There are several nutrients a horse requires such as water, protein, vitamins, and minerals. Specific amounts for each of these nutrients will depend largely on your horse’s weight and activity level or physiologic status (such as if an animal is growing or lactating). Detailed values of these requirements are found in the National Research Council’s Nutrient Requirements of Horses, and are summarized in the table below:
Nutrient Requirements of Horses at Maintenance
Water is by far the most important nutrient, and is most often overlooked. All horses should have access to fresh, clean water at all times. Without it, colic, dehydration, and even death could result. Horses’ water requirements depend greatly on their physiologic state—as a lactating horse will require significantly more water than a horse at “maintenance.” In general, a 500 kg horse will drink approximately 30-45 liters per day. However, how much horses actually drink will largely depend on diet; for example, a horse at pasture likely won’t drink as much as a horse eating hay because the pastured horse takes in water with each blade of grass.
Protein’s main function as a nutrient is to provide the building blocks for tissues, muscle, hormones, and enzymes. With respect to equine diets, we often classify protein requirements based on quantity and quality. Quantity refers to grams of protein required in the diet. Most horse owners think in terms of percentage of protein in a given feed, but how much the horse actually gets would depend on how much of that feed it gets. (Example: Feeding 5 kg of a 10% protein diet would give a horse 500 grams of protein [5,000 grams x 0.10]; feeding 2.5 kg of a 20% protein diet would also give a horse 500 grams of protein [2,500 grams x 0.20 = 500 grams].)
Horse owners should consider the total grams of protein intake per day, not the percentage. In addition to being aware of the quantity of protein a horse is getting, being aware of the quality of the protein is equally important. Protein quality refers to the amino acid make up of a feed. Some amino acids can actually be made by the body and are not essential from a dietary standpoint. Amino acids that cannot be produced by the body, such as lysine, are considered essential and must be provided for in the diet.
A high-quality protein should provide these essential amino acids. Good-quality sources of protein include the seed meals (such as soybean meal or linseed/flaxseed meal) and legume (alfalfa, clover, etc.) hays. The essential amino acid lysine is of particular importance because of its requirements for growth. Some equine feeds are relatively low in one or more of the key amino acids, with lysine being considered the first limiting amino acid (meaning that if insufficient quantities of lysine are present, the body's protein synthesis abilities are limited ). Thus, if a horse were easily meeting its protein quantity requirements but wasn’t getting enough lysine, the diet wouldn’t be suitable.
Let’s use this analogy: If amino acids were letters, and protein a word (a chain of letters), lysine could be thought of as the letter “E”; it is very important for the formation of many words and certainly important in writing a sentence or paragraph.
Fats and Carbohydrates
The main nutritional property of fats and carbohydrates is their ability to generate energy through being metabolized. However, specific types of carbohydrates and fats serve additional important functions for the horse. For example, complex carbohydrates such as fiber are extremely important for digestive tract health; the microbial ecosystem is highly sensitive to an insufficiency of fiber.
Furthermore, in humans it is now recognized that some types of fats are essential parts of the diet; namely the omega fatty acid group, including , omega-3 and omega-6. These fats are important for their anti-inflammatory properties and their roles in immune function. Horses also likely benefit from these omega fatty acids and research is ongoing, though these fats are not considered essential nutrients.
Equines require several minerals to meet a variety of functional needs, including skeletal integrity and cellular communication. The macro minerals (those needed in relatively high amounts) include calcium, phosphorus, sodium, potassium, chloride, magnesium, and sulfur.
Trace minerals (those needed in relatively small amounts) include cobalt, copper, zinc, selenium, iron, iodine, etc. Horse feeds tend to be variable in many minerals, and as they are usually low in sodium and chloride (salt), it is recommended all horses be offered some kind of salt source, such as a salt block.
Another important point about minerals is the significance of several ratios among these minerals, as the amount of one mineral in the diet may affect the use of another. For example, there should always be more calcium in the diet than phosphorus, ideally in the ratio of approximately 2:1. If this ratio is imbalanced, the horse may not be able to use the calcium in its diet and may develop bone problems.
The only way to know how many minerals are present in your feeds (particularly hay and/or pasture) is to have them analyzed at a local agriculture lab. Most commercially available feeds will have minerals added in quantities to meet the needs of the type of horse the feed is designed for.
Vitamins are classified as water-soluble or fat-soluble. The fat-soluble vitamins (they can dissolve in fat) include A, D, E, and K while the water-soluble vitamins include the B complex (niacin, thiamin, etc.) and vitamin C. The horse is unique with respect to some of its vitamin requirements in that the microbes located within the large intestine have the ability to synthesize the B complex vitamins and vitamin K. The microbes do so in quantities sufficient to meet most horses’ needs such that deficiencies of these vitamins are very rare and even difficult to induce experimentally.
Horses, unlike humans (and fruit bats, primates, or guinea pigs), can synthesize their own vitamin C and therefore generally do not require it in their diet. Vitamin D, synthesized upon the skin’s exposure to sunlight, is found in good amounts in sun-cured forages. Therefore, providing you feed good-quality hay (i.e., not last year’s batch) and your horse gets some outdoor exposure, it should be getting plenty of vitamin D.
Vitamins A and E are found in variable amounts in pasture and hay, with higher amounts found in pasture during the spring months and in hay that hasn’t been stored for too long. Most of the fat-soluble vitamins will degrade over time in stored hay.