The horse is a nonruminant herbivore. Nonruminant means that horses do not have a multi-compartmented stomach as cattle do. Instead, the horse has a simple stomach that works much like a humans. Herbivore means that horses live on a diet of plant material. Figure 1 is a diagram of the equine digestive tract.
|Figure1. The equine digestive tract.|
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 horses 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 (overbite) or a monkey mouth (underbite) may also have difficulty in harvesting and chewing feeds.
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 up to 10 gallons (85 lb.) of saliva per day.
This is a simple muscular tube that takes food from the mouth to the stomach. Because the muscles in the cardiac sphincter valve leading into the stomach are very strong, it is almost impossible for a horse to vomit. It is more likely that the stomach will rupture in the event of an overconsumption of 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. 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 horses 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.
In the stomach, food is mixed with pepsin (an enzyme to digest proteins) and hydrochloric acid to help break down solid particles. 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.
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.
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, he will usually drink a little as he eats, if consuming dry feeds. The best recommendation is to offer water free choice at all times.
The small intestine of the horse is approximately 70 feet long and can hold up to 48 quarts. This is the major organ of digestion in the horse. There are many components to this digestive process. Pancreatic enzymes will help digest the food; carbohydrases digest sugars and starches; proteases break proteins down into amino acids; 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.
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 5070% 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.
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 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.
The cecum is a blind sack approximately four-feet long that can hold up to 40 quarts of food and fluid. The cecum 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 cecum 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 cecum, which in turn produces pain (colic). The microbial population in a cecum is somewhat specific as to what feedstuffs it can digest.
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:
Week 1: Feed a mix of three-fourths of the old ration and one-fourth of the new ration.
Week 2: Feed a mix of one-half of the old ration and one-half of the new ration.
Week 3: Feed a mix of one-fourth of the old ration and three-fourths of the new ration.
Week 4: Feed all new ration.
Feed 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.
The colon or large intestine is about 12 feet long and will hold 80 quarts. Food may reach here in a little as seven hours and will stay here for 4865 hours. 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 in fecal ball formation. These fecal balls, which are the undigested and mostly indigestible portion of what was fed, are then passed from the rectum.