Equine nutrition isn’t simple. When nutritionists evaluate equine rations, we are looking beyond the feed tag or guaranteed analysis. We’re looking at the specific requirements of that horse for protein, minerals, trace elements, and vitamins in the total diet.
That means accounting for what is in the forage along with any complementary feeds and supplements being given.
Of course, activity level, health status, and life stage are important considerations as well. But here, the discussion will be about a few basics of equine nutrition focusing on protein, vitamins, and minerals.
Equine Nutrition: Amino Acids
Technically speaking, horses do not have protein requirements. They have amino acid requirements. Amino acids are the building blocks of all protein. Think of amino acids like letters of the alphabet, that when combined can create thousands and thousands of words.
There are 22 amino acids that can mix and match to form many different proteins with various functions in the body. Of these 22 amino acids, eight are considered “essential,” meaning they must be consumed in the diet.
The three main essential amino acids for horses we believe are lysine, methionine, and threonine.
Lysine, known as the “first limiting amino acid,” is critical for every protein in the body. Without lysine, there will be no protein (i.e., without fuel, a vehicle will not run). The rain barrel analogy is also often used to illustrate this point. Think of a barrel made up of staves. If the barrel is to collect rain, it can only hold rainwater up to the level of the shortest stave.
Think of lysine as that shortest stave—once lysine becomes deficient or used up, no more protein can be made.
Thus, appropriate lysine intake is very important for overall protein synthesis.
Equine Nutrition: Proteins
As mentioned, amino acids combine to form a variety of proteins. Proteins are in turn the building blocks of all cells and tissues in the body. They also are important for many biological processes. These include growth, tissue repair, hormone and enzyme production, proper immune system function, neurotransmitters, and transport of substances throughout the body. Protein is more than just muscle!
If a horse consumes more protein that is required, the body simply breaks it down and eliminates it in the urine. Excess dietary protein is not stored in the body if it is not used. It is degraded to urea in the liver and excreted by the kidneys via urine.
Hence, any excess protein and its subsequent degradation requires additional water intake.
Ammonia is an end product of protein breakdown. That means a strong ammonia smell in a horse’s stall could mean the horse is taking in more protein than is needed.
A nutritionist can evaluate the diet to determine if protein is appropriate in the diet.
On the other hand, protein deficiency has negative consequences. These include muscle wasting and reduced growth, performance, and endurance. Insufficient protein intake can also contribute to slowed hair and hoof growth, slowed shedding, and decreased appetite.
Protein is not a nutritionally preferred option as an energy source in horses. While it is possible to do so, protein does not produce much usable energy and proportionally higher amounts of waste energy (heat) is produced instead. In other words, protein is metabolically “expensive” as an energy source. That means a lot is used for not much return.
Malnourished and starving horses are more likely to break down proteins in their own bodies (from muscle, for example) for energy if they do not receive enough calories and nutrients.
Equine Nutrition: Vitamins
Vitamins are organic (containing carbon) compounds that are essential in the diet. Vitamins are not degraded for energy but have various metabolic functions. They are essential for life and are only required in relatively small amounts.
Vitamins are either water soluble (Vitamins B and C) or fat/fat solvent soluble (Vitamins A, D, E, and K). Metabolism, growth, and enzyme production are just some of the diverse roles that vitamins are used for in the body.
Water soluble vitamins are generally not stored in the body. Any excess that the horse consumes is eliminated in urine.
The horse has the ability to make Vitamin C in the liver, something humans can’t do! However, under certain circumstances, additional Vitamin C support from the diet might be advantageous.
The B vitamins (i.e., niacin, riboflavin) are produced by bacteria in the gut. It is thought that the horse’s needs are met that way. It might be necessary to supplement B vitamins if the horse is experiencing digestive upset or if food is moving very rapidly though the digestive tract.
Fat soluble vitamins need to be eaten along with some fat for proper absorption.
Vitamins A, D, E, and K can be stored in adipose tissue in the body and must be carefully balanced. Because the body can store fat-soluble vitamins, there is a risk of toxicity for some vitamins if high amounts are taken in over time.
For example, Vitamin A consumed in excess can cause birth defects (if a mare is pregnant), dull hair coat, depression, and can be fatal if very high doses are consumed over time.
Vitamin D toxicity can cause calcification of soft tissue.
Vitamins E and K have low toxicity risk, but even so should be fed according to scientifically established nutritional guidelines and individual needs (NRC 2007).
Only some fat-soluble vitamins are typically consumed in the diet. Raw feedstuffs for horses don’t typically contain Vitamin A, but rather the “pro-vitamin” beta-carotene. Beta-carotene can then be metabolized into Vitamin A in the horse’s small intestine.
Horses can also make Vitamin D in their skin when they are exposed to sunlight, similar to humans.
Fresh forage is a good source of Vitamin E, as are most commercially fortified concentrates. However, preserved forages might have very low levels.
Vitamin K is found in plants in the form of phylloquinone. Intestinal bacteria can produce another form of Vitamin K known as menaquinone, but the exact contribution to a horse’s dietary needs is unknown (NRC 2007).
Equine Nutrition: Minerals
Minerals are inorganic (do not contain carbon). They are essential in the diet and are also required in relatively small amounts.
Minerals are important for a wide variety of physiological functions. These include growth, acid-base balance, bone formation, and support of other structural tissues in the body.
From a dietary perspective, minerals are classified as macrominerals or microminerals/trace elements.
Macrominerals are measured in grams and include calcium, phosphorus, sodium, chlorine, magnesium, potassium, and sulfur. For context, a small paperclip and a single raisin each weigh approximately 1 gram.
Microminerals are measured in milligrams. One milligram is 1/1000 of a gram, so imagine that raisin or paper clip in 1,000 tiny pieces! Microminerals include iodine, zinc, copper, cobalt, iron, and selenium.
Because minerals are not required in large doses, caution must be taken when choosing supplements. Multiple supplements containing minerals could inadvertently lead to imbalances or toxicity. On the other hand, under-supplementation can also cause imbalances (i.e., if iodine is over-supplied or deficient, thyroid issues can result).
In the digestive tract, minerals sometimes compete with each other for absorption. Different minerals, for example, might use the same transporter to cross the membranes of the small intestine to get absorbed into the body. There is only room for so much on one transporter.
Think of a car that can seat five people. If you are trying to move seven friends, not everyone will fit in the car at the same time, and the friends might have to compete for space in the vehicle.
For this and other reasons, minerals often have to be fed in certain ratios to each other. For example, in a mature horse it is recommended that 2 parts calcium be fed for every 1 part phosphorus to ensure the proper amounts of each mineral are absorbed.
Final Words
Protein is second to water in terms of what makes up the body. The amino acids that make up proteins are important in and of themselves to make sure that the proper proteins can be assembled.
High-quality protein sources include certain fresh forages, soybean meal, canola meal, and alfalfa meal.
Vitamins and minerals are found naturally in fresh, quality forage, but might need to be supplemented if feeding dried hay or unfortified forage products (cubes, beet pulp, etc.).
Not all forages are created equal, so horses on a forage-only diet most likely should receive a ration balancer to ensure no nutrients are deficient.
Vitamins and minerals are critical for a healthy diet but are only required in relatively small amounts. Balance is key. Be cautious of over-supplementation, and seek help from a qualified equine nutritionist if you have questions.
Further Reading
- 7 Management Tips to Reduce the Risk of Feed-Associated Equine Colic. Dr. Pat Harris. MySeniorHorse.com
- Dr. Nettie Liburt author bio. MySeniorHorse.com
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Nettie Liburt, MS, PhD, PAS, earned her Master's of Science and PhD in Animal Science (Equine Exercise Physiology and Nutrition) from Rutgers University studying with Drs. McKeever and Malinowski. Her research focused on the effects of age and exercise training on endocrine control of the stress response and glucose metabolism. S has authored or co-authored several scientific articles, including a chapter on feeding and care of the adult and senior horse in the Second Edition of the textbook, Equine Clinical Nutrition. She is an active member of the Equine Science Society and is a registered Professional Animal Scientist.View all posts