Horses and other members of the genus Equus are adapted by evolutionary biology to eating small amounts of the same kind of food all day long.
In the wild, horses ate prairie grasses in semi-arid regions and traveled significant distances each day in order to obtain adequate nutrition.
Therefore, their digestive system was made to work best with a small but steady flow of food that does not change much from day to day.
Digestion begins in the mouth. First, the animal selects pieces of forage and picks up finer foods, such as grain, with sensitive, prehensile, lips. The front teeth of the horse, called incisors, nip off forage, and food is ground up for swallowing by the premolars and molars.
The esophagus carries food to the stomach. The esophagus enters the stomach at an acute angle, creating a one-way valve, with a powerful sphincter mechanism at the gastroesophageal junction, which is why horses cannot vomit. The esophagus is also the area of the digestive tract where horses may suffer from choke.
Horses have a small stomach for their size, which limits the amount of feed that can be taken in at one time. The average sized horse has a stomach with a capacity of only 4 US gallons (15?l), and works best when it contains about 2 US gallons (7.6?l).
One reason continuous foraging or several small feedings per day are better than one or two large meals is because the stomach begins to empty when it is two-thirds full, whether the food in the stomach is processed or not.
The small intestine is 50 to 70 feet (15 to 21 m) long and holds 10 US gallons (38?l) to 12 US gallons (45?l). This is the major digestive organ where 50 to 70 percent of all nutrients are absorbed into the bloodstream. Bile from the liver acts here, combined with enzymes from the pancreas and small intestine itself.
Equids do not have a gall bladder, so bile flows constantly, an adaptation to a slow but steady supply of food, and another reason for providing fodder to horses in several small feedings.
The cecum is the first section of the large intestine. It is also known as the "water gut" or "hind gut."
It is a blind-ended pouch, about 4 feet (1.2?m) long that holds 7 US gallons (26?l) to 8 US gallons (30?l). The small intestine opens into the cecum, and the cellulose plant fiber in the food is fermented by microbes for approximately seven hours. The fermented material leaves the cecum through another orifice and passes to the large colon.
The microbes in the cecum produce vitamin K, B-complex vitamins, proteins, and fatty acids. The reason horses must have their diets changed slowly is so the microbes in the cecum are able to modify and adapt to the different chemical structure of new feedstuffs. Too abrupt a change in diet can cause colic, because new materials are not properly digested.
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The large colon, small colon, and rectum make up the remainder of the large intestine. The large colon is 10 to 12 feet (3.0 to 3.7 m) long and holds up to 20 US gallons (76?l) of semi-liquid matter. Its main purpose 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.
The small colon is also 10 to 12 feet (3.0 to 3.7 m) long, holds about 5 US gallons (19?l), is the area where the majority of water is absorbed, and where fecal balls are formed. The rectum is about one foot long, and acts as a holding chamber for waste, which is then expelled from the body via the anus.
Equine nutrition is the feeding of horses, ponies, mules, donkeys, and other equines. Correct and balanced nutrition is a critical component of proper horse care.
Horses are non-ruminant herbivores of a type known as a "hindgut fermenter." This means that horses have only one stomach, as do humans. However, unlike humans, they also have to digest plant fiber (largely cellulose) that comes from grass and hay.
Therefore, unlike ruminants, which digest fiber in plant matter by use of a multichambered stomach, horses use microbial fermentation in a part of the digestive system known as the cecum (or caecum) to break down the cellulose.
In practical terms, horses prefer to eat small amounts of food steadily throughout the day, as they do in nature when grazing on pasture. Although this is not always possible with modern stabling practices and human schedules that favor feeding horses twice a day, it is important to remember the underlying biology of the animal when determining what to feed, how often, and in what quantities.
The digestive system of the horse is somewhat delicate. Horses are unable to regurgitate food, except from the esophagus. Thus, if they overeat or eat something poisonous, vomiting is not an option.? They also have a long, complex large intestine and a balance of beneficial microbes in their cecum that can be upset by rapid changes in feed.
Because of these factors, they are very susceptible to colic, which is a leading cause of death in horses. Therefore, horses require clean, high-quality feed, provided at regular intervals, and may become ill if subjected to abrupt changes in their diets. Horses are also sensitive to molds and toxins. For this reason, they must never be fed contaminated fermentable materials such as lawn clippings.
Fermented silage or "haylage" is fed to horses in some places; however, contamination or failure of the fermentation process that allows any mold or spoilage may be toxic.
Like all animals, equines require five main classes of nutrients to survive: water, energy (primarily in the form of fats and carbohydrates), proteins, vitamins, and minerals.
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Water makes up between 62-68% of a horse's body weight and is essential for life. Horses can only live a few days without water, becoming dangerously dehydrated if they lose 8-10% of their natural body water. Therefore, it is critically important for horses to have access to a fresh, clean, and adequate supply of water.
An average 1,000?lb (450?kg) horse drinks 10 US gallons (38?l) to 12 US gallons (45?l) of water per day, more in hot weather, when eating dry forage such as hay, or when consuming high levels of salt, potassium, and magnesium.
Horses drink less water in cool weather or when on lush pasture, which has a higher water content. When under hard work, or if a mare is lactating, water requirements may be as much as four times greater than normal. Though they need a great deal of water, horses spend very little time drinking; usually 1?8 minutes a day, spread out in 2-8 episodes.
Water plays an important part in digestion. The forages and grains horses eat are mixed with saliva in the mouth to make a moist bolus that can be easily swallowed. Therefore, horses produce up to 10 US gallons (38?l) or 85 lb. of saliva per day.
Horses that are heavily exercised, growing, pregnant or lactating need increased energy and protein in their diet.
?However, if a horse has too much energy in its diet and not enough exercise, it can become too high-spirited and difficult to handle.
Fat exists in low levels in plants and can be added to increase the energy density of the diet. Fat has 9 megacalories (38?MJ) per kilogram of energy, which is 2.25 times that of any carbohydrate source.
Because equids have no gall bladder to store large quantities of bile, which flows continuously from the liver directly into the small intestine, fat, though a necessary nutrient, is difficult for them to digest and utilize in large quantities. However, they are able to digest a greater amount of fat than can cattle.
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Horses benefit from up to 8% fat in their diets, but more does not always provide a visible benefit. Horses can only have 15-20% fat in their diet without the risk of developing diarrhea.
Carbohydrates, the main energy source in most rations, are usually fed in the form of hay, grass, and grain.
Soluble carbohydrates such as starches and sugars are readily broken down to glucose in the small intestine and absorbed. Insoluble carbohydrates, such as fiber (cellulose), are not digested by the horse's own enzymes, but are fermented by microbes in the cecum and large colon to break down and release their energy sources, volatile fatty acids.
Soluble carbohydrates are found in nearly every feed source; corn has the highest amount, then barley and oats. Forages normally have only 6-8% soluble carbohydrate, but under certain conditions can have up to 30%. Sudden ingestion of large amounts of starch or high sugar feeds can cause at the least an indigestion colic, and at the worst potentially fatal colitis or laminitis.
Protein is used in all parts of the body, especially muscle, blood, hormones, hooves, and hair cells. The main building blocks of protein are amino acids. Alfalfa and other legumes in hay are good sources of protein that can be easily added to the diet. Most adult horses only require 8-10% protein in their diet; however, higher protein is important for lactating mares and young growing foals.
Horses that are not subjected to hard work or extreme conditions usually have more than adequate amounts of vitamins in their diet if they are receiving fresh, green, leafy forages. Sometimes a vitamin supplement is needed when feeding low-quality hay, if a horse is under stress (illness, traveling, showing, racing, and so on), or not eating well.
Grain has a different balance of nutrients than forage, and so requires specialized supplementation to prevent an imbalance of vitamins and minerals.
Minerals are required for maintenance and function of the skeleton, nerves, and muscles. These include calcium, phosphorus, sodium, potassium, and chloride, and are commonly found in most good-quality feeds.
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Horses also need trace minerals such as magnesium, selenium, copper, zinc, and iodine. Normally, if adult animals at maintenance levels are consuming fresh hay or are on pasture, they will receive adequate amounts of minerals in their diet, with the exception of sodium chloride (salt), which needs to be provided, preferably free choice.
Some pastures are deficient in certain trace minerals, including selenium, zinc, and copper,and in such situations, health problems, including deficiency diseases, may occur if horses' trace mineral intake is not properly supplemented.
Calcium and phosphorus are needed in a specific ratio of between 1:1 and 2:1. Adult horses can tolerate up to a 5:1 ratio, foals no more than 3:1. A total ration with a higher ratio of phosphorus than calcium is to be avoided. Over time, imbalance will ultimately lead to a number of possible bone-related problems such as osteoporosis.
Foals and young growing horses through their first three to four years have special nutritional needs and require feeds that are balanced with a proper calcium:phosphorus ratio and other trace minerals. A number of skeletal problems may occur in young animals with an unbalanced diet.
Hard work increases the need for minerals; sweating depletes sodium, potassium, and chloride from the horse?s system. Therefore, supplementation with electrolytes may be required for horses in intense training, especially in hot weather.
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