02.10.20
People often wonder how many stomachs does a horse have, but 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 nor 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.
Horses grasp food using a combination of the lips, tongue and 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 bottom 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 assists 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 horse's 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 that 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 takes 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 affects 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 a large amount of grain is 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 is not masticated correctly, the bolus (feed and saliva) may lodge in the oesophagus and cause choke.
This is a simple muscular tube that takes food from the mouth to the stomach. The oesophagus is around 1.5m in length in a mature horse. As the oesophagus is quite long and the horse has very little reflux capacity, incorrectly chewed large pieces of feed, such as carrots, etc, can lodge inside the horse's oesophagus 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. Adding chaff to a horse's feed or putting a brick or large stone into a horse's feed bin will slow a horses rate of intake and reduce the risk of choke from a horse 'bolting' its feed.
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 roughage 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 digests 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 proven 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 initiate 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 lactobacteria. 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 when horses' stomachs are virtually empty.
The mixture of feed and saliva mixes with the acid produced by the stomach. When the horse's stomach is empty, the acid destroys the unprotected 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 a horse's appetite, behaviour and performance. Feeding horses a higher proportion of roughage in their diet, small frequent meals, and allowing them the ability to graze will dramatically reduce the frequency and severity of stomach ulcers.
Digester passes from the stomach into the small intestine. The small intestine is approximately 28% of the horse's 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, is 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 are 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 bloodstream 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 phosphorus. Changing the structure of carbohydrates of the feed by processes such as micronization and extrusion greatly increases the grain's digestibility in the small intestine. This reduces the burden on the large intestine and can reduce the risk of overloading 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 takes 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 horse's diet has been 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 maximising the small intestine's 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 bloodstream 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 utilised 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 (such as foals and lactating mares) 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 at sufficient levels to meet the demands of the horse's life stage.
The hindgut, often referred to as the large intestine, consists of the caecum and large colon and makes up approximately 60% of the horse’s total gastrointestinal (GIT) volume. This highly specialised section of the digestive tract has evolved to process and extract nutrients from plant components that cannot be digested in the small intestine.
Unlike the foregut, where digestion is largely enzymatic, hindgut digestion is primarily microbial. Billions of symbiotic bacteria ferment carbohydrate-rich material, mainly plant fibre, into volatile fatty acids (VFAs), which serve as an important energy source for the horse. This process is known as microbial fermentation, giving horses the name “hindgut fermenters.” These microbes not only assist in fibre breakdown but also support the horse’s overall health and wellbeing. A balanced hindgut microbiome is essential for protecting digestive function, supporting immunity, and optimising nutrient absorption.
The caecum is a blind sac 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 feed without adequate water or if a rapid change of diet occurs. Both may cause a compaction in the lower end of the caecum, which in turn produces pain (colic). The microbial population in the 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 large colon consists of the right and left ventral colons and the 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 phosphorus. 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.
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 faecal balls being formed. These faecal 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.
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 bacterial 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, has 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 our horses and their general health. Trying to feed your horses as close to their natural grazing habit as possible (small meals frequently) and ad lib access to long-stemmed roughage will greatly reduce the risk of gastrointestinal tract disorders. This will allow you to enjoy your horse to its fullest potential.
If you are keen to keep exploring how nutrition shapes the horse's digestive system, you can continue your reading with our guides on feeding and gut health at Hygain®.
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