Magnesium Supplements for Horses

Magnesium is one of the nutrients that is getting more attention in both human and equine nutrition. This is because, as a mineral, magnesium plays a vital role in both muscle and nerve function. Magnesium is also critical to many biochemical reactions within the horse’s body. Thus, there is a real interest in ensuring horses get enough magnesium each day in their diet. Research is also evaluating if supplementing magnesium in the diet can be beneficial to horses.

Why Do Horses Need Magnesium Supplements?

Whether a horse needs supplemental magnesium in their diet is still under debate. The current National Research Council (NRC) daily minimum recommendations for magnesium in the diet for a 500 kg (1100 lb.) adult mature horse is 7.5 grams per day. This amount of magnesium should be easily achieved by most standard equine diets.

For example, it is estimated that common horse feedstuffs range from 0.1 to 0.3% percent magnesium. We also know most grass hays are about 0.2% magnesium in total, with alfalfa (legume) hay about 0.3%. If we calculate how much a horse receives each day in an average diet of grass hay, we can identify if they could possibly be deficient.

If, for example, we only calculate the lowest estimate for magnesium (0.1%) in a hay only diet for a 500 kg (1100 lb.) horse that eats 2% of its body weight per day, we can:

  • Calculate 2% x 500 kg (1100 lb.) = 10 kg (22 lb.) of hay per day.
  • Thus, 10 kg (22 lbs.) x 0.1% = 10 grams (0.022 lbs.) of magnesium per day from hay alone.

What this means is, that even calculating at the lower end of magnesium of 0.1% in common feeds for horses, they should still exceed the minimum recommendations for daily magnesium intake. This also means that horses should rarely, if ever, experience a magnesium deficiency in their diet. 

There is some debate today about adding more magnesium to the diet above the daily recommendations because there is some research and anecdotal evidence that claims adding more magnesium to the diet can benefit certain horses.

It is proposed that horses that experience tying up, or muscle cramping after exercise, can benefit from excess magnesium in the diet. There are no studies in horses to back these claims.

There are published research studies examining magnesium and its calming effect on horses. For example, in one study, horses that were introduced to novel stimuli were less startled after receiving a magnesium supplement. Another study found horses supplemented with magnesium did less head shaking compared to others without it. While more research is needed in this area, there is some evidence that feeding more magnesium may benefit some horses under certain conditions.

As a mineral, magnesium can be overfed and can be toxic to the horse. Thus, caution must be taken when feeding any magnesium supplement. How much magnesium is toxic to horses is still not known. The NRC states that the maximum tolerable level for horses is at 0.8% of the diet. Thus, using the example above:

  • 10 kg (22 lbs.) feed per day x 0.8% = 80 grams (0.176 lbs.) of magnesium per day appears to be the upper safe threshold of horses’ daily magnesium uptake.

Signs of magnesium toxicity in horses are:

  • Most common sign for excess magnesium in the diet. However, be aware there are many causes of diarrhea in the horse.
  • Muscle weakness is often observed with horses having difficulty walking or even standing.
  • Just overall lack of energy or interest in training.
  • Loss of appetite. Decreased interest in food or eating.

If you notice any of the abovementioned signs, it is highly recommended to speak with your veterinarian as soon as possible.

How to Feed Magnesium Supplements?

If you decide to add a magnesium supplement to your horse’s diet, there are some tips you should take into consideration. First, you should identify the type of magnesium you are supplementing your horse with. The kind of magnesium fed can impact just how much magnesium they are getting from the supplement. The types of magnesium you will see in different products include:

  • Magnesium Oxide. Generally considered the most cost effective and safest type of magnesium to supplement. This is because the horse can absorb about 50-60% of it, is economical, and difficult to cause toxicity.
  • Magnesium sulfate. Common source of magnesium for horses in the form of Epsom salt and it can cause diarrhea in horses.
  • Magnesium citrate. A more expensive source of magnesium and horses absorb about 16% of it when supplemented.
  • Magnesium gluconate. Less than 6% is absorbed by the horse.
  • Other magnesium sources are generally best to avoid.

When supplementing your horse with magnesium, it is always recommended to follow the manufacturer’s instructions. Some other tips include:

  • Start with small doses to allow the horse’s digestive system to adjust, like any new horse feed or supplement.
  • Mix with a horse’s feed to ensure maximum uptake.
  • Divide doses between the horse’s meals during the day.
  • Ensure horses always have access to clean and fresh water.

The final and most important tip is to always monitor your horse. If you notice any abnormalities (i.e., diarrhea), it is best to speak with your veterinarian for advice.

Take Home Message

Does your horse need a magnesium supplement? For most horses, the answer would most likely be no. If fed high-quality hay with at least a ration balancer, most horses will get more than enough magnesium each day. If your horse is excitable or suffers from muscle cramping and/or tying up, then a magnesium supplement may be beneficial. However, more research is needed before most equine nutritionists will suggest adding more magnesium to a horse’s diet. If you have any questions or are wondering about magnesium in your horse’s diet, please feel free to contact us for a free consultation.


Dodd, J.A., et al. (2015). Magnesium aspartate supplementation and reaction speed responses in horses. J. Equine Vet. Sci. 35:401-2.

National Research Council. (2007). Nutrient requirement of horses. 6th rev. ed. National Academies Press.

Sheldon, S.A. et al. (2019). Effects of magnesium with or without boron on headshaking behavior in horses with trigeminal-mediated headshaking. J. Vet. Internal Med. 33:1464-72.

Article By: Chris Mortensen, Ph.D.
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