing into the large colon, fibre continues to be digested by microbes. The large colon doubles back on itself twice and becomes quite narrow at these points. If a blockage occurs at either of these flexures colic can result.

The final part of the large intestine is the small colon. this is intermingled with part of the small intestine and moves freely making it susceptible to twisting. A twisted gut is a severe colic requiring immediate veterinary attention – an operation is usually required.

Water and nutrients are absorbed through the walls of the small colon. More water is absorbed from the rectum and the remaining waste materials expelled through the anus.

Rules of feeding

In order to minimise digestive problems we need to try to feed our domestic horse to mimic as far as possible their natural eating habits. This summarised in the following rules:

  • Feed little and often
  • Feed plenty of forage (ad lib if possible)
  • Feed according to work (many leisure horses need little or no hard feed)
  • Make clean water available at all times
  • Introduce diet changes gradually
  • Have teeth checked regularly (at least once a year)

More information

Posted on 9 July, 2011

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