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The Horse's Digestive System Overview
Horses and their unique digestion result in a precarious dietary balance and most equine problems are related to diet and digestion. An understanding of the horse’s digestive system is vitally important to safely and properly caring for horses. In nature the difference between life and extinction depends on a successful survival strategy. The ancient ancestors of Equidae (The horse family) developed the ability to consume the recently adapted dry climate vegetation; grasses. Eating grass, which is a difficult to digest, was a successful survival strategy. During this period many species were unable to adapt to eating grasses and became extinct. However this survival strategy came with a price. Horses are non-ruminant herbivores of a type known as a "hind-gut fermentor." This means that horses have a simple stomach, as do humans. However, unlike humans, they also have the ability to digest plant fiber (largely cellulose) that comes from grass and hay. They differ from ruminants, which digest fiber in plant matter by use of a. Ruminants (cattle, sheep and deer) which use bacteria in complex multi-chambered stomach to digest fiber by fermentation and use enzymatic digestion in the small intestines. A far more efficient system which allows ruminants much broader feed options.
Horses have a delicate but unique dual system which allows them to digest simple carbohydrate sources such as starch from grain in the fore gut as well as fibrous sources such as oat hulls, soy hulls, beet pulp, hay and pasture are digested in the hind gut. In the horse, all true digestion is by microbial enzymatic fermentation and takes place in the fore gut ahead of the cecum. This accounts for 52-58% of the crude protein digestion and virtually all soluble carbohydrate digestion (fiber excluded). In addition, bacterial or microbial digestion of fibre occurs in the cecum and colon where large quantities of volatile fatty acids are produced through fermentation and are subsequently absorbed. Unlike ruminants the horse’s delicate and simplified digestion system requires close observation and care to avoid deadly complications.
Equine Digestion
Equine Digestion - Mouth: Digestion begins in the mouth, which is also called the "oral cavity." It is made up of the teeth, the hard palate, the soft palate, the
tongue and related muscles, the cheeks and the lips. Horses also have three pairs of salivary glands, the parotid (largest salivary gland and located near the poll), sub maxillary (located in the jaw), and sublingual (located under the tongue). Horses select pieces of forage and pick up finer foods, such as grain, with their sensitive, prehensile lips. The front teeth of the horse, called incisors, clip forage. The food is then moved in the mouth by the tongue and ground up for swallowing by the premolars and molars. 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 sub maxillary, and the sublingual. Horses will produce up to 10 gallons (85 lb.) of saliva per day.
Equine Digestion - Teeth: For enzymatic and microbial action to digest feed efficiently, the horse needs healthy teeth to grind feed and allow enzymes and bacteria to attack the plant cell walls. Teeth should be examined during the annual health check to ensure that they are wearing normally and are effectively grinding the feed. The mouth contains 36 (females) to 40 (males) teeth. The wolf teeth are not counted as not all horses have them. Teeth are important in harvesting and chewing feed. The horse’s upper jaw is wider than the lower jaw, so sharp points often develop on the molar teeth. These points may prevent normal chewing which reduces the food value received from the feed and may predispose a horse to colic. Filing (or floating) the teeth will remove the points. Horses with a parrot mouth (over bite) or a monkey mouth (under bite) may also have difficulty in harvesting and chewing feeds.
Equine Digestion – Esophagus: The esophagus is a simple muscular tube that takes food from the mouth to the stomach and is about 4-5 feet in length, and carries food to the stomach. A muscular ring, called the cardiac sphincter, connects the stomach to the esophagus. This sphincter is very well developed in horses and because of these muscles in the cardiac sphincter valve leading into the stomach are very strong, this and the oblique angle at which the esophagus connects to the stomach explain why it’s almost impossible for a horse to vomit.
Equine Digestion – 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. 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 concentrate once or twice a day. This greatly undermines the horse’s digestive capabilities. It has been established that we can improve the digestive efficiency of a horse by feeding small meals often, but this has to be weighed against the labor costs of doing so. Horses have a relatively small stomach for their size, and this limits the amount of feed a horse can take in at one time. The average sized horse (800 to 1200 lb) has a stomach with a capacity of only four gallons, and works best when it contains about two gallons. Because the stomach empties when 2/3 full, whether stomach enzymes have completed their processing of the food or not, and doing so prevents full digestion and proper utilization of feed. To avoid this potential problem continuous foraging or several small feedings per day are preferable to one or two large ones.
In the stomach, assorted acids and the enzyme pepsin break down food. Pepsin allows for the further breakdown of proteins into amino acid chains. Other agents include resin, lipase and hydrochloric acid to help break down solid particles. During digestion there are also bacteria present that produce lactic acid. Additionally, the stomach absorbs some water, as well as ions and lipid soluble compounds. The end product is food broken down into chyme (The term used to refer to partially digested food as it moves through the digestive tract). It then leaves the stomach through the pyloric valve, which controls the flow of food out of stomach. The rate of passage of food 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.
Small Intestine (Upper Gut): The horse’s small intestine is 50 to 70 feet (21 m) long and holds 10 to 12 gallons. This is the major digestive organ. After the food 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 50—70% of carbohydrate digestion and absorption and almost all amino acid absorption occur in the small intestine. It can take as little as 30 to 60 minutes for food to pass through the small intestine.
Most food is digested and absorbed into the bloodstream from the small intestine, including proteins, simple carbohydrate, fats, and vitamins A, D, and E. Any remaining liquids and roughage move into the large intestine.
The small intestine contains three sections; the duodenum, jejunum and ileum.
Duodenum: Duodenum is derived from the Latin term "duodena" meaning 12. The majority of digestion occurs in the duodenum as bile from the liver aids in digesting fats in the duodenum combined with enzymes from the pancreas and small intestine. Horses do not have a gall bladder, so bile flows constantly.
Jejunum: Jejunum is derived from the Latin term "jejunum" meaning "fasting" or "empty". Galen named the second part of the small intestine in humans the name because he believed it was always found empty after death. The Jejunum is important in re-absorption process of bile salts and absorption of nutrients during digestion and is the part of the small intestine where the majority of nutrient absorption occurs. The Jejunum connects the duodenum and ilium.
Ilium: Ilium is the Latin word for flank and is used to designate the iliac bone in the pelvis. The third segment of the small intestine was called the ileum because in man the small intestines are largely supported by the iliac bone. This part of the upper gut is primarily a transition area between large and small intestines.
The large Intestine (Hind Gut): The large intestine contains two sections; the Cecum and the Colon.
Cecum: The cecum is the first section of the large intestine. It is also known as the "water gut" or "hind gut." It is a pouch, about 4 feet (1.2 m) long that holds 7 to 8 gallons of digestive chyme. The cecum is a marvel of symbiosis (Beneficial relationship between several disparate organisms) as it is a microbial fermentation vat, similar to the rumen in a cow. These microbes produce specialized enzymes which ferment and break down the cellulosic structure of primarily fibrous feeds like hay or pasture, effectively any material that was not digested in the small intestine. These bacteria feed upon the chyme, which will remain in the cecum for about seven hours, allowing bacteria time to start breaking it down. 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 by the horse.
Cecum – The Biological Log Jam: The reason horses must have their diets changed slowly is that the microbial population in a cecum is somewhat specific as to what feedstuffs it can digest. The result; the bacteria in the cecum are slow to modify and adapt to the different chemical structure of new feeds. Too abrupt a change in diet can cause colic, as the new food is not properly digested.
Another potential problem is because or the unique design of the cecum. 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 impaction in the lower end of the cecum, which in turn produces pain and potentially colic.
Colon: The large colon, small colon, and rectum make up the remainder of the large intestine. Microbial digestion continues, and most of the nutrients made through microbial digestion are absorbed here. In addition to the vitamins and fatty acids absorbed in the colon, water is also absorbed, resulting into the formation of fecal balls. These fecal balls, which are the undigested roughage and mostly indigestible portion of what was fed, are then passed from the rectum.
Large Colon: The large colon is 10-12 feet long and holds up to 20 gallons of semi-liquid matter. It is made up of the right lower (ventral) colon, the left lower (ventral) colon, the left upper (dorsal) colon, the right upper (dorsal) colon, and the transverse colon, in that order. The main purpose of the large colon is to absorb carbohydrates, which were broken down from cellulose in the cecum. Due to its many twists and turns, it is a common place for a type of horse colic called an impaction.
Small Colon: The small colon is 10-12 feet in length and holds only 5 gallons of material. It is the area where the majority of water in the horse's diet is absorbed, and is the place where fecal balls are formed.
Rectum: The rectum is about one foot long, and acts as a holding chamber for waste matter, which is then expelled from the body via the anus.
Equine Dietary Cautions
Avoid Sudden Feed Changes:If a change of feed occurs, it takes about three weeks to develop a microbial population that can digest a new feed and maintain a normal flow through the cecum. A general rule for safely changing feeds:
Equine Digestions in Foals: The foal and growing horse have undeveloped cecal and colonic digestion as compared to the adult horse. There is very little microbial digestion before three months of age. Therefore, the foal requires a diet low in fiber and easily digested in the fore gut. Foals who are seen eating their mothers manure are thought to be obtaining a bacterial culture necessary for future microbial digestion. As noted during digestion there are also bacteria present that produce lactic acid. It is believed that these bacteria may be important in the case of a ruptured stomach. Ruptured stomachs occur most often in foals at the time of weaning. The foal stops eating at weaning due to the stress associated with the dam being taken away. A few days later the foal decides to eat. If there is a large amount of grain available, the foal often overstuffs itself. Lactic-acid-forming bacteria ferment this mass of carbohydrate, producing high levels of lactic acid. This lactic acid causes paralysis of the pyloric sphincter, which normally lets the food out of the stomach. The stomach bursts from gas produced in the bacterial fermentation of the feed.
Feed or Hay First which is Better? 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. Another question is whether a horse should get water before or after a meal. If you leave it up to the horse, they will usually drink a little as they eat, if consuming dry feeds. The best recommendation is to offer water free choice at all times.
Feeding Cautions: 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 moldy or spoiled feeds. Urea is a mineral block additive and feed supplement which is fed to cattle. Urea 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 a horse.