Why Do Horses Need Magnesium Supplements?

Magnesium is beneficial not only to humans but to horses as well. Being an essential mineral in over 300 metabolic processes in a horse’s body, it qualifies as a vital macromineral.

Supplementation with magnesium is indicated especially for horses that are growing or which are heavily exercised.

A horse that’s deficient in magnesium can suffer a host of problems from muscle issues to colic and nervousness.

In what follows, I’ll discuss the benefits of magnesium for horses and how to supplement a horse’s diet with adequate levels of magnesium.

The Benefit of Magnesium for Horses

Magnesium deficiency is likely to occur in spring with heavy grass growth that does not allow for the accumulation of magnesium. At least not enough to meet the magnesium requirements of horses.

Therefore, adding magnesium to a horse’s diet will supplement this deficiency and come with several health benefits including:

1. Exerts a calming effect on horses

Suboptimal levels of magnesium in the diet can cause nervousness in horses. Horses that exhibit signs of excitability, nervousness and mood swings can benefit from magnesium supplementation.

2. Improves muscle relaxation

On the levels of the muscles, magnesium and calcium have antagonistic roles. Calcium causes the muscles to contract, while magnesium induces relaxation. When a horse is deficient in magnesium, muscles can chronically tie-up. Feeding magnesium to your horse will ease the symptoms.

3. Helps prevent laminitis in horses

Magnesium also has anti-inflammatory properties and a protective effect against endotoxins. It is believed that magnesium helps reduce the occurrence of laminitis in horses. Horses with laminitis routinely have low magnesium levels in their blood.

4. Supports healthy bones

Magnesium works in synergy with calcium in improving the bone structure. Magnesium helps with the absorption of calcium from the diet, supporting both healthy bones and the overall well-being of horses.

5. Improves insulin sensitivity

Magnesium may also play a role in helping insulin clear glucose from the blood. While further research is still needed on this front, anecdotal evidence suggests a beneficial effect of magnesium in improving insulin sensitivity in horses.

6. Boosts performance

Apart from its role in relaxing muscles and improving bone health, magnesium can also have a role in boosting the performance of horse athletes. Magnesium helps with the delivery of oxygen to muscles and it’s involved with the metabolic processes of protein synthesis.

7. Eases colic in horses

Another benefit of magnesium for horses is that it has been found to ease the symptoms of colic in horses. Nearly 80% of horses that have been diagnosed with enterocolitis had low magnesium levels.

Therefore, the benefits of magnesium for horses are multifold. As you can see, magnesium is involved in several processes from muscle and bone health all the way through to digestive and even mental well-being.

In this context, monitoring your horse’s magnesium intake is essential, especially during periods when it’s not naturally available in their primary food source (e.g. green pastures in spring).

Magnesium Supplements for Horses

Although magnesium is available in varying degrees in the forage and feed of horses, it may still not be enough, especially for horses with a high workload.

Magnesium supplements for horses often come in an inorganic form like magnesium sulfate or magnesium oxide or an organic form such as chelated magnesium.

Of these, magnesium oxide is probably the most commonly used variety in magnesium supplements designed for horses. Bought in bulk powdered form, it’s also a good value for money.

Unfortunately, not all magnesium supplements have a high bioavailability and horses will absorb it at a 50%-70% rate, depending on its type. On the flip side, however, it’s difficult to overdose on it, but not impossible, of course.

Magnesium sulfate has a relatively high bioavailability, but because it draws water into the bowels, daily and regular use may cause diarrhea.

Chelated magnesium or magnesium aspartate is often referred to as the gold standard for magnesium supplementation in horses due to its high bioavailability. But they can be more expensive than magnesium oxide, for example.

Therefore, absorption rate, costs, and even palatability are all aspects you must consider when adding magnesium supplements to your horse’s diet.

But just because you’re supplementing your horse’s diet with magnesium, it doesn’t mean you’re actually making a noticeable impact.

The amount of magnesium that you feed your horse is important in drawing the benefits I discussed above.

How Much Magnesium Should a Horse Have?

Horses need, on average, between 13-20 milligrams of magnesium per kg of body weight.

Horses with a higher workload should receive a higher amount while horses with normal exercise levels can be given less.

A horse that’s 500 kg (1100 pounds) should have an intake of 10 grams of magnesium per day.

Because magnesium is absorbed at different rates, depending on its type, and because feeding your horse 20 mg of magnesium per kg of body weight does not mean your horse will actually absorb 100% of that amount, you’ll obviously need to feed your horse more than this amount.

Horses that are training or in the competition will need their dosage adjusted to account for magnesium lost in sweat. Similarly, lactating mares may also have depleted magnesium, so their dosage also needs to be adjusted.

Follow the dosage recommendation on the label of the magnesium supplement you’ve chosen for your horse to make sure that they receive the right quantity for their size.

But it’s not always the lack of magnesium in their forage or feed that’s the problem. Often, magnesium deficiency is caused by a high intake of potassium, which can inhibit magnesium uptake.

The ability of a horse to use magnesium from their diet can also be detrimentally influenced by the amount of calcium, phosphorus and fats in their diet. Sodium, on the other hand, aids the uptake of magnesium.

Therefore, supplementing your horse’s diet with various minerals and vitamins should be carried out with the involvement of a vet, so that the correct feeding ratio between the different minerals is maintained.

How to Give Your Horse Magnesium?

Magnesium supplements for horses are usually available in powdered form. This helps tremendously in mixing them into their food, which is the easiest way to get horses to consume it.

Try mixing magnesium into the feed or your horse or mix it into a treat like applesauce or molasses, especially if it’s magnesium oxide, which can have a bitter flavor.

Mineral blocks can also help, but they don’t provide adequate amounts of magnesium for your horse and you can’t actually monitor their intake level.

Feeding your horse magnesium supplements through their food is the best way to ensure they get the right amount of magnesium.

Because some magnesium supplements are unpalatable, you may need to use treats to mask the flavor, so your horse will not reject it.

Can You Give a Horse too Much Magnesium?

Although you can give too much magnesium to your horse, toxicity isn’t something you need to actively worry about since excess magnesium is excreted in the urine.

Horses with impaired kidney function or other kidney problems, however, may have trouble excreting magnesium, which could cause magnesium levels in blood serum to increase.

Therefore, horses with known kidney issues should only be given magnesium under the supervision of a vet.

Symptoms of magnesium toxicity in horses include:

  • Increased heart rate
  • Muscle tremors
  • Agitation
  • Rapid breathing
  • Diarrhea (when magnesium sulfate or Epsom salt is administered in excess).

The onset of these symptoms is usually within a couple of hours after the administration of the high-dose magnesium.

Horses experiencing magnesium toxicity are usually administered calcium intravenously to counter the effects of the magnesium.

It’s important to maintain the balance of magnesium and calcium in the body. Therefore, you should always consult your vet or an equestrian nutritionist if you want to give your horse magnesium supplements or any other supplements for that matter.

How Long Does it Take for Magnesium to Work in Horses?

Usually, magnesium supplementation will start to show its beneficial effects within 7 to 10 days. This, of course, if your horse’s symptoms were caused by magnesium deficiency or it was determined by your vet that your horse has a magnesium deficiency.

Because the symptoms of magnesium deficiency can be related to other health issues as well, it’s important to consult a vet if you don’t see an improvement or if you notice a worsening of symptoms.


As you can see, the beneficial effects of magnesium for your horse should not be discounted. From bone health to muscle relaxation, supplementing magnesium in your horse’s diet provides your horse with numerous health advantages.

Whether you just want to make sure your horse is not deficient in magnesium or you have a horse athlete, magnesium is a vital mineral that should not be missing from your horse’s diet.

Make sure to feed your horse the correct dosage relative to its weight and don’t give too much magnesium for quicker results, because you may cause magnesium toxicity in your horse.

As with everything, balance is key, so it’s best to keep a close eye on your horse’s magnesium intake.

avatar Noah
I’m Noah, chief editor at VIVO Pets and the proud owner of a playful, energetic husky (Max). I’ve been a volunteer at Rex Animal Rescue for over 2 years. I love learning and writing about different animals that can be kept as pets. read more...

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