Magnesium supports muscle relaxation, nerve signaling, bone structure, and activates over 300 enzyme systems in your horse's body. It has gained particular attention in the equine world for its calming properties — magnesium helps regulate nerve excitability, and supplementation is widely used for horses that are anxious, spooky, or prone to muscle tension. About 60 percent of your horse's magnesium is stored in bones, with the rest distributed in muscles and soft tissues. Magnesium is also an essential electrolyte lost in sweat, though in smaller quantities than sodium and potassium.
A 500kg horse needs about 7.5 grams of magnesium per day — roughly the weight of a heaped teaspoon. Ten kilograms of grass hay provides approximately 10 to 20 grams, which typically exceeds the requirement. Horses in work lose additional magnesium through sweat and may benefit from supplementation during heavy training or hot weather. Many calming supplements contain magnesium oxide or magnesium aspartate at 5 to 10 grams per day.
0.42% of daily nutrient intake
Magnesium makes up 0.42% of your horse's total daily nutritional requirements by weight. That's a tiny amount — but it matters.
Nervousness, muscle tension or twitching, increased spookiness, poor exercise recovery, stiff movement, and in severe cases, synchronous diaphragmatic flutter (thumps). Chronic marginal deficiency may manifest as a horse that is persistently anxious or tight through the topline.
Excess magnesium from food sources is efficiently excreted by healthy kidneys. Very high supplemental doses can cause loose stools. Horses with kidney impairment should not receive additional magnesium supplementation without veterinary guidance.
| Life Stage | Size | Min | Max | Unit | Notes |
|---|---|---|---|---|---|
| Adult | — | 7.5 | 15 | g | For a 500kg horse. Most forage-based diets exceed this requirement. Magnesium supplementation at 5-10g daily is sometimes used for anxious horses. |
| Working / Active | — | 10 | 20 | g | Working horses lose magnesium in sweat and have increased needs. Supplementation may benefit horses that are tight or spooky during work. |
Source: NRC 2007
Calcium and magnesium compete for absorption pathways and work in opposition in muscle tissue — calcium triggers contraction while magnesium promotes relaxation. Excess calcium can interfere with magnesium absorption, potentially contributing to muscle tension and anxiety.
What this means: If your horse is on a high-calcium diet (heavy alfalfa) and seems tense or spooky, consider whether magnesium is being crowded out. Adding a magnesium supplement at 5-10g daily may help restore the balance. The ideal Ca:Mg ratio in the total diet is roughly 2:1 to 3:1.
