The Role of Micronutrients in Equine Nutrition

While water and other macronutrients, such as carbohydrates, fats, and proteins, make up the majority of the equine diet, micronutrients are no less important. Vitamins and minerals make up a very small portion of the diet; however, they play major roles in important physiological functions including bone and muscle function, digestion, and metabolism. In this fact sheet, we discuss the different micronutrients and the roles they play, along with best practices when incorporating them into horses’ diets.

Minerals

Minerals are naturally occurring, pure, inorganic (without carbon) substances. The major site of mineral digestion and absorption is the small intestine, while some mineral absorption also occurs in the hindgut. After they are absorbed, minerals are transported throughout the body where they assist with almost every function in the body. Minerals are involved in bone structure, muscles, nerves, hormone signaling, oxygen transport, metabolism, and more. (Table 1 and Table 2). Required minerals in the correct amounts and ratios are essential in equine diets.

Minerals in the Diet

Equine diets should be made up of at least 50% forage. Horses get most of their daily nutrient requirements from forages, such as pasture and hay. Testing forage is important because quality varies based on maturity, species, soil fertility, weather, etc. The only way to know mineral content of your forage is to test it. In some cases, such as in mature horses at maintenance, high-quality forage alone can provide the animal’s daily mineral requirements. However, horses in other production stages (e.g., pregnant/lactating mares, growing horses, and performance horses) may need complementary feeds to bridge the gap between what the forage provides and the total mineral requirement. Knowing which nutrients the forage supplies helps identify nutrient deficiencies. Some geographic locations lack minerals in the soil (e.g., copper, selenium, and zinc), which then causes a decreased concentration of these minerals in the forages grown in those areas. If the main dietary forage doesn’t provide the necessary amounts of required minerals, try adding a commercial concentrate, ration balancer, specific forages (e.g., alfalfa for calcium), or a salt/mineral block. Working with an equine nutritionist is an excellent way to ensure your horse is consuming minerals in sufficient quantities and in the correct ratios.

Providing horses access to a salt/mineral supplement either in loose, granular form or in a block (Figure 1) is a good idea, along with unlimited access to water. Electrolytes may be added to water, but there should always be access to electrolyte-free water. With so many products on the market, keep these things in mind. Purchasing salt/mineral blocks that are specifically labeled for horses is vital. If salt/mineral blocks for other species are used (e.g., cattle), they may contain ingredients that are unsafe for horses. Trace mineral blocks, salt blocks, and Himalayan salt blocks provide mainly salt (NaCl). Sodium (Na) and chloride (Cl) are key electrolytes and essential to horse health. While trace mineral blocks also provide additional minerals e.g., copper, iron, zinc, and manganese), note that some of those minerals are most likely not consumed in amounts large enough to meet requirements or ensure the proper balance of minerals in the diet. The amount horses consume from blocks is highly variable between individuals. Horses on pasture typically do not have a preference between salt blocks, trace mineral blocks, or Himalayan salt blocks.

The horse is made up of approximately 4% mineral. Minerals needed by the horse fall into two categories:

1. macrominerals
2. microminerals (also known as trace minerals)

As the names imply, macrominerals are needed in the diet in larger quantities than microminerals—though still relatively small amounts compared to the total diet. The National Research Council’s Nutrient Requirements of Horses (2007) lists macromineral daily requirements as grams per day (g/d) and micromineral requirements as milligrams per day (mg/d). Macrominerals are generally expressed as a percentage (%) on a feed tag, whereas microminerals are listed as parts per million (ppm = mg/kg).Keep in mind that minerals work together in delicate balance. Adequate amounts of all minerals and in the correct ratios must be supplied in the diet or there can be negative consequences to the horse’s health and performance.

Mineral Toxicity and Deficiency

Mineral toxicity and deficiency can result in various issues, such as decreased growth rate, poor hair coat, reduced hoof integrity, impaired performance and reproduction, and impaired mineral absorption. Over-supplementation is a common culprit in mineral toxicities. Mineral deficiency can be caused by malnutrition, glucocorticoid treatment (e.g., for recurrent airway obstruction), and improper ratios of other minerals with which the deficient mineral may have interactions. Other culprits of mineral toxicity/deficiency include:

• unfortified diets (phosphorus imbalance and other mineral deficiencies)
• laxatives and calming supplements (magnesium toxicity)
• diarrhea/hard work (electrolyte deficiencies, e.g., sodium, potassium, and chloride)

Other Considerations

The ratio of calcium (Ca) to phosphorus (P) is arguably one of the most important nutrient ratios in the horse’s diet. Forages are typically higher in Ca than P, especially legumes, but cereal grains are often much higher in P than Ca. The calcium to phosphorus ratio should be 2:1 ideally, never below 1:1 or higher than 6:1. Calcium and phosphorus are found in bone at a 2:1 ratio. Over time, too much Ca relative to P can lead to a phosphorus deficiency, resulting in rickets, osteomalacia, and reproductive issues. Too much P compared to Ca can lead to a calcium deficiency, resulting in “Big Head’s Disease” or Nutritional Secondary Hyperparathyroidism (NSH), along with other bone issues (Figure 2). Feeding forage first and complementary feeds only if needed will reduce the chances of an inverted calcium to phosphorus ratio.

Vitamins

Vitamins are another group of essential micronutrients, differing from minerals in that they are organic (containing carbon). After they are absorbed or synthesized in the body, vitamins are transported throughout the body where they assist with many bodily functions. Vitamins are involved in vision, bone growth, reproduction, immune response, organ function, and more, and are vital components of the equine diet (Table 3 and Table 4).

Vitamins in the Diet

Vitamins are most commonly found in the leafy portion of forages. As with minerals, keeping in mind that forage quality determines vitamin content is important. Sunlight exposure and how forage is cured (sun, heat, etc.) also plays key roles in vitamin content. Grazing pasture typically provides the daily vitamin requirement for most horses (Figure 3). However, as soon as forage is cut and stored, vitamin losses occur. Horses not on pasture, especially those housed indoors, may need other sources of vitamins. Most commercial concentrate mixtures include fat-soluble vitamins. Additional vitamin supplements are also available. Remember that vitamin requirements will vary based on the production stage of the horse. Mares in gestation/early lactation have the greatest requirement, though performance level, age, and health issues may also constitute an increased need.

Vitamins are classified based on solubility:

1. fat-soluble
2. water-soluble

Fat-soluble vitamins require the presence of fat or lipids to be absorbed into the bloodstream from the small intestine. Vitamins A, D, E, and K are fat-soluble, can be stored in the body’s fat stores and liver for later use, and are excreted in the feces. Water-soluble vitamins include vitamin C and B-vitamins. These vitamins need to be supplied continuously (either via the diet or synthesized in the body) because water-soluble vitamins are not stored in the body and are excreted primarily via urine. Horses can synthesize (either directly or indirectly by microorganisms in the hindgut) or store most vitamins within the body. However, if a deficiency occurs, supplementation can become necessary. In general, more research is needed to determine vitamin requirements for horses, though some (i.e., A, D, E, thiamin, and riboflavin) have been established (NRC 2007).

*Adenosine triphosphate

Vitamin Toxicity and Deficiency

Overall, vitamin toxicity is rare, though toxicity of fat-soluble vitamins is more likely due to the horse’s ability to store these vitamins in the body. Signs of toxicity can range from bone issues to organ damage. Signs of deficiency are vitamin dependent and can negatively impact vision, bone and muscle function, fertility, and mineral absorption (e.g., decreased calcium absorption). There can be various causes of vitamin deficiencies in horses including:

• use of antimicrobial medications
• feeding a high-grain, low-forage diet
• feeding poor-quality forages, or those stored for over 6 months
• malnutrition (e.g., using feeds not designed for horses and starvation)
• health issues impairing vitamin absorption

Key Points

Horse owners and managers need a basic understanding of micronutrients to provide optimal nutrition and management for animals in their care. Micronutrients, vitamins, and minerals, are indispensable, supporting a variety of physiological functions essential to horse health and performance. Follow these general guidelines to provide horses with adequate micronutrients:

• Maintain a forage-based diet (at least 50%) whenever possible.
• Analyze pasture and hay to ensure that micronutrient requirements are being met.
• Provide access to salt and minerals as a loose supplement or in a block along with unlimited access to water.
• Ensure that the calcium to phosphorus ratio in equine diets is balanced—2:1 ideally.
• When forage and cereal grains are unable to provide adequate micronutrients, use commercial concentrate mixtures formulated for horses based on type of forage consumed, age of the animal, and the animal’s activity and performance levels.
• Consult an equine nutritionist or your Ohio State University Extension representative on questions about your horse’s diet.

References

Geor, Raymond, Pat Harris, and Manfred Coenen. 2013. Equine Applied and Clinical Nutrition. London: Elsevier.
doi.org/10.1016/C2009-0-39370-8

Kunath, H., K. Bennett-Wimbush, and S.L. Mastellar. 2019. “Equine and Wildlife Use of and Preference for Salt Blocks in Pastures.” Journal of Equine Veterinary Science, Volume 76, May 2019: 124–125. doi.org/10.1016/j.jevs.2019.03.198

National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press.
doi.org/10.17226/11653