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Chapter 12 Digestion and nutrition.

Digestion--the process that releases nutrients from feeds for use by the horse's body--begins in the mouth, where food is ground and mixed with saliva. Feed then moves to the stomach, where the chemical breakdown starts releasing the nutrients.

The science of nutrition draws heavily on findings of chemistry, biochemistry, physics, microbiology, physiology, medicine, genetics, mathematics, endocrinology, cellular biology, and animal behavior. To the individual involved with horses, nutrition represents more than just feeding. Nutrition becomes the science of the interaction of a nutrient with some part of a living organism, including the composition of the feed, ingestion, liberation of energy, elimination of wastes, and all the syntheses for maintenance, growth, and reproduction.


After completing this chapter, you should be able to:

* List six categories of nutrients

* Define terms associated with energy

* List the sources of energy nutrients

* Describe the functions of energy nutrients

* Describe the symptoms of energy nutrient deficiencies

* Describe the energy needs of horses for milk production, pregnancy, and work

* List the most important compound sugars

* Describe the digestion of fiber

* Explain the function of protein

* Describe digestible protein

* Explain the difference between essential and nonessential amino acids

* Identify at what stages of the horse's life protein requirements are the greatest

* Describe the functions of minerals in horse nutrition

* Describe the deficiency symptoms caused by the lack of minerals in the ration

* List the macrominerals needed by horses

* Identify the microminerals needed by horses

* List the vitamins that are essential in horse nutrition

* Describe the functions of vitamins

* Describe deficiency symptoms caused by the lack of three vitamins in a ration

* List and discuss factors affecting the amount of water a horse will consume

* Describe the ways by which horses lose water from the body



amino acids

as-fed basis


crude protein (CP)


digestible energy (DE)


dry-matter basis

fat-soluble vitamins







metabolic alkalosis

metabolizable energy (ME)


National Research Council (NRC)

net energy (NE)







total digestible nutrients (TDN)

water-soluble vitamins


Much credit has been attributed to oats and timothy hay for the nutrition of horses. But researchers have been unable to substantiate a need for either, when substitutions of other grains and hays were made. The Arabian horse, progenitor of most domestic breeds, reached its excellence in a country that produced no oats or timothy hay. As early as 1911, Trowbridge completed 365-day tests with hardworking mules that showed less weight loss and 28 percent less feed cost with corn compared to oats fed with mixed hay. Respiration counts showed no difference in heat tolerance. However, over the year-long test, mules seemed to tire of corn more than oats.

Horses relish oats. This fact, combined with the knowledge that less care is needed to avoid digestive problems with oats than with corn because of the higher fiber content of oats, has always made oats popular.

Timothy hay and good oats fed together make a satisfactory ration for adult horses. But they are too low in protein, calcium, and vitamins for broodmares and growing horses.

The subcommittee on Horse Nutrition of the Committee on Animal Nutrition of the National Research Council (NRC) examines the literature and current practices in the nutrition and feeding of horses and publishes recommendations on horse nutrition. The latest NRC publication on horse nutrition was issued in 1989. Many of the recommendations in this chapter and Chapter 13 are based on this NRC publication.


The anatomy of the horse's digestive system was discussed in Chapter 5. Figure 12-1 serves as a review of the general anatomy.


The small intestine is a major site of digestion and absorption of many nutrients. Good parasite control is necessary for optimum function of the small intestine. Parasites not only reduce feed utilization, but can cause colic.

The large intestine consists of the cecum and colon. It has a large population of micro-organisms (bacteria and protozoa) that ferment the fiber in plant materials. If feed changes are made rapidly, the microorganisms do not have time to adapt. Fiber digestion is dependent on the efficiency of microbial fermentation in the cecum and colon. Horses do not digest low-quality forages (cellulose) as well as cattle do, so they need to eat immature, high-quality hay or pasture. Besides supplying energy, dietary fiber helps regulate the flow of nutrients in the digestive tract and guards against behavioral problems related to boredom. The only nutritional problem to feeding high-quality forage as the sole source of feed is that some classes of horses need more energy than an all-forage diet can supply.

To a degree, the cecum and colon serve the same purpose for the horse that the rumen does for the cow. However, the cecum's location toward the end of the digestive tract probably reduces its contribution to the horse's overall digestive efficiency. Feed passes through the horse's gastrointestinal (GI) tract much faster than through the GI tract of ruminants. This faster rate of food passage is largely responsible for lower digestion efficiency in horses than in ruminants.


Horses need the same feed ingredients as other livestock. These ingredients are carbohydrates, fats, protein, minerals, vitamins, and water. The first three of these can be converted to yield energy. Major sources of energy and protein are grains and roughages, including pasture.

Feeding horses is both an art and a science. There is considerable variation in individual horses' nutrient requirements, but a table of these requirements forms a useful basis for formulating rations. Feeding practices are discussed in Chapter 13.

All horses require nutrients to maintain body weight and to support digestive and metabolic functions. They need additional nutrients for growth, work, reproduction, and lactation.

Tables of nutrient requirements for horses are expressed in two ways: (1) daily nutrient requirements and (2) nutrient concentration in the feed. This may be expressed on an as-fed basis or a dry-matter basis.

Most horses receive their daily ration in two parts: roughage (hay or pasture) and concentrates. The concentrate portion contains grain and may include a protein supplement, minerals, and vitamins. It may also include bran, cane molasses, and/or dehydrated alfalfa.


Feeds and feedstuffs contain the energy and nutrients essential for the growth, reproduction, and health of horses. Deficiencies or excesses can reduce growth and/or lead to disease. Dietary requirements set the necessary levels for energy, protein, amino acids, lipids (fat), minerals, and vitamins.

Table 12-1 indicates how the major nutrients are measured on a daily basis or in terms of their concentration in the feed.


Horses are nonruminant herbivores who use carbohydrates for their main energy supply. Carbohydrates eaten by the horse first pass through the stomach and intestine, where nonstructural carbohydrates--starch, maltose, and sucrose--are removed and enter the portal vein (Figure 12-2). These carbohydrates are used for energy or converted to other biochemicals needed for life. The liver stores carbohydrates in the form of glycogen (Figures 12-2 and 12-3).

The remaining nonstructural carbohydrates, as well as the structural carbohydrates--cellulose and hemicellulose--then reach the areas of the digestive tract that carry out fermentation. Fermentation results in the production of volatile fatty acids--acetic, propionic, isobutyric, isovaleric, and valeric acid. These volatile fatty acids are easily absorbed and converted into energy.

As a side note, lactose--the sugar found in milk--is tolerable to horses up to 3 years of age. After that, the addition of milk or its by-products to the feed may disturb the gastrointestinal tract, causing diarrhea.




Fat is a concentrated source of energy that can be readily utilized by the horse. Fat contains 2.25 times more energy per unit of weight than do carbohydrates or proteins. During exercise, especially strenuous activity such as galloping, body fat will be mobilized and converted to energy.

Fat in the diet seems to spare the glycogen storage; increasing the fat in the diet increases performance and maintains body condition. Horses accept fat addition to the diet as long as the fat is not rancid. Often, performance horses are fed a percentage of their digestible energy concentration in the form of corn oil. The proportion of energy generated from fat and carbohydrate can be altered in exercising horses by dietary manipulation, but the ideal proportion is not yet known.

Energy Requirements

Energy can be measured as digestible energy (DE), metabolizable energy (ME), net energy (NE), and total digestible nutrients (TDN). Most nutrient requirement tables for horses now use DE. Digestible energy is expressed in calories, usually megacalories (million) or kilocalories (thousand). Energy requirements are needed for or altered by:

* Maintenance

* Reproduction

* Gestation

* Lactation

* Growth

* Work

* Old age

* Stalling

Maintenance. The maintenance requirement of the horse is described as the energy needed to keep the animal from gaining or losing weight.

Reproduction. Energy is an important factor in the success of reproduction in mares. A mare that is in poor condition but gaining weight is twice as likely to conceive as a mare that is in poor condition while maintaining weight. Mares that are in the range of good condition to fat condition have higher conception rates, whether they maintain or even lose body weight.

Gestation. During the last 3 months of pregnancy, the fetus experiences the greatest amount of growth. The 9th, 10th, and 11th months of gestation require an increase in DE.

Lactation. Milk production and composition varies between breeds, as well as individual horses. The NRC recommendations for horses (1989) determined that the conversion of milk to DE was 792 kcal of DE per kilogram milk. This means lactating mares must be provided daily with more DE. Table 12-2 shows the required increase in DE for mares of different weights and stages of lactation.

Growth. The tendency in the past has been to push the growth of foals to try to produce the biggest horse possible, as quickly as possible. This is not always in the best interest of the animal. Requirements for energy during growth are based on the age of the animal and the amount of weight being gained each day. The NRC recommendation for foals is

DE(mcal/day) = maintenance + (4.81 + 1.17 X + 0.023X2) (ADG)

Where X equals age in months, and ADG is the average daily weight gain.

Work. A working horse naturally needs more energy than a horse at rest does. Many studies have been done to calculate the exact requirements of exercising horses. The huge variety in the types of work that horses do, as well as the ability of a given breed, or individual, to perform those tasks prevents us from having one formula that fits every animal. The NRC recommends that ponies and light horses (200 to 600 kg) need an increase of 25 percent, 50 percent, or 100 percent for light, moderate, or heavy work, respectively. Draft horses should increase maintenance by 10 percent for every hour of fieldwork.

Old Age. The older horse can be treated as a maintenance animal. However, the decrease in activity and use may lead to obesity and related health problems. It is wise to monitor the geriatric horse for weight gain and adjust its diet to include more forage.

Stalling. A stalled horse has a lower requirement for energy but is more likely to develop bad habits and vices. The stalled animal is better off with larger quantities of a lower-energy roughage. This keeps the feed present for a greater portion of the day and gives the stalled horse an activity similar to normal behavior--grazing.
What Is a Calorie?

Humans count calories and humans avoid calories. Horses need
calories in the form of digestible energy. Actually, both humans
and horses need calories, but neither needs calories in excess. The
excess is stored as fat.

A calorie is a measure of the heat energy in food or feed. Digested
food is actually burned in the body, so the number of calories in a
food determines how much heat energy will be released when the food
is burned in the body.

To directly measure calories in a feedstuff, a sample of feed is
completely burned in a controlled environment. If the feedstuff
contains one calorie, it has enough heat energy to raise the
temperature of water exactly 1 degree centigrade from
14.5[degrees]C to 15.5[degrees]C.

For nutrition, one calorie is out of the realm of discussion. The
energy in feedstuffs is expressed as thousands of calories or
kilocalories (kcal), or it is expressed as millions of calories or
megacalories (mcal). The fires of life burn the feeds at a slow,
controlled burn, releasing the calories for energy to maintain
life, reproduce, grow, and work.

Underfeeding and Excess of Energy

Energy and protein are the major factors in evaluating a horse's ration. Underfeeding of either nutrient will cause a reduction in health and performance. Overfeeding can result in excessive fat deposition. Overfeeding of protein can be wasteful and sometimes causes stress. A depressed appetite can be an indication of a protein deficiency and then cause an energy deficiency.


The protein requirement of the horse is related to the quality and digestibility of the protein and the requirements of the individual. Young animals undergo stages of rapid growth and require proteins to provide building blocks for their bodies.

Proteins are composed of amino acids. These amino acids are used by the horse to build the proteins in its body. Various studies compared the growth of horses in different stages of life to the compositions of the proteins that the animals were eating. These studies indicated that the greatest growth was achieved when the horses were fed proteins high in lysine (an amino acid). Figure 12-4 shows diagrams of several amino acids. Mature horses do not need protein of as high a quality as younger animals do.

Protein in the diet is expressed as crude protein (CP). Digestible protein (DP) is a more accurate estimate of how much protein the animal is actually able to use. The DP of individual feeds has not been calculated for horses. Instead, DP values are estimated from CP values.

Deficiency. Insufficient dietary protein decreases production of protein in foals. This results in smaller, less-healthy foals, sometimes called "poor doers."


In an adult animal, a dietary deficiency of protein increases problems in areas of high protein turnover. The hair coat and hoof wall may be adversely affected, and tissue wasting may occur.

Excess. Excess protein has not been found to cause adverse effects on horses when fed in moderation. For adult horses, especially for hardworking horses, there is some debate about the value of excess protein.


Minerals are important for energy transfer and as an integral part of vitamins, hormones, and amino acids. The horse obtains most of its necessary minerals from pasture, roughage, and grain. Depending on the amount required by the body, minerals in the diet are classified as macrominerals or microminerals (sometimes called trace minerals). The seven macrominerals include:

1. Calcium (Ca)

2. Phosphorus (P)

3. Potassium (K)

4. Sodium (Na)

5. Chloride (Cl)

6. Magnesium (Mg)

7. Sulfur (S)

Eight microminerals important in equine nutrition include:

1. Copper (Cu)

2. Iodine (I)

3. Iron (Fe)

4. Selenium (Se)

5. Cobalt (Co)

6. Manganese (Mn)

7. Fluorine (F)

8. Zinc (Zn)

Table 12-3 summarizes some of the mineral requirements and the signs of their deficiency in horses.

Calcium. Calcium (Ca) is involved in homeostasis--the functions that maintain life-- blood clotting mechanisms, and muscle contractions. Calcium also makes up 35 percent of the horse's bone structure.

Calcium deficiency in developing foals may lead to rickets, which shows up as poor mineralization of bone tissue, enlarged joints, and crooked long bones. Calcium excess does not seem to be detrimental, as long as the level of phosphorus is adequate.

Phosphorus. This mineral makes up 14 to 17 percent of the horse's skeleton. Phosphorus (P) is required for many energy-transfer reactions and for the synthesis of some lipids and proteins. Phosphorus requirements increase during late gestation and lactation. In the horse's diet, calcium and phosphorus are considered together in the calcium-to-phosphorus ratio.

Calcium-to-Phosphorus Ratio. Calcium and phosphorus are considered together because they work together and affect one another's availability. The ratio and level of calcium and phosphorus must both be considered. Adequate vitamin D must be available for proper calcium and phosphorus use. Bone growth problems are a symptom of problems with the calcium, phosphorus, and vitamin D complex.

A calcium-to-phosphorus (Ca:P) ratio of less than 1:1 may be detrimental to calcium absorption. Even if the calcium intake is sufficient, excessive phosphorus intake will cause malformations of the skeleton. On the other hand, very high ratios of calcium to phosphorus (as high as 6:4) in growing horses will not be detrimental as long as the phosphorus intake is adequate. Phosphorus deficiency will produce rickets in the developing horse and weakening of the bones (osteomalacia) in the mature horse, similar to what is found in deficiencies of calcium and vitamin D. Excessive phosphorus intake will lead to reduction of calcium absorption, chronic calcium deficiency, and nutritional secondary hyperparathyroidism. Hyperparathyroidism is an increase in the function of the parathyroid glands, which lie next to the thyroid gland in the neck.

Table 12-4 gives the proper ratios of calcium to phosphorus for various classes of horses.

Potassium. This mineral maintains the acid-base balance and osmotic pressure inside the cells. Forages and oilseed meals generally contain 1 to 2 percent potassium in the dry matter. Cereal grains (corn, oats, wheat) contain 0.3 to 0.4 percent potassium. Required potassium concentration in a diet for growing foals is 1 percent. A mature horse requires 0.4 percent in the diet.

Because forages usually constitute a significant portion of the diet, the horse should get its potassium requirement from the diet. If only cereal grains are fed, potassium chloride and potassium carbonate can be used as supplements.

Potassium deficiency in foals causes loss of appetite and weight loss. These symptoms promptly reverse when potassium carbonate is given. Excess potassium in the diet is readily excreted, provided water intake is normal.

Sodium. Sodium maintains the acid-base balance outside the cells and regulates the osmosis of body fluids. Sodium is also involved in nerve and muscle function. Since the sodium concentration of natural feedstuffs for horses is often lower than 0.1 percent, salt is often added to concentrates at rates of 0.5 to 1.0 percent or fed free-choice as plain, iodized salt.

Chronic sodium deficiency in horses results in decreased elasticity of the skin, a tendency to lick sweat-covered tool handles, decreased appetite, and decreased water intake. Eventually, the horse will stop eating. If the deficiency is acute, the horse will have uncoordinated muscle contractions, impaired chewing, and an unsteady gait. In the blood, the sodium and chloride will be low and the potassium high. Horses are tolerant of high levels of salt in their diets as long as there is free access to fresh drinking water.

Chloride. In the diet, chloride normally accompanies sodium as NaCl or salt. This is an important extracellular anion (negative charge) involved in acid-base balance and osmotic regulation. Chloride is an essential component of bile, hydrochloric acid, and gastric secretions. The chloride requirement is assumed adequate when the sodium requirement is met.

Magnesium. More than half of the magnesium found in a horse's body exists in the skeleton. Magnesium is an activator of many enzymes. Magnesium concentrations in common feedstuffs range from 0.1 to 0.3 percent.

Sulfur. Sulfur is a component of many biochemicals in a horse's body, including amino acids, biotin, thiamine, insulin, and chondroitin sulfate. Requirements for the horse have not been established, and a deficiency has not been described.

Copper. This mineral is essential for several copper-dependent enzymes. Deficiency of copper may cause bone disease and bone malformation. A deficiency may also cause a dullness of the coat color. Horses are tolerant of excess copper.

Iodine. Iodine is essential for the production of the thyroid hormones. These hormones regulate basal metabolism. If iodine has to be supplemented, iodized or trace mineralized salt containing 70 mg of iodine per kilogram can be used.

Iodine deficiency in pregnant horses, which may show no symptoms themselves, may lead to foals that are either stillborn or born too weak to stand and suckle. These foals have an enlargement on the front side of the neck due to enlargement of the thyroid gland--a condition called goiter.

An excess of dietary iodine may result from using feedstuffs high in iodine, such as kelp (a seaweed), or from adding excessive supplemental iodine. This may lead to baldness in horses.

If a pregnant mare has had too much iodine in the feed, the foal will be born with an iodine-induced goiter. The milk of the mare will contain excess iodine. The foal needs to recover from the excess of iodine received before birth. An alternate source of milk, low in iodine, has to be found.

Before giving iodine supplementation to the pregnant mare or the foal with a goiter, the owner must determine whether the horse was fed too little or too much iodine.

Iron. In the body of a horse, 60 percent of the iron is in the red blood cells and 20 percent is in the muscles. Common feedstuffs should meet the horse's iron requirements.

Iron deficiency causes anemia when too little iron is available for the formation of red blood cells. Because milk is low in iron, young, milk-fed foals are most likely to have deficient iron. Iron supplements have not proven effective in improving the oxygen-carrying capacity of red blood cells.

Excess iron is very toxic to young animals, and death can result in a foal that has been given supplemental iron by mouth. Before death, the foal shows diarrhea, jaundice, dehydration, and coma.

Selenium. This mineral is essential for detoxification of certain peroxides that are toxic to cell membranes. Selenium is closely connected with vitamin E in that the two work together to scavenge free radicals.

Selenium deficiency is linked to the status of vitamin E. Selenium-vitamin E deficiency causes white muscle disease that involves both the skeletal muscles and the heart muscle. The symptoms are weakness, difficulty in suckling and swallowing, troubled breathing, and heart dysfunction.

Excess selenium can be acute or chronic. The acute form (blind staggers) exhibits itself in blindness, head pressing, perspiration, colic, diarrhea, and lethargy. This acute form is seen if selenium is ingested from some toxic plants. Chronic selenium toxicity results in hair loss about the mane and tail and cracking of the hoofs around the coronary band.

Cobalt. Cobalt is a part of vitamin B12. Microflora in the cecum and colon use dietary cobalt to make vitamin B12. Specific dietary cobalt requirements have not been studied in the horse.

Manganese. Manganese is necessary for metabolizing carbohydrate and fat and for synthesizing cartilage. Manganese requirements in horses are not established, but some recommendations are made based on information from other species.

Fluorine. Fluorine is involved in bone and tooth development in other species. The dietary necessity is not established in horses. Excesses of fluorine in the diet can cause colored teeth, bone lesions, lameness, and unthriftiness (loss of condition).

Zinc. Zinc is a component of many enzymes. Experimentally, a zinc deficiency can be produced in foals. Signs of the deficiency include loss of appetite, reduced growth rate, and rough, scaly skin.

Common equine feedstuffs contain 15 to 40 mg of zinc per kilogram. If a supplement is needed, zinc sulfate, zinc oxide, zinc chloride, zinc carbonate, and various zinc chelates can be used.


Vitamin requirements, like other nutrient requirements, are affected by age, stage of production, gastrointestinal infections, and muscular activity (Figure 12-5). Type and quality of the diet and extent of vitamin absorption determine the need for vitamin supplements. Bacteria in the gut produce vitamins while breaking down feedstuff. These vitamins are also available for absorption. Forages contain mostly fat- and water-soluble vitamins so that horses grazing high-quality pastures should not need vitamin supplementation. Vitamins are classified as fat-soluble or water-soluble.

The fat-soluble vitamins are vitamins A, D, E, and K. Conditions interfering with fat absorption also adversely influence absorption of these vitamins. Vitamin K is synthesized in the intestines. Vitamin D requires exposure to ultraviolet light. The other fat-soluble vitamins must be present in the diet. Table 12-5 summarizes some of the vitamins, their functions, and signs of a deficiency.


Vitamin A. Vitamin A is important for vision. Metabolites of vitamin A are found in visual pigments within the retina. This vitamin also plays a basic role in cell differentiation and, in the growing animal, in bone remodeling. Green plants and hays contain carotene, which the body normally converts to vitamin A.

Deficiency of vitamin A causes night blindness, excessive tearing of the eye, thickening of the horn layer of the skin and the cornea, lack of appetite, poor growth, respiratory infections, abscesses under the tongue, convulsive seizures, and progressive weakness.

Excess vitamin A, given over a long time, may cause fragile or thick bones, flaking skin, and tumors. In some animals, hair and skin are lost. These horses are severely depressed and lie down on their sides.

Vitamin D. Dietary vitamin D seems to be sufficiently present, especially if the horse is exposed to sunlight. Deficiency of vitamin D in the diet, while the horse is deprived of sunlight, seldom produces rickets; but it does produce loss of appetite and slower growth. Supplementary vitamin D in the diet promotes absorption of calcium and phosphorus.

Excess vitamin D in the diet leads to calcification of blood vessels, the heart, and other soft tissues, and to bone abnormalities. Besides accidental addition of excess vitamin D to the diet, an excess may be caused by ingestion of certain toxic plants, such as day jasmine.

Vitamin E. An interrelationship exists between vitamin E and selenium. They both function as a part of a multicomponent antioxidant defense system.

Vitamin E activity in feedstuffs is reduced by moisture, mold growth, and grinding of the feedstuff during processing. Deficiencies of vitamin E and of selenium are difficult to distinguish separately. A deficiency of both nutrients in the foal will show pale areas in degenerating skeletal and cardiac muscles, as well as swelling of the tongue. If the deficiency is not corrected, pulmonary congestion occurs.

Claims that vitamin E is beneficial in restoring fertility in horses have not been verified by research. No symptoms of excess vitamin E in the horse are known.

Vitamin K. Vitamin K plays an important role in blood clotting. Deficiency of vitamin K results in decreased production of thrombin, which in turn interferes with the formation of fibrin clots. This leads to excessive bleeding with blood that will not clot. Too much vitamin K does not seem to cause problems in the horse.

Water-soluble Vitamins. Water-soluble vitamins include thiamine, riboflavin, niacin, pantothenic acid, biotin, folacin, ascorbic acid (vitamin C), choline, and vitamin B12. Some of the water-soluble vitamins are often grouped as the B vitamins. This includes thiamine, riboflavin, niacin, pantothenic acid, biotin, and folacin. The water-soluble vitamins are available in feedstuffs or synthesized by microorganisms in the intestine. Only a couple of the water-soluble vitamins have a required level in the diet.

A need for thiamine in the diet has been demonstrated. Deficiency of riboflavin contributes to periodic ophthalmia (moon blindness). Common sources of B vitamins are green plants, dried legumes, and soybean meal.

Less is known about vitamins than minerals, but supplementation is easy and inexpensive.

Horses with access to good pasture, if only for a brief time, and those receiving good-quality hay, especially if it is half legume, will probably need no vitamin supplementation (Figure 12-6). Deficiencies are more likely to appear with horses confined for long periods of time on poorquality roughage.

Establishments with horses confined for long periods should consider an economical commercial source of vitamins as insurance against deficiencies when the roughages are not of top quality. On the other hand, "stuffing" an animal with vitamins many times beyond the known requirement increases expenses and contributes nothing to its health.


A source of fresh, clean drinking water is essential for horses at all times. Daily consumption can average 10 to 12 gallons, with much higher amounts consumed during hard work and/or hot weather conditions. When water is not available by free choice, idle animals should be taken to it at least twice daily at regular intervals. Impaction is a common and rather serious problem resulting from infrequent drinking. Hot horses should be cooled out or permitted small amounts of water before drinking their fill. To refresh the animal and reduce heat exhaustion, those at work should be watered frequently whenever possible.


Dehydration and Electrolyte Balance

Dehydration from sweating results in the loss of both water and electrolytes--sodium, chloride, and some potassium. During extended workouts in hot, dry weather, the losses can be significant. In heavy sweating, the loss of chloride can result in hypochloremia, a condition in which there are low levels of chloride in the blood, and metabolic alkalosis, which is when pH increases above normal.

An adequate water supply, a balanced diet, and free-choice mineralized salt should provide all the necessary fluid and electrolytes for racing or extended work.


The Subcommittee on Horse Nutrition of the NRC's Committee on Animal Nutrition last made recommendations in 1989 for the nutrient allowance and requirement of horses, based on all the available information about their nutritional needs and by making inferences from other species. Tables 12-6, 12-7, and 12-8 are adapted from the NRC information. Table 12-6 gives the minimum daily requirements for digestible energy, crude protein, calcium, phosphorus, and vitamin A for different conditions and weight. Table 12-7 presents the same minimum daily requirements for growing horses with different mature weights, with or without training. Finally, Table 12-8 lists the mineral and vitamin recommendations for maintenance, growth, pregnancy, and work.

The next task is to put all of this information about digestion and nutrition into action and feed horses. The feeding of horses is covered in Chapter 13.


The purpose of digestion and nutrition is to supply the horse with the proper amounts and kinds of nutrients for maintenance, growth, reproduction, lactation, and work. These nutrients include carbohydrates, fats, protein, minerals, and vitamins. Water is also an important part of proper nutrition. Carbohydrates, fats, and protein provide energy, which is measured in calories. Protein in the diet also provides amino acids, which are the building blocks for protein in the body of the horse. Depending on the quantity in the body, minerals are classified as macrominerals or microminerals. Minerals become a part of the skeleton, function in energy production, and become a part of enzymes. Vitamins are either fat-soluble or water-soluble. Forages contain vitamins, and some vitamins are produced by microoganisms in the digestive tract. Vitamins serve in biochemical reactions that influence how the other nutrients are used in the body. Deficient, out-of-balance, or excess nutrients can reduce production and/or lead to health problems.


Success in any career requires knowledge. Test your knowledge of this chapter by answering these questions or solving these problems.

True or False

1. Carbohydrates are made up of amino acids.

2. Macrominerals supply most of the energy in the diet.

3. Vitamin A is a fat-soluble vitamin. 4. Fats contain more energy than proteins or carbohydrates do.

5. A lactating mare needs more energy than a pregnant mare in the early stages of gestation does.

Short Answer

6. Name four fat-soluble vitamins and six water-soluble vitamins.

7. List five macrominerals and five microminerals.

8. Name five conditions that alter the energy requirements of a horse.

9. Give two terms used to express energy in a feed and two terms used to express protein in a feed.

10. What other element is always associated with sodium in the diet?

Critical Thinking/Discussion

11. Compare the energy needs of a mature mare that is being maintained to those of a mare that is lactating and a mare that is working out each day.

12. What is a calorie?

13. Describe two functions of protein in the diet.

14. Explain the importance of the calcium-to-phosphorus ratio.

15. Describe the symptoms of three vitamin deficiencies.


1. Obtain five samples of high-protein feeds common to your area. Take these samples to a laboratory for protein analysis.

2. Fecal material is a reality of any livestock production operation. Research the differences or similarities in the composition of horse, sheep, beef, and dairy fecal waste.

3. Compare the nutrition information contained on a cereal box for humans to the information contained on a feed tag for horses. What did you learn about human nutrition?

4. Obtain feed labels from horse, dairy, pig, and chicken feed. Compare the contents of each and compare the price.

5. Collect samples of horse feed and develop a display using small bottles or plastic bags. Label the type of feed and its protein, energy, and mineral content. Use a feed composition table to find this information.

6. Develop a report or presentation on the digestion of protein, carbohydrates, fats, or fiber in horses.



Asimov, I. (1954). The chemicals of life. New York: New American Library. Cheek, P. R. (2004). Applied animal nutrition: Feeds and feeding (3rd ed.). Upper Saddle River, NJ: Prentice Hall.

Frandson, R. D., Fails, A. D., & Wilke, W. L. (2003). Anatomy and physiology of farm animals (6th ed.). Philadelphia: Lippincott Williams & Wilkins.

Lewis, L. D. (1996). Feeding and care of the horse (2nd ed.). Media, PA: Lippincott Williams & Wilkins.

Subcommittee on Horse Nutrition, National Research Council. (1989). Nutrient requirements of horses (5th ed.). Washington, D.C.: National Academy Press. (1)

Worth, M. (2004). Storey's guide to feeding horses: Lifelong nutrition, feed storage, feeding tips, pasture management. North Adams, MA: Storey Publishing.


Internet sites represent a vast resource of information, but remember that the URLs (uniform resource locator) for World Wide Web sites can change without notice. Using one of the search engines on the Internet such as Yahoo!, Google, or, find more information by searching for these words or phrases:

amino acids


digestion of fiber

digestion of horses

digestive system

energy requirements of horses

fat-soluble vitamins



nutrition of horses

trace minerals


Table A-18 in the appendix also provides a listing of some useful Internet sites that can serve as a starting point for further exploration.


(1.) A great deal of new information has accumulated since the publication 17 years ago of the last edition of Nutrient Requirements of Horses. This new edition features a detailed review of scientific literature, summarizing all the latest information, and provides a new set of requirements based on revised data. Unfortunately, this new edition was not available at the time of publication but the reader can order it or view it at The National Academies Press (http://www. 11653.html#description)
TABLE 12-1 Requirements for the Major
Nutrients Expressed and Their Units

                       Unit of Measure        Unit of Measure
Nutrient               (amount/day)           (concentration)

Digestible energy      megacalories           mcal/kg

Crude protein          kilograms (kg)/day     percent (%)

Calcium                grams/day              %

Phosphorus             grams/day              %

Sodium, potassium      grams/day              %

Copper, zinc, iron,    milligrams (mg)/day    parts per million
other trace minerals                          (ppm) or mg/kg

Vitamins A, D, and E   international unit     IU/kg

Thiamine,              mg/day                 mg/kg
other B vitamins

Note: 1 kg (kilogram2.2 lbs.

TABLE 12-2 Increase of Digestible Energy for Mares of
Different Weights and Stages of Lactation

Formulas                             Foaling to 3 Months

DE = maintenance + 0.04 BW x 0.792   200-299 kg BW

DE = maintenance + 0.03 BW x 0.792   300-900 kg BW

                                     3 Months to Weaning

DE = maintenance + 0.03 BW x 0.792   200-299 kg BW

DE = maintenance + 0.02 BW x 0.792   300-900 kg BW

Note: 1 kg = 2.2 1 bs; BW is body weight.

TABLE 12-3 Horses: Requirements, Functions,
and Deficiency Signs of Minerals

Mineral (Requirement)   Functions

Calcium1                Bone mineral; blood clotting; nerve,
                        muscle, and gland function

Phosphorus (1)          Bone mineral, part of many proteins
                        involved in metabolism

Iron (50 mg/kg) (2)     Part of hemoglobin and some
                        enzymes; oxygen transport

Copper (9 mg/kg)        Iron absorption; hemoglobin synthesis;
                        skin pigments; collagen metabolism

Magnesium (.1%)         Bone mineral, enzyme activator:
                        energy metabolism

Sodium, potassium,      Tissue fluid pressure and acid-base
and chloride            balance; passage of nutrients and water
                        into cells; nerve and muscle function

Zinc (36 mg/kg)         Activator of many enzymes

Iodine (.1 mg/kg)       Thyroid function

Manganese (36 mg/kg)    Synthesis of bone and cartilage
                        components; cholesterol metabolism

Selenium (.2 mg/kg)     Removal of peroxides from tissues;
                        enzyme activation

Mineral (Requirement)   Deficiency Signs

Calcium1                Rickets, osteomalacia, Nutritional
                        Secondary Hyperparathyroidism (NSH),
                        osteoporosis; bones may be soft and
                        easily deformed or broken

Phosphorus (1)          Bone disease; decreased growth; repro-
                        ductive problems; low blood phosphorus

Iron (50 mg/kg) (2)     Anemia: lack of stamina, poor growth

Copper (9 mg/kg)        Anemia; hair pigment loss; bone disease:
                        swollen joints, deformed thin bones

Magnesium (.1%)         Nervousness; muscle tremors; ataxia;
                        convulsions; mineralization of blood
                        vessels; low serum magnesium

Sodium, potassium,      Craving for salt; hyperexcitability,
and chloride            decreased growth rate; loss of appetite

Zinc (36 mg/kg)         Hair loss, scaly skin, poor wound healing;
                        reproductive, behavioral, and skeletal

Iodine (.1 mg/kg)       Goiter; poor growth; low body temperature;
                        impaired development of hair and
                        skin; foals weak at birth

Manganese (36 mg/kg)    Reproductive problems: delayed estrus,
                        reduced fertility, spontaneous abortion,
                        skeletal deformities in the newborn

Selenium (.2 mg/kg)     White muscle disease; low serum selenium
                        and serum glutathione peroxidase

(1) See Table 12-1.

(2) Units per kg of air-dried feed.

TABLE 12-4 Calcium-to-Phosphorus Ratios

Status         Minimum Ca:P   Maximum Ca:P   Optimum Ca:P

Nursing foal       1:1           1.5:1          1.2:1
Weaning            1:1             3:1          1.5:1
Yearling           1:1             3:1            2:1
Mature             1:1             5:1            2:1

TABLE 12-5 Horses: Requirements, Functions,
and Deficiency Signs of Vitamins

Vitamins           Functions                 Deficiency Signs

Vitamin A (2,000   Growth and development    Night blindness, poor
IU/kg) (1)         of bone and epithelial    conception rate,
                   cells, vision             abortion, loss of
                                             libido, testicular
                                             convulsions, elevated
                                             cerebrospinal fluid

Vitamin D          Absorption of dietary     Poor mineralization of
(250 IU/kg)        calcium and phosphorus    bone, bone deformities

Vitamin E          Antioxidant in tissues    Decreased serum
(15 IU/kg)                                   tocopherol, increased
                                             red blood cell fragility;
                                             muscular dystrophy

Thiamine           Coenzyme in energy        Loss of appetite and
(3 mg/kg)          metabolism                weight; incoordination,
                                             muscular weakness and

Riboflavin         Coenzyme in many enzyme   Conjunctivitis,
(2 mg/kg)          systems                   lacrimation, aversion
                                             to bright light

(1) Units per kg of air-dried feed.

TABLE 12-6 Minimum Daily Nutrient Requirements for Mature Horses (1)

                       Mature     Digestible       Crude Protein
                       Body       Energy
                       Weight     Megacalories   Pounds    Percent
                       (pounds)   Daily          per Day   of Diet

Mature horse at rest      440            7.4      0.65         8
(maintenance)             880           13.4      1.18         8
                        1,100           16.4      1.45         8
                        1,980           24.1      2.13         8

Mature horse at           440           11.1      0.98        10
moderate work (2)         880           20.1      1.77        10
                        1,100           24.6      2.17        10
                        1,980           36.2      3.20        10

Mares, last 30            440            8.9      0.86        11
days of pregnancy         880           16.1      1.56        11
                        1,100           19.7      1.91        11
                        1,980           29.0      2.81        11

Mares, peak of            440           13.7      1.52        13
lactation (3)             880           22.9      2.52        13
                        1,100           28.3      3.15        13
                        1,980           45.5      5.67        13

                       Mature          Calcium
                       Weight     Grams     Percent
                       (pounds)   per Day   of Diet

Mature horse at rest      440         8       .25
(maintenance)             880        16       .25
                        1,100        20       .25
                        1,980        36       .25

Mature horse at           440        14       .30
moderate work (2)         880        25       .30
                        1,100        30       .30
                        1,980        44       .30

Mares, last 30            440        17       .50
days of pregnancy         880        31       .50
                        1,100        37       .50
                        1,980        55       .50

Mares, peak of            440        27       .50
lactation (3)             880        45       .50
                        1,100        56       .50
                        1,980       101       .50

                       Mature         Phosphorus      Vitamin A
                       Weight     Grams     Percent   1,000 IUs
                       (pounds)   per Day   of Diet   per Day

Mature horse at rest      440         6       .20         6.0
(maintenance)             880        11       .20        12.0
                        1,100        14       .20        15.0
                        1,980        25       .20        27.0

Mature horse at           440        10       .25         9.0
moderate work (2)         880        17       .25        18.0
                        1,100        21       .25        22.0
                        1,980        32       .25        40.0

Mares, last 30            440        13       .40        12.0
days of pregnancy         880        13       .40        24.0
                        1,100        23       .40        30.0
                        1,980        28       .40        54.0

Mares, peak of            440        18       .35        12.0
lactation (3)             880        29       .35        24.0
                        1,100        36       .35        30.0
                        1,980        65       .35        54.0

(1) From Nutrient Requirements of Horses, by the National
Research Council, 1989.

(2) Examples are horses used in ranch work, roping,
cutting, barrel racing, jumping, etc.

(3) Lactation level is assumed to be 3 percent of
body weight/day.

TABLE 12-7 Minimum Daily Nutrient Requirements for Growing Horses (1)

Current Age               Current Body   Expected     Megacalories
(months)                  Weight         Daily Gain   Daily

Growing horses (2)
440 lb. mature weight
4                               165         0.88            7.3
12                              308         0.44            8.7
18NT (3)                        374         0.22            8.3
24T (4)                         407         0.11           11.4

Growing horses
880 lb. mature weight
4                               319         1.87           13.5
12                              583         0.88           15.6
18NT                            726         0.55           15.9
24T                             803         0.33           21.5

Growing horses
1,100 lb. mature weight
4                               385         2.00           14.4
12                            1,100         1.98           31.2
18NT                          1,463         1.54           33.6
24T                           1,672         0.99           42.2

                            Crude Protein          Calcium

Current Age               Pounds    Percent   Grams     Percent
(months)                  per Day   of Diet   per Day   of Diet

Growing horses (2)
440 lb. mature weight
4                            0.81      16        16       .70
12                           0.86      14        12       .55
18NT (3)                     0.83      13        10       .45
24T (4)                      1.07      12        13       .45

Growing horses
880 lb. mature weight
4                            1.49      16        33       .70
12                           1.55      14        23       .55
18NT                         1.58      13        21       .45
24T                          2.02      12        27       .45

Growing horses
1,100 lb. mature weight
4                            1.59      16        34       .70
12                           3.10      14        49       .55
18NT                         3.34      13        49       .45
24T                          3.96      12        61       .45

                              Phosphorus      Vitamin A

Current Age               Grams     Percent   1,000 IUs
(months)                  per Day   of Diet   per Day

Growing horses (2)
440 lb. mature weight
4                             9       .50           3
12                            7       .40           6
18NT (3)                      6       .35           8
24T (4)                       7       .35           8

Growing horses
880 lb. mature weight
4                            18       .50           7
12                           13       .40          12
18NT                         12       .35          15
24T                          15       .35          16

Growing horses
1,100 lb. mature weight
4                            19       .50           8
12                           27       .40          22
18NT                         27       .35          30
24T                          34       .35          34

(1) From Nutrient Requirements of Horses,
by the National Research Council, 1989.

(2) Moderate rate of gain.

(3) Long yearling (18 months) not in training (NT).

(4) Two-year-old (24 months) in training (T).

TABLE 12-8 Minerals and Vitamins for Horse Rations (1)

                               Adequate Levels
Nutrient                         Growth and             Tolerance
                   Maintenance   Broodmares   Working    Levels


Sodium %               .10          .10         .30         3.0%
Chloride %             .3           .40         .40         5.0%
Magnesium %            .10          .10         .15          .5
Sulfur %               .15          .15         .15        1.25%
Iron ppm (2)           40            50         40        1,000
Zinc ppm               40            40         40          500
Manganese ppm          40            40         40        1,000
Copper ppm             10            10         10          800
Iodine ppm            .1            .1         .1           5.0
Cobalt ppm            .1            .1         .1            10
Selenium ppm          .1            .1         .1           2.0
Fluorine ppm           --            --         --           50


Vitamin A IU/lb.       910         1,667        910       7,273
Vitamin D IU/lb.       135           135        365       1,000
Vitamin E IU/lb.        25            37         37         450
Thiamine ppm             3             3          5       3,000
Riboflavin ppm           2             2          2          --

(1) From Nutrient Requirements of Horses, by the
National Research Council, 1989.

(2) Parts per million (ppm) = mg/kg = mg/2.2 lb.
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Author:Parker, Rick
Publication:Equine Science, 3rd ed.
Date:Jan 1, 2008
Previous Article:Chapter 11 Reproduction and breeding.
Next Article:Chapter 13 Feeds and feeding horses.

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