Q: How do different forms of magnesium (e.g., magnesium oxide vs. magnesium sulfate) compare in terms of bioavailability and efficacy in managing stress or muscle tension in performance horses?

A: Magnesium is a macromineral involved in various bodily functions in horses. It contributes to bone crystalline structure and plays a key role in cellular and metabolic processes such as energy production and muscle contraction. Additionally, magnesium supports neurotransmitter and receptor function. Researchers studying various species, including horses, have linked magnesium deficiency to cellular membrane destabilization and neurological impairment. Muscle and nervous system component malfunction could cause horses to exhibit signs of nervousness, irritability, or increased sensitivity to unfamiliar situations.

Daily Magnesium Requirement for Horses

Horses absorb magnesium in both the small and large intestine, with the small intestine serving as the primary site of absorption. According to the National Research Council (2007), the estimated magnesium requirement for a 1,100-pound horse at maintenance is 15 milligrams/kilogram body weight or 7.5 grams per day. Most horses obtain magnesium from forage and feedstuffs. However, in certain cases, these ingredients’ magnesium content might not meet dietary needs. For instance, working horses need 1.5 to 2 times more magnesium than maintenance horses primarily due to losses through sweat. In this case, adding a daily, well balanced electrolyte supplement is appropriate. Growing horses, especially young foals, and lactating mares also have higher magnesium needs. Utilizing growth feeds formulated specifically for these classes of horses is an excellent way to ensure requirements are being met.

Adding Magnesium to Your Horse’s Diet

You can add magnesium to your horse’s diet through supplementation using both inorganic and organic forms, which all vary in bioavailability and concentration. Researchers have shown inorganic forms have an absorption rate of approximately 70% in horses. Horse owners commonly use inorganic sources, such as magnesium oxide, due to cost-effectiveness and availability. Horses do not absorb excess dietary magnesium oxide, making it difficult for them to overdose on the macromineral. However, nutritionists do not recommend magnesium sulfate (Epsom salt) for daily use in horses due to its laxative effect. Researchers believe that organic magnesium, which is bound to another molecule such as an amino acid, may have higher absorption rates than inorganic forms, but these claims are primarily based on studies in humans and rodents.

While absorption remains important, consider the concentration of magnesium in each form. For instance, inorganic magnesium oxide contains approximately 60% magnesium by atomic weight and has a 70% absorption rate. Therefore, for a horse to absorb 30 mg of magnesium, he would need to consume 72 mg of magnesium oxide. In comparison magnesium aspartate hydrochloride, an organic form, is approximately 10% magnesium by atomic weight. Even if magnesium aspartate hydrochloride were 100% absorbable, the horse would have to consume 300 mg of it to get the same 30 mg of magnesium.

To date no well-designed studies prove magnesium supplementation beyond requirements reduces muscle tension or stress in horses. I suspect the anecdotal evidence suggesting extra magnesium reduces muscle excitability might be due to resolving an existing suboptimal magnesium status.

Take-Home Message

Ensure your horse’s daily magnesium needs are met by feeding a fortified, well-balanced diet appropriate for his life stage and workload. Check the tag on your concentrate or ration balancer for magnesium inclusion because the manufacturer might not have added it. In performance horses excessive sweating without replenishment usually causes magnesium deficiency, so supplementing with a quality electrolyte might be appropriate. If you are unsure about your horse’s magnesium needs, work with a qualified equine nutritionist.

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