A Quick Guide to Macronutrients for Horses

Learn how these vitamins and minerals affect nearly every physiological function in the horse’s body

Micro: a prefix originating from the Greek letter “µ,” meaning small. Though they make up only a tiny part of horses’ diets, micronutrients play big roles in major physiological functions, ranging from bone and muscle performance to digestion to hormone signaling. Let’s dive into the world of micronutrients to learn more.

Minerals

By definition, minerals constitute any naturally occurring, pure inorganic (meaning without carbon) substance. Remember that periodic table from chemistry class? On it you will find a collection of minerals that are present on earth. After countless hours of research, nutritionists determined the proportion of these minerals necessary to sustain equine life.

Minerals required in the equine diet fall into two categories: macrominerals and microminerals. What makes a mineral macro or micro? The amount a horse requires daily. If you glance at the National Research Council’s Nutrient Requirements of Horses (2007), you’ll find macromineral requirements listed as grams per day and microminerals as milligrams per day.

Macrominerals include:

Microminerals include:

The small intestine serves as the major site of mineral digestion and absorption, after which minerals proceed to various tissues where they’re needed to assist almost every physiological function in the body. Many are multifunctional, serving horses’ structure, fluid balance, muscles, nerves, immunity, protein synthesis, carbohydrate metabolism, enzymes, and hormones.

Selenium Balance

Mieke Holder, MS, PhD, assistant professor of nutrition and environmental impact in the University of Kentucky’s Department of Animal and Food Sciences, in Lexington, knows selenium. She dedicated her doctoral research to better understanding how a horse’s selenium balance affects his antioxidant status and immune function.

Antioxidants in the body help protect cells from damage by molecules known as free radicals. Selenium is a vital part of an antioxidant known as glutathione peroxidase.

“What sets selenium apart from other trace minerals is how it is incorporated into the functional parts of various proteins,” she says.

Through these ‘selenoproteins,’ the body’s selenium concentration affects the antioxidant system, immune function, and other physiological pathways. Selenium is toxic at relatively low levels, and a horse only requires about 1 milligram of selenium per day (for a 1,100-pound idle horse; NRC, 2007). However, soil selenium concentrations and, therefore, plant selenium content, vary across the United States (see map). For example, about 80% of forages and grains produced in areas up and down the East Coast contain less than 0.1 parts per million of selenium.

Researchers have found that blood selenium levels decrease below adequate levels in unsupplemented idle horses grazing pastures with marginal selenium.

However, “due to a high toxicity risk, supplementation of selenium in addition to that provided by your horse’s forage, commercial feed, or balancer pellet should only be done in consultation with a nutritionist,” says Holder.

—Kristen Janicki, MS

Because forages constitute most of the equine diet, they do the heavy lifting of supplying minerals. But, just like forage quality, forage mineral content depends on various factors, such as the species of grass or legume, maturity at cutting or consumption, and region in which it was produced, to name a few. For mature pasture pets a good-quality forage source can provide most, if not all, of the daily required minerals. In all other cases you’ll need to fill mineral gaps with either concentrates or supplements. This includes young, growing horses, lactating broodmares, and performance horses. In some cases—and this is often dictated by geographic location—forage levels of certain minerals (such as copper, selenium, or zinc) can be low enough to warrant additional supplementation even to idle horses, says Mieke Holder, MS, PhD, assistant professor of nutrition and environmental impact in the University of Kentucky’s Department of Animal and Food Sciences, in Lexington.

“Having your forage tested and working with a nutritionist simplifies these supplementation decisions,” she says.

“A ration balancer pellet is a simple option, designed to meet the mineral requirements of horses kept on a forage-only diet,” she adds.

When manufacturers produce commercial concentrates, they take into consideration the mineral content an average grass or legume delivers and design a specific blend of minerals to complement it.

Commercial concentrates, when fed according to the manufacturer’s ­recommendations (listed by class and weight of horse—e.g., a performance horse weighing 1,100 pounds—for each quantity), will meet most mineral requirements. Depending on your objective, you might use other forages as supplemental sources of minerals, says Holder.

“In the case of the growing horse or lactating mare, some breeders choose to add good-quality alfalfa hay for the supplemental calcium benefits that legumes provide, in addition to other nutrients, like digestible fiber and protein,” she says.

It’s important to understand that minerals work together as a whole, in careful balance with each other. Supplying adequate macrominerals without also supporting the microminerals will affect the utopian atmosphere in horses’ bodies and potentially the way they perform, reproduce, or grow.

Table 1. Macrominerals

Mineral  Function Common Dietary Source
Sodium Component of acid-base balance; promotes nerve and muscle function Supplemental salt
Chloride Component of acid-base balance; mineral in gastric acid Supplemental salt
Potassium Component of acid-base balance; promotes nerve and muscle function Forages
Calcium Major component of bone and teeth; facilitates muscle function; plays a role in blood pressure and clotting; supports immune system Forages, especially legumes (alfalfa, clover)
Phosphorus Component of bone and teeth and of acid-base balance Cereal grains
Magnesium Component of bone and teeth; facilitates protein synthesis; aids nerve and muscle function; supports the immune system Forages and cereal grains
Sulfur Aids in protein synthesis Forages and cereal grains

Table 2. Microminerals

Mineral  Function Common Dietary Source
Iron Part of hemoglobin (a protein in red blood cells); aids muscle and enzyme function Forages
Zinc Component of many enzymes; aids protein synthesis Forages and cereal grains
Iodine Component of thyroid hormone Forages and cereal grain
Selenium An antioxidant Forages and cereal grain
Copper Component of connective tissues; vital to iron metabolism; component of cellular enzymes Forages and cereal grains
Manganese Component of enzymes for metabolism; facilitates cartilage formation Forages and cereal grains
Cobalt Supports B-vitamin synthesis Forages and cereal grains

Vitamins

Vitamins are essential organic (containing carbon) micronutrients needed for a diverse range of metabolic functions. They fall into two categories depending on their solubility: fat-soluble or water-soluble.

Fat-soluble vitamins, including vitamins A, D, E, and K, cannot be absorbed by the small intestine and transported into the bloodstream without the presence of fat. The body can also store them in the liver and adipose (fat) tissue for later use.

Water-soluble vitamins, including vitamin C and the B-complex vitamins, dissolve readily in water. The horse needs to consume or synthesize these continuously for use because they are not stored in the body.

Fresh forages provide most of the vitamins horses need, and grazing pasture covers the daily vitamin requirements for most classes of horses. But once cut and conserved, forages experience vitamin losses, so horses not on pasture might need extra vitamin support. Fortunately, horses can make or store most of the necessary vitamins themselves and, even in times of dietary deficiency, can synthesize or mobilize enough to meet their needs.

Most commercially produced concentrates include at least some fat-soluble vitamins in their formulations. For the diet do-it-yourselfer, the ever-growing pool of vitamin supplements offers individualized options, as well. As with other nutrients, vitamin requirements vary depending on the horse’s life stage, with broodmares and their fetuses needing the most throughout gestation and for milk production. Aged, diseased, or performance horses might need supplemental vitamins to provide additional immune support.

Table 3. Fat-Soluble Vitamins

Vitamins Function Common Dietary Source
Vitamin A (retinol) Aids vision; Supports bone and muscle development; Facilitates fertility and reproduction Beta-carotene, the precursor to vitamin A, found in abundance in fresh pasture and yellow vegetables like corn and, most importantly, carrots
Vitamin D Facilitates calcium absorption from the small intestine; promotes calcium and phosphorous homeostasis in bone Exposure to sunlight, sun cured hay
Vitamin E An antioxidant that works in conjunction with selenium Fresh pasture
Vitamin K Component in blood clotting; supports bone and skin health Green, leafy forages; possibly microbial synthesis

Table 4. Water-Soluable Vitamins

Vitamin  Function Common Dietary Source
Thiamine (vitamin B1) Supports enzymes needed for carbohydrate metabolism and nervous system function Microbial synthesis; cereal grains and byproduct feeds, such as brewer’s yeast
Riboflavin (vitamin B2) Precursor for enzymes needed in energy synthesis and protein and lipid metabolism Forages, especially legumes (alfalfa, clover); Microbial synthesis
Niacin (B-vitamin) Supports enzymes and coenzymes for energy metabolism Forages, microbial synthesis
Biotin (B-vitamin) Supports enzymes and cell proliferation and growth Fresh forages, microbial synthesis
Folate Aids red blood cell synthesis; Supports rapid cell growth and turnover in tissues Green forages
Vitamin C (ascorbic acid) An antioxidant Synthesized from glucose in the liver

Take-Home Message

The two micronutrient classes, minerals and vitamins, are small but mighty valuable. These essential nutrients ­support all aspects of life, including reproduction, performance, maintenance, and growth and development. Work with an equine nutritionist to ensure the micronutrients in your horse’s diet are meeting his daily needs. If he’s deficient in any area, you might need to add a concentrate or supplement to his ration.