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The horse’s digestive system

Horse's Digestive System

Reproduced with the permission of QA International from the book The Visual Dictionary. © QA International, 2003. All rights reserved.
Click to enlarge.

The horse is a non-ruminant herbivore. Non-ruminant means that horses do not have multi-compartmented stomachs as cattle do. Instead, the horse has a simple stomach that works much like a human’s. Herbivore means that horses live on a diet of plant material. The equine digestive tract is unique in that it digests portions of its feeds enzymatically first in the foregut and ferments in the hindgut.

The horse’s digestive system really should be thought of as being in two sections. The first section has similarities to the pre-caecal digestive system of a monogastric animal such as the dog, man or pig. The second section is more like the rumen of a cow. This has profound effects on the way we need to think about feeding the horses in our care. However, the horse is neither a dog nor a ruminant or even a direct combination of both. It is unique and needs to be considered as such.

The cow benefits by having the microbial breakdown of fibrous food at the start of the GIT (gastrointestinal tract) and nutrient absorption can then take place along the entire intestine. Dietary protein is not utilised efficiently because the microbial fermentation breaks down protein plus some carbohydrate.

In the horse unlike in the ruminant the microbial fermentation occurs after the ‘monogastric’ like section rather than before. This has a great impact on how we should feed a horse and explains in part why the horse and cow differ so much in their nutritional efficiencies and requirements.

Mouth

Horses grasp food using a combination of the lips, tongue and the teeth. Horses’ lips are extremely tactile when it comes to consuming feed. They can be quite selective as many of us would have seen powdered supplements or pellets in a nice little pile at the bottle of the feed bin.

Feeds are mixed with saliva in the mouth to make a moist bolus that can be easily swallowed. Three pairs of glands produce saliva – the parotid, the submaxillary, and the sublingual. Horses will produce between 20-80 litres of saliva per day. Salvia contains bicarbonate which buffers and protects amino acids in the highly acidic stomach. Saliva also contains small amounts of amylase which assist with carbohydrate digestion.

The mouth contains 36 teeth (females) and 40 teeth (males). Wolf teeth are not included as not all horses have them. The horses upper jaw is wider than the bottom jaw to allow for a chewing motion that is quite complex. The chewing action of the horse is a sweeping action which incorporates both lateral forward and backwards motions and vertical motions. This allows the feed to be effectively ground and mixed with saliva to initiate the digestive process.

The texture of the feeds fed to horses will dramatically influence the chewing rate (jaw sweeps) and rate of ingestion. An average horse with general take 60,000 jaw sweeps per day when grazing. This amount will be dramatically reduced when confined to a stable and large amounts of grain are fed.

The size of the horse also effects the time and amount of jaw sweeps it takes to sufficiently masticate the feed. The average 500kg horse generally takes 40 minutes and 3400 jaw sweeps to consume one kilogram of hay. Ponies will generally take twice as long to consume this amount of hay. Oats on the other hand only take 10 minutes and 850 jaw sweeps for the mature horse and up to five times longer for ponies.

When horses chew fibrous feeds such as hay or pasture it is a long jaw sweep action. This is why horses continually out on pasture rarely develop sharp edges on their teeth. Grains are consumed in a shorter sweep which does not extend past the outer edge of the teeth. When large amount of grain are fed, horses chewing action will be changed and the teeth will not be worn evenly. Hooks or sharp edges will start to form on the outside edge of the teeth. If teeth are not properly ‘floated’ or rasped the rate of intake, chewing efficiency, appetite and temperament can be seriously affected. If feed are not masticated correctly the bolus (feed and salvia) may lodge in the oesophagus and cause choke.

Esophagus

This is a simple muscular tube that takes food from the mouth to the stomach. The esophagus is around 1.5m in length in a mature horse. As the esophagus is quite long and the horse has very little reflux capacity, incorrectly chewed are large pieces of feed such as carrots etc can lodge inside the horse’s esophagus and can cause choke. This is why it is important to maintain horses’ teeth correctly to ensure horses chew their feed sufficiently as well as stopping horses ‘bolting’ their feed down without chewing their feed. By adding chaff to a horses’ feed or putting a brick or large stone in to a horses feed bin will slow horses rate of intake and reduce the risk of choke from a horse ‘bolting’ its’ feed.

Stomach

The stomach of the horse is small in relation to the size of the animal and makes up only 10% of the capacity of the digestive system or 9-15 litres in volume. The natural feeding habit of the horse is to eat small amounts of roughages often. Domestication has brought a change to all this. Horses are now expected to eat large amounts of grain feed once or twice a day to suit our lifestyle. This greatly undermines the horse’s digestive capabilities and health. It has been established that we can improve the digestive efficiency of a horse by feeding small meals often (assimilate natural grazing), but this has been weighed against the labour costs of doing so.

In the stomach, feed is mixed with pepsin (an enzyme to digest proteins) and hydrochloric acid to help break down solid particles. The rate of passage of feed through the stomach is highly variable, depending on how the horse is fed. Passage time may be as short as 15 minutes when the horse is consuming a large meal. If the horse is fasted, it will take 24 hours for the stomach to clear.

It has long been a question as to what you should feed a horse first, grain or hay. Because of their density, grains tend to stay in the stomach longer, but it has not been proved to be advantageous to feed either first. For fast eaters chaff can be added to the feed to bulk the feed out which slows the rate of consumption. Another question is whether a horse should get water before or after a meal. If you leave it up to the horse, it will usually drink a little as it eats, if consuming dry feeds. The best recommendation is to offer fresh clean water at all times.

The stomach has 3 main areas; the saccus caecus, fundic and pyloric regions. Each is quite unique in structure and function. The saccus caecus region is located at the entrance of the stomach and the oesophagus. When food enters the stomach it begins to come under the influence of hydrochloric acid and pepsin – a protein digesting enzyme.
However, this feed, (especially if it is predominantly grass), is already releasing soluble sugars for absorption and undergoing bacterial fermentation to produce lactic acid. Under normal circumstances, as the hydrochloric acid mixes in with the stomach ingesta, the pH drops, fermentation slows down and eventually stops. This is an important process – because if it does not occur and fermentation continues, the relatively non-distensible, fixed-volume stomach will very quickly fill with gas and, with little ability to release pressure through the oesophagus gastric colic may result or in extreme cases a ruptured stomach lining.

As the feed moves through the stomach the next section of the stomach is the fundic region. The pH level decreases to around 5.4 and fermentation begins to halt. Pepsin and stomach acid initiates the digestion and degradation of lipids (fats) and proteins (amino acids). The final section of the stomach is the pyloric region where the stomach joins the small intestine. The pH drops further to 2.6 which virtually eliminates all fermentable lacto-bacteria. The proteolytic activity (protein digestion) in this area is 15-20 times that of the fundic region.

Changed feeding practices have led to long periods of the day where horses’ stomachs are virtually empty. The mixture of feed and saliva mixes with the acid produced by stomach. When the horses’ stomach is empty the acid destroys the non protected squamous cells of the saccus caecus region of the stomach. This causes the stomach lining to ulcerate. Studies have shown that over 80% of thoroughbreds have some degree of stomach ulceration. Stomach ulcers can affect horse’s appetite, behaviour and performance. Feeding horses a higher proportion of roughage in their diet, small frequent meals and allow them ability to graze will dramatically reduce the frequency and severity of stomach ulcers.

Small Intestine

Digesta passes from the stomach into the small intestine. The small intestine is approximately 28% of the horses’ digestive tract, is 15-22m long and has a volume of 55-70 litres. This is the major site of digestion in the modern performance horse. The small intestine is broken into 3 sections; the duodenum, jejunum and the ileum.

The saliva of a horse contains only small amounts of amylase and there is little actual digestion that occurs in the stomach of most horses. Most digestion therefore occurs in the small and large intestines. Although the intestine itself secretes some enzymes, the pancreas releases by far the greatest amount. In the small intestine the digestive processes (enzymatic breakdown of proteins, fats, starches and sugars) are similar to those of other monogastric animals but the activity of several of the enzymes in the chyme (food mix), in particular amylase, are lower than in other monogastric animals.

There are many components to this digestive process. Pancreatic enzymes will help digest the food; carbohydrates digest sugars and starches; proteases break proteins down into amino acids; lipases and bile from the liver is added to emulsify (break into smaller units) fats and to suspend the fat in water. Bile constantly flows into the small intestine from the liver because the horse does not have a gall bladder in which to store it. The pancreatic juice also contains some alkali and bicarbonates, which buffer the acid ingesta (feed bolus) leaving the stomach, and help maintain an optimal environment for the functioning of the digestive enzymes.

After the feed has been digested, it is absorbed through the walls of the small intestine and carried off by the blood stream to whatever cells need the nutrients. Nearly 30-60% of carbohydrate digestion and absorption and almost all amino acid absorption occur in the small intestine. Fat soluble vitamins A, D E and K are absorbed in the small intestine as well as some minerals such as calcium and some phosphorous. Changing the structure of carbohydrates of the feed by processes such as micronization greatly increases the grains digestibility in the small intestine to around 90%. This reduces the burden on the large intestine and can reduce the risk of over loading the digestive tract and incidences of colic, laminitis and acidosis.

It can take as little as 30 to 60 minutes for food to pass through the small intestine, as most digesta moves at a rate of approximately 30cm per minute. However, feed generally take 3-4 hours to pass through the small intestine. The quicker the digesta moves through the small intestine the less time the enzymes have to act. The addition of oil to a horses’ diet has shown to reduce the flow of feed through the small intestine thus allowing the digestive enzymes more time to process starches, proteins and fats thereby increasing the total tract digestibility of these nutrients and maximizing the small intestines digestive efficiency.

Horses are very susceptible to colic or death from toxic materials in the feed. Unlike the cow that has bacteria in the rumen that can detoxify materials before they reach the small intestine, toxic material a horse may consume enters the intestine and is absorbed into the blood stream before it can be detoxified. Therefore, it is very important not to feed horses mouldy or spoiled feeds. Urea is a feed supplement fed to cattle that can be utilized in their rumen to make protein. Horses cannot use this feed supplement because it is absorbed in the small intestine before it can get to the cecum where it could be used. Urea can be toxic to the horse, but the horse can tolerate the level at which it is added to most cattle feeds.

Microbial protein, which is synthesised in the large intestine, cannot be utilised to any great extent by the horse. This means that animals with a high demand for protein (foals, lactating mares and probably intensively exercising horses) must be fed high quality protein which can be broken down and absorbed primarily in the small intestine. In a practical sense this does not mean we need to necessarily increase the crude protein content of our horses feed but to increase the quality of it. This may mean ensuring that the correct levels of essential amino acids such as lysine, methionine and threonine are in sufficient levels to meet the demands of the horse.

The Hind Gut

The hindgut or large intestine, to which it is commonly referred to, consists of the caecum, large (or ascending colon, small colon, rectum and anus. Here is where a bulk of the digestive more is done. The hindgut comprises of 62% of the entire gut is approximately 7 meters in length and has a volume of 140-150L. Digestion in the hindgut is largely microbial rather than enzymatic. Digestion in the hindgut is performed by billions of symbiotic bacteria which efficiently break down plant fibres and undigested starches into simpler compounds call volatile fatty acids (VFA’s) which can be absorbed through the gut wall.

Compared with the digestive tract of ruminants the horse is not as well suited to digesting products of grass with high crude fibre content, low-grade protein and low levels of carbohydrates, starch and fat. They are however much better at it than man or pigs! And equids have reduced these disadvantages by selectively grazing large amounts of feed each day.

Caecum

The caecum is a blind sack approximately 1.2m long that can hold around 28-36 litres of feed and fluid. The caecum is a microbial inoculation vat, similar to the rumen in a cow. The microbes break down feed that was not digested in the small intestine, particularly fibrous feeds like hay or pasture.

The caecum is odd in design because its entrance and exit are both at the top of the organ. This means that the feed enters at the top, mixes throughout, and is then expelled up at the top. This design is the cause of problems if an animal eats a lot of dry feeds without adequate water or if a rapid change of diet occurs. Both may cause a compaction in the lower end of the caecum, this in turn produces pain (colic). The microbial population in a caecum is somewhat specific as to what feedstuffs it can digest. It can take up to 2-3 weeks for the microbial population of the caecum to adjust to a new diet and return to normal function. This is why you will read on bag labels to slowly introduce new feeds to a horse over 7-14 days.

Feed will remain in the caecum for about seven hours, allowing bacteria time to start breaking it down using the fermentation process. The microbes will produce vitamin K, B-complex vitamins, proteins, and fatty acids. The vitamins and fatty acids will be absorbed, but little if any protein will be absorbed.

Large Colon

The large colon consists of the right and left ventral colons and dorsal colon is about 3-3.5m long and will hold 86 litres. Microbial digestion (fermentation) continues, and most of the nutrients made through microbial digestion are absorbed here as well as B group vitamins produced by the bacteria and some trace minerals and phosphorous. The ventral colons have a “sacculated” construction that resembles a series of pouches. This design facilitates the digestion of large quantities of fibrous materials but due to its design can become a large risk factor for colic. The pouches can easily become twisted and fill with gas due to fermentation of the feed. Feed may reach here in as little as seven hours and will stay here for 48-65 hours.

Small Colon, Rectum and Anus

The small colon is approximately the same length as the large colons but only has diameter of roughly 10cm. By now the vast majority of the nutrients have been digested, and what is left can not be digested or used by the horse. The main function of the small colon is to reclaim excess moisture and return it to the body. This results in fecal balls being formed. These fecal balls, which are the undigested and mostly indigestible portion of what was fed some 36-72 hours ago are then passed to the rectum and expelled as manure through the anus.

Digestive Tightrope

The equine gastrointestinal tract functions well under normal constant conditions. However as all horse people know the equine GUT is extremely sensitive and easy to upset and colic is the number one cause for equine death.

Any sudden change in diet can compromise and change the bacteria population in the horse’s hindgut, potentially resulting in colic and at least a reduced digestive efficiency of the diet. Keeping the microflora happy can be difficult if a horse is under stress, travelling large distances, suffered illness or injury, received antibiotics, weaned foal or a high performance horse being fed large amounts of grain. It is imperative that we treat the horse hindgut with respect and monitor the diet of ours horses and there general health. Trying to feed your horses as close to their natural grazing habit as possible, (small meals frequently) will greatly reduce the risk of gastrointestinal tract disorders. This will allow you to enjoy your horse to its fullest potential.

For further information on the equine digestive tract or nutritional advice please contact HY GAIN FEEDS on 1300 HYGAIN (494246) or email us.

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One Response to The horse’s digestive system

  1. Hannah April 22, 2014 at 11:38 am #

    Thanks so much for this information it was a great help to my Equine Studies project the only information missing was information on the Kidneys.

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