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." Horses have only one stomach, as do humans. However, unlike humans, they also need to digest plant fiber (largely cellulose) that comes from grass or hay. Ruminants like cattle are foregut fermenters, and digest fiber in plant matter by use of a multi-chambered stomach, whereas horses use microbial fermentation in the hindgut (cecum and large colon) 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 lands."Horse Nutrition - Frequency." Bulletin 762-00, Ohio State University. Web site accessed February 9, 2007. 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 hindgut 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 and water at regular intervals.March, Linda. "Feeding Your Horse To Avoid Problems," from University of Illinois, College of Veterinary Medicine. Web site accessed February 16, 2007 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."Horse Nutrition - Carbohydrates and fats." Bulletin 762-00, Ohio State University. Web site accessed February 9, 2007.
The digestive system
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.
Chewing and swallowing 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 stomach and small intestine Horses have a small stomach for their large size, which limits the amount of food that can be taken in at one time. The average sized horse has a stomach with a capacity of only , and works best when it contains about . 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 long and holds to . This is the major digestive organ where 50 to 70 percent of all nutrients are absorbed into the bloodstream."Horse Nutrition - The Horse's Digestive System." Bulletin 762-00, Ohio State University. Web site accessed February 9, 2007. 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 and large intestine 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 long that holds to . 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."Digestive Aids: Does Your Horse Need Them?." Equine Review Accessed July 4, 2009. 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.
The large colon, small colon, and rectum make up the remainder of the large intestine. The large colon is long and holds up to 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 long, holds about , 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.
Nutrients 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.
Water
Water makes up between 62-68% of a horse's body weight and is essential for life.Kane, Ed. "What's new about water." Equus issue 359, August 2007, pp 61-64. Horses can only live a few days without water, becoming dangerously dehydrated if they lose 8-10% of their natural body water.
An average horse drinks 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."Horse Nutrition - Diet Factors - Water." Bulletin 762-00, Ohio State University. Web site accessed February 9, 2007. In the winter, snow is not a sufficient source of water for horses. 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 or 85 lb. of saliva per day.
Energy nutrients and protein Nutritional sources of energy are primarily fat and carbohydrates. Protein is primarily a source of amino acids, which are critical building blocks for muscles and other tissues."Horse Nutrition - Protein." Bulletin 762-00, Ohio State University. Web site accessed February 9, 2007. Horses that are heavily exercised, growing, pregnant or lactating need both increased energy sources 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,Warren, Lori K. "Horse Feeding Myths and Misconceptions" Horse Industry Section, Alberta Agriculture, Food and Rural Development Web site accessed February 16, 2007 as well as gain excessive weight.
Fat exists in low levels in plants and can be added to increase the energy density of the diet. Fat has per kilogram of energy, which is 2.25 times that of any carbohydrate source.Mowrey, Robert A. "Horse Feeding Management - High-Fat Diets for Horses". from North Carolina Cooperative Extension Center , web site accessed February 14, 2007 Because equids have no gall bladder to store bile, it 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."Digestive System 2" , web site accessed August 15, 2011 However, they are able to digest a greater amount of fat than can cattle. Horses can 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.
Digestible carbohydrates are found in nearly every feed source; corn has the highest amount, with an average starch content of 72%, 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 and/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.
Vitamins and minerals
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/mineral 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.