Feeding magnesium to horses

Three sources of magnesium: from left, magnesium L-aspartate, Epsom Salts (magnesium sulfate), and dolomite. Magnesium L-aspartate has excellent bio-availability. Epsom Salts has good bio-availability, but can cause diarrhoea in excess. The magnesium in dolomite is not readily taken up by horses.
Three sources of magnesium: from left, magnesium L-aspartate, Epsom Salts (magnesium sulfate), and dolomite. Magnesium L-aspartate has excellent bio-availability. Epsom Salts has good bio-availability but can cause diarrhoea in excess. The magnesium in dolomite is not readily taken up by 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.

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.

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.

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.

Horses have a limited ability to store magnesium. Problems arising from any deficiency are therefore likely to manifest themselves quite quickly.

The most common forms of supplementation are magnesium oxide (MgO), magnesium carbonate (MgCO3), or magnesium sulfate (MgSO4) — also known as Epsom Salts.

In each case, horses can expected to be able to make use of about 70 per cent of the available magnesium in each of these minerals.

Thus, to get the 13mg of elemental magnesium per kilogram of bodyweight, will require:

• 31 mg/kg/day of MgO (Magnesium Oxide) or
• 64 mg/kg/day of MgCO3 (Magnesium Carbonate) or
• 93 mg/kg/day of MgSO4 (Magnesium Sulfate – Epsom Salts)

There are 1000mg to a gram, so the total daily magnesium requirements of a 500kg horse would be 15.5 grams of magnesium oxide, 32 grams of magnesium carbonate, and 46.5 grams of magnesium sulfate.

The equation makes it clear that magnesium oxide provides the greatest available amount of magnesium among these three.

The newcomer, at least for horse owners, is magnesium aspartate, more properly known as magnesium-L-aspartate.

It is the magnesium salt of aspartic acid and is highly water-soluble. When dissolved, it is readily absorbed through the intestine wall.

Its biological uptake is much higher than the aforementioned magnesium supplements.

Magnesium aspartate has been approved for use in many countries for the maintenance of normal magnesium levels in humans.

It is actually 20 per cent magnesium, so delivering 13mg/kg/day of magnesium for a 500kg horse will require 32.5 grams a day.

Dolomite is another material commonly given to horses. It is made up of about 80% calcium carbonate (of which 23% is calcium), and 20% magnesium carbonate (of which 12% is magnesium)

In all, just 5.7% of dolomite is actually magnesium. This means a level tablespoon of dolomite will deliver just 1 gram of magnesium. Even then, its bio-availability is considered low.

There is evidence that feeding salt (sodium chloride) at the same time as a magnesium supplement increases an animal’s uptake of magnesium. Some equine specialists recommend supplementing with a mix of 95 per cent salt and 5 per cent magnesium oxide to a horse’s diet.

With so many factors potentially affecting a horse’s ability to use magnesium, the answer must surely be to take a common-sense approach.

Otherwise, you run the risk of supplementing your horse’s diet with all manner of minerals, leaving your horse’s hapless kidneys to secrete the myriad additives you only think your horse needs.

You can always order a blood test. If not, supplement at no more than the recommended rate and monitor the result.

Where possible, monitor one thing at a time. For example, if you are concerned about spooky or erratic behaviour, you might try a magnesium supplement. You might also try a mycotoxin binder.

If you try both at once, and you get a positive result, you have no way of knowing which is delivering the desired outcome, short of taking the horse off one and seeing what happens.

Supplementation of magnesium in a deficient horse will have a huge effect on its wellbeing.

Just be sure your horse needs it, and don’t deliver too much of a good thing.

First published in July, 2007



9 thoughts on “Feeding magnesium to horses

  • November 11, 2012 at 10:09 am

    Thought some of you horse owners might like to know that often the cheapest source of Mag Sulphate (epsom salts) is your local stock and station firm such as CRT, RD1 etc.. They sell it in 20kg bags for a pittance.

  • June 1, 2013 at 5:04 am

    Cheap Epsom salts can be contaminated with heavy metals. If you want the best health for your horse only feed the purest Epsom salts. You should be able to get a chemical analysis with the salts you buy. The cheaper the salt the more cheaply it was made.

  • August 31, 2013 at 12:38 am

    Just buy loose magnesium for cattle it’s the same you’d buy at a suppliment/tack store but because its for “cows” it’s generally not taxed and its way cheaper to buy and is safe for horses. I have my hypp mare on it.

  • September 18, 2013 at 6:45 pm

    The safest magnesium to feed your horses is actually magnesium chloride.

    • November 13, 2013 at 4:18 pm

      Totally agree. 🙂

  • November 13, 2013 at 4:18 pm

    As per Micky above – MgCl is a wonderful source of bioavailable Magnesium – is also water soluble like Mg Aspartate but far more readily available and cheaper. I would only ever feed Magnesium Sulphate or Epsom salts in an emergency if nothing else available. Very irritating to the stomach and intestines at the levels needed for supplementation.

    The one in stock feed stores is Magnesium Oxide (as fed to cows) and sets like rock if left in damp conditions; has been known to cause enteroliths if fed in significant quantities to horses.

    • November 24, 2013 at 10:21 am

      So Micky and Kerry, what rate would you feed Magnesium chloride at? Do you dissolve it in water first?

    • April 15, 2019 at 11:18 pm

      Research finds enteroliths to be made up of magnesium phosphate.

      It does not point to which form of magnesium is more prone to causing this as both phosphorus and magnesium are present in the body to form such stones as mentioned above.

      One study found that horses fed majority alfalfa tend to develop these stones more than others so perhaps the culprit is a high calcium intake and ratio in the diet?
      I’m speculating and would love to know more.

  • February 10, 2015 at 10:54 am

    can you give a pregnant mare epson salts ?


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