How the Horse Eats and Processes Food
The horse uses his incisors (front teeth) to bite off grass. His back teeth chew and grind food, mixing it with saliva to make it easier to chew and swallow. If teeth wear unevenly, he can’t chew properly; this can lead to indigestion, colic, impaction, or even starvation. The horse has three sets of paired salivary glands, secreting up to ten gallons of saliva daily. Some authorities state that horses on a diet of continual grazing or roughages produce twenty-five to thirty gallons of saliva per day. Not only does saliva moisten feed so it can be swallowed and passed through the gut, but it also acts as a buffering solution to help maintain proper pH — to counteract bile in the small intestine and keep the ingesta at a pH between 6.8 and 6.5 (for optimal fermentation in the cecum and large colon).
The esophagus is a muscular tube going from the pharynx down the left side of the neck (alongside the centrally located windpipe) into the stomach. The esophagus is basically a one-way street, with peristalsis (contracting movements that go along the tube in rhythmic waves) traveling only toward the stomach. This one-way movement, along with the acute angle at which the esophagus joins the stomach and the one-way valve at the stomach, makes the horse practically unable to vomit.
If a horse eats spoiled feed or has a blockage between stomach and intestine and tries to vomit, the food may come out through his nose and get into his windpipe. After food moves into the stomach, digestive enzymes secreted by the stomach walls begin breaking down the feed and start the digestion of proteins. The stomach is small, holding only about 8 percent of the total capacity of the digestive tract, and most feeds pass through it rapidly, often entering the small intestine within fifteen minutes of being eaten.
The stomach is healthiest when food is passing through it more or less continually. The outer lining of the stomach has little protection against the hydrochloric acid produced for digestion by the inner glandular lining. If horses are fed often (or grazing at pasture), the stomach always has food in it to absorb the acid, greatly reducing the risk of ulcers. The type of feed given to a horse also can be a factor in whether he develops ulcers. Diets containing grain can put a horse at risk of ulcers. After food leaves the stomach, digestion of protein continues in the small intestine. Breakdown of starches and sugars, and to some extent fats, begins. Even though feed passes through the small intestine rapidly, most of the digestion and absorption take place there; the majority of proteins and starches are absorbed through intestinal walls. The end products of digestion of soluble carbohydrates are glucose and other sugars.
It is important that they be digested efficiently in the small intestine. Large amounts of carbohydrates passing to the large intestine raise the risk for colic. This can happen when a horse is fed too much grain or is worked hard too soon after eating grain; exercise can hinder the digestion process. Inadequately digested carbohydrates passing into the large intestine can disrupt the fermentation process and alter the population of microbes. Because the horse has no gall bladder, bile is secreted continually in the small intestine. It was earlier thought that horses could not use fat due to the absence of a gall bladder, but research has shown they can handle diets containing as much as 18 percent fat.
Weanlings gain weight faster with less total feed when fat is added to their rations, and performance horses can use fat for energy, reducing the amount of grain needed (thus reducing risks of founder and colic). Fat as an energy source can help a hard-working horse exert longer without fatigue. Since fat slows the emptying of the stomach, it should not be fed just before strenuous exertion or during an athletic event. A horse doing sustained activity needs a good roughage fill instead and can be started on fats again after the event. Some nutrients pass through the digestive tract without being absorbed, because they’re in a form not readily used by the horse or he doesn’t need them.
The healthy horse’s fine-tuned system supervises absorption of certain nutrients on an as-needed basis; the intestine acts as a regulatory gateway to help maintain a constant blood level of important nutrients. Overfeeding can cause some problems with the regulatory system, resulting in high levels of trace elements that can be toxic. Enzymes aid in digestion in the first part of the digestive tract.
In the latter part of the tract, microbes break down and digest fiber through fermentation. The cecum and large colon have a unique structure; their walls are pursed out with many small ridges and sacs that slow food passage so microbes have more time to work. The remaining water from the food material is absorbed from the residue passing through the small colon, forming balls of feces that then pass out through the rectum and anus. The horse normally passes thirty to fifty pounds of manure daily. The quantity of manure and its moisture content depend on the types of feed being eaten and whether the horse is drinking sufficient water. The color, smell, and texture of a horse’s manure can give a clue as to the digestive system’s health and function. Check the horse’s teeth if long stems of roughage or pieces of grain are coming through. Manure that is too fluid could indicate infection or irritation that needs medical attention.
Laxative feeds such as lush green grass, rich alfalfa hay, or bran produce softer manure than a grass hay diet. If feces are small and hard, the horse may be suffering from impaction and constipation or might not be drinking enough water. The horse does not start life with a digestive system that can handle roughage. At birth his cecum is not yet functional (having no microorganisms to facilitate fermentation), and he depends on his simple stomach and small intestine for digesting milk. As he comes into contact with bacteria, he develops the ability to digest starches and cellulose.
Foals instinctively eat some of their mother’s fresh manure to gain the proper microflora that set up housekeeping in the cecum and large intestine. A foal utilizes roughages quite well by two months of age, though the amount of hay or pasture he can consume is still somewhat limited. As the hindgut increases in size and function as he grows, he has more need for forage and less dependency on milk.
**Basic Nutritional Needs
The horse in nature has all his requirements met by natural forages, salt, and water. Native grasses provide all the vitamins and minerals needed except for salt; no forages contain sufficient sodium or chloride. The wild horse meets this need by using natural salt licks (salt deposits in soil) and by developing body systems that retain and conserve salt. Horses are also well suited to handling fluctuations in vitamin and mineral content of feed since various regions and seasons affect availability of some of these micronutrients. Dry grass in winter does not have carotene (from which the horse manufactures vitamin A), but this vitamin can be stored in the liver for long periods; the horse’s body has enough to last until green grass appears the next spring. Horses are very good at storing, conserving, and manufacturing vitamins and minerals; pasture, hay, and grain provide most horses with more than enough of the essentials
. The exception might be a sick horse that could benefit from C and B-complex vitamins (since illness may have impaired his ability to create them) or a horse with more athletic demands on his body than any wild horse. The energy content of grasses provides adequate fuel for the wild horse’s activities. Foals grow up healthy and fit on grass and milk, nursing their dams until just before birth of the next foal. They might have been thin during winter but grow up sound with none or few of the structural problems of overfed and underexercised domesticated youngsters, though it takes the wild horse longer to reach full maturity. With domestication, we demand more of horses, wanting youngsters to grow fast and be ready for sale or athletic careers at a young age, pushing them far beyond the normal activities of their wild counterparts. We confine horses and provide very little natural exercise. In many instances a horse is not allowed to graze but is expected to eat concentrated feeds at widely spaced intervals, leaving him nothing else to do.
Others are worked much harder than any wild horse and would not have time to consume enough grass to provide the needed energy. The amount of exercise a horse gets affects how much feed he needs and how well he digests it. Mild exercise can aid the digestive process (it’s healthier for the horse to travel slowly grazing than to stand all day in a stall), while strenuous exercise can hinder it. Roughage benefits a horse that works hard all day by providing nutrients to the hindgut. These horses get more than half their energy from volatile fatty acids (created by fiber breakdown in the hindgut), and the forage also provides electrolyte reserves and fluid storage. The main problem with green grass for hard work is a high moisture content that limits the total amount the horse can eat. Horses can get more total nutrients in hay. Horses that compete in strenuous sports often cannot give peak performances on fuel that green grass provides (due to inability to eat enough), and their bodies may not be able to handle stresses involved in these activities without additional minerals and other nutrients to balance losses.
How much supplementation with concentrated energy feeds, vitamins, and minerals a horse needs depends upon how hard he is working and upon the efficiency of his metabolism. Some horses require more dietary additions than others for peak performance. Sprinters generally need a more concentrated and readily metabolized energy source than do distance horses; many endurance horses and ranch horses that travel many miles a day do well on a basic roughage diet, especially if they have an efficient body metabolism. Horses bred for endurance can perform well on good hay alone or with a small amount of concentrate added to the ration. The amount of high-energy feed required by any horse depends on the individual as well as on how he is being used.
Protein
If protein is added to the diet, signs of protein deficiency in an adult horse can be resolved in about a week. Young growing horses may suffer more serious damage, however, if they receive insufficient protein. The National Research Council recommends that weanlings’ diets consist of 14.5 percent protein; yearlings, 12.6 percent; and two-year-olds, 10.4 percent. A mare needs extra protein during the last trimester of pregnancy (10.6 percent) when the fetus is growing fastest.
She needs the most protein, however, when she is producing milk (11 percent). Excess protein in the diet is metabolized as energy and produces heat when digested; high protein feeds are a good choice in cold weather but detrimental when a horse is working hard in hot weather. Mature horses have very low protein needs, even when working hard (8 to 10 percent). Protein is a poor energy source compared with carbohydrates, fats, and volatile fatty acids from roughages — all metabolized far more efficiently. Once a horse matures, his protein needs can usually be met by low-protein feeds such as pasture or grass hay.
Amino Acids
Protein is created from building blocks called amino acids. Chemically, protein resembles a knotted and tangled bead necklace, each bead being an amino acid. The order in which amino acids are combined determines the protein and its structure and functions. In the horse, there are twenty-two amino acid beads in this necklace, ten of which must be supplied by diet. The body can synthesize the others from any source of nitrogen.The body can change some amino acids into different combinations, but others can only be created by green plants, which a horse must eat. The ten that can’t be synthesized by the body are essential amino acids. If one essential amino acid for a certain protein is missing from the diet, the synthesis of that particular protein cannot continue. Its absence may prevent normal growth even if the diet contains an adequate supply of all the other ingredients. The growing horse is similar to a house being built. Even if all the other materials are there, the structure cannot progress if the carpenter runs out of nails.
But the other materials cannot be stored until the necessary amino acid shows up. Unable to create the growth protein needed, the body converts the rest of the amino acids into energy instead — an inefficient form of fuel. Lysine is considered the most important essential amino acid for growth; without it the body has trouble using any of the other amino acids. Feeds that are rich in lysine include alfalfa, soybean meal, and milk casein (such as dried skim milk); grains and grass hay are much lower in this important nutrient.
How Much Protein Is Enough?
By the time the foal is weaned, his protein needs are much less. Domestic horses are more often overfed than underfed. Many people assume that if a little protein is good, more is better. It’s true that a deficiency in protein can make a young horse slow growing, but most well-fed horses receive more protein than they need. The overfed mature horse on a generous ration of alfalfa hay and high-protein sweet feed must break down the excess to be used as energy or stored as fat, an inefficient process that produces six times more heat than the breakdown of carbohydrates or fats, while yielding less energy.
Protein not used immediately by the body is broken down to release nitrogen atoms, which then combine with other atoms to become ammonia and urea molecules, which are excreted in urine. A horse on a high protein diet drinks more water than normal to help flush the by-products of protein breakdown, and thus produces more urine. This can lead to wet bedding in stalls, plus irritation of respiratory passages from increased ammonia. Some researchers think the high levels of ammonia in stalls where racehorses are confined — on high-protein diets — contribute to exerciseinduced lung bleeding, due to the respiratory irritation.
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