Magnesium oxide for horses

Epsom salts, or magnesium sulfate, is becoming an increasingly common supplement for horses.

Magnesium plays an important part in nerve and muscle function, and horses deficient in this important element can show signs of nervousness, wariness, excitability, and muscle tremors.

This gives magnesium its reputation for having a calming influence on equines.

A deficient horse is likely to have a poor tolerance to work and its muscles will tie up quite quickly.

Magnesium is also known to play an important part in reducing equine obesity, and can lessen the risk of laminitis in animals prone to it during periods of strong spring grass growth.

But like most things, you can easily end up giving your horse too much.

Epsom salts is cheap and there is a danger that horse owners may be providing too much in their horse’s diet.

Epsom salts is best known as a laxative. Give your horse an overly generous amount and, just like people, they’ll be feeling the effects of diarrhoea. Anything greater than one level tablespoon a day per 100kg of your horse’s bodyweight is likely to result in a case of the runs.

Supplementation rates:
• 31 mg/kg/day of MgO or
• 64 mg/kg/day of MgCO3 or
• 93 mg/kg/day of MgSO4

While at the correct rate, it is an acceptable source of magnesium, you will probably be better feeding magnesium oxide or – the Rolls-Royce of magnesium supplements – magnesium aspartate.

Excessive magnesium will be excreted in the urine, but major overdoses have been linked to heart conduction problems and renal trouble, so it’s important you don’t overdo it.

A study looking at magnesium uptake in horses was conducted by six veterinarians at the Department of Veterinary Biosciences, College of Veterinary Medicine, at Ohio State University.

The magnesium requirement of a typical horse was put at 13 milligrams per kilogram of bodyweight per day. Horses that are growing, lactating, or in work will use more each day.

For example, a lot of magnesium can be lost in sweat. For such animals, the quantity could be increased 1.5 to 2 times the maintenance dose.

Opinion appears to vary on whether magnesium supplementation is needed at all. It will, of course, depend in part on whether the soils on which a horse is grazing are deficient in the element. Any such deficiency will be reflected in the grass grown.

In general, a horse is likely to get between 60 per cent and 100 per cent of its daily magnesium needs through a normal forage diet.

Deficiencies are most likely in spring, during periods of strong grass growth, and even in winter on pastures in milder areas where grass is being pushed along with fertiliser.

Grass in both circumstances is likely to be low in magnesium, sodium, and soluble carbohydrates, and most likely high in nitrogen and potassium. This is a double whammy, as high potassium levels can slow the absorption of what little magnesium there is, while sodium (which is low in these situations) is known to help its uptake.

Mechanisms affecting magnesium uptake in a horse are complex, and not always related to too little magnesium in the diet. It is just as likely that magnesium deficiency is caused by too much potassium in the diet inhibiting uptake.

Potassium is not the only potential player in this complex equation. The presence and proportions of calcium, phosphorous, and fats in the diet can also play a part in the ability of a horse to use the magnesium in its diet.