Why magnesium type matters

Walk into any feed store and you'll see a dozen "magnesium" buckets. They are not the same product. The form of magnesium on the label decides how much your horse actually absorbs, and how much passes straight through.

The forms you'll actually see

Magnesium oxide is the cheapest, most common, and least bioavailable form. Absorption rates in horses sit somewhere around 15–25%. It's what makes generic calming supplements look impressive on price-per-kg, and underwhelming in the paddock.

Magnesium citrate sits in the middle. Better absorption than oxide, gentler on the gut, but still not the gold standard.

Chelated magnesium (magnesium glycinate, magnesium bisglycinate) is bonded to an amino acid, which the small intestine recognises and absorbs efficiently. Real-world absorption is roughly 3–4x that of oxide, with far less loose-manure risk at working doses.

Magnesium sulfate (Epsom salts) is a laxative first and a magnesium source a distant second. Useful for a soak, not for daily feeding.

Why this matters for behaviour

Most "calming" responses to magnesium are really the body finally getting enough of it. A horse fed 10g of magnesium oxide may functionally receive 1.5–2.5g of elemental magnesium. The same horse fed 10g of magnesium glycinate may receive 6g or more. Same scoop. Different horse.

If you tried a magnesium calmer and "it didn't work," there's a good chance the form, not the mineral, was the problem.

How to read a label

• Look for the **form** named explicitly: glycinate, bisglycinate, citrate, oxide.
• Look for **elemental magnesium per dose**, not just total compound weight.
• Be cautious of blends that hide oxide behind a proprietary name.
• Cross-check against your forage analysis if you have one, most Canadian hay is borderline on magnesium to start with.

A reasonable starting point

For an average 500 kg horse in light work, 5–10g of elemental magnesium per day from a chelated source is a sensible floor. Sport horses, hard keepers, and horses on high-potassium grass often need more. Always introduce over 7–10 days and watch manure consistency.

When magnesium isn't the answer

Magnesium fixes a magnesium deficiency. It does not fix pain, ulcers, poor saddle fit, or a horse that needs more turnout. If you're three weeks into a quality chelated product and nothing has shifted, the next conversation is with your vet, not a bigger scoop.