Trace Mineral Basics: Magnesium

Magnesium (Mg), which is classified as a macromineral, plays several important roles in a horse’s body.

Approximately 60% of a horse’s Mg is found in bone and another 30% in skeletal muscle tissue. Not surprisingly then, this mineral plays a key role in muscle contraction and relaxation. Additionally, Mg helps maintain appropriate blood acid-base and electrolyte-water balance and activates many enzymes throughout the body. Examples of enzymes include those involved in the production of cellular energy and interpreting genetic information.

Widely touted online as a “super mineral” exerting a number of additional beneficial effects, including calming (as described previously by The Horse) and improving heart and hoof health, and improving hormonal imbalances, etc.; however, research supporting these claims is limited.

Requirements and Sources

According to the National Research Council’s Nutritional Requirements of Horses (NRC, 2007), a 500-kilogram (1,100-pound) horse should consume approximately 7.5 g of Mg per day (or 15 mg/kg body weight per day). Most commonly used feedstuffs contain 0.1-0.3% Mg, with about half of that magnesium readily absorbed, primarily via the small intestines. Supplements usually offer magnesium sulfate, oxide, or carbonate.

As with any dietary component, nutrient requirements will vary depending on the horse’s age, breed, and use. Mares in late gestation, for example, require 287-450 mg of Mg per day to support fetal development.

Deficiency and Excess

Insufficient Mg intake presents as muscle tremors, nervousness, ataxia (incoordination), and in rare cases, collapse and death.

The NRC suggests that a Mg concentration of 0.8% of any feed could lead to toxicity. However, research showed horses tolerated an alfalfa hay containing 0.5% Mg well.

Take-Home Message

As with all minerals, Mg fulfills specific roles and must be offered in balance with other nutrients taking the horse’s age, weight, breed, growth/gestation/lactation, and exercise into consideration. If in doubt, consult your veterinarian or an equine nutritionist for help balancing your horse’s diet.