Does My Horse Need a Magnesium Supplement?

Does My Horse Need a Magnesium Supplement?

Why do horses need magnesium?

Magnesium is an essential macromineral that plays a crucial role in various biological processes in the horse’s body. These include:

  • Assisting in muscle contraction and relaxation
  • Supporting the proper function of the nervous system
  • Aiding in the formation and maintenance of strong and healthy bones
  • Helping maintain electrolyte balance within the body


On average, a horse in light-medium work requires 20mg of magnesium per kilogram of body weight per day (equivalent to 10g per day for an average 500kg horse). 


Where does magnesium come from?

Magnesium is abundant in the Earth's crust and is often found in soil within grazing  paddocks and pastures used for forage production. Grass that grows in magnesium rich soil will also have a high magnesium content. In areas where soil magnesium levels are low, land owners will often consider using fertilisers with a higher magnesium content. Soil in the UK is typically naturally rich in magnesium, therefore magnesium fertilisation is not often required.


Magnesium can also be found in various foods, including green leafy vegetables, nuts, seeds, grains and legumes which are commonly found in many commercial feeds. Some mineral water sources also contain significant amounts of magnesium.


Concentrated forms of magnesium are a common ingredient in a vast range of horse supplements, often found in high volumes in calming and hormone supporting supplements as it is believed to have a calming effect on the horse.


Apart from supplements, this concentrated form of magnesium can also be found added to concentrate feeds and balancers, typically in high quantities.


Should you feed magnesium to your horse?

Whilst it is true that magnesium deficiency in horses can have a negative effect on the nervous system, which may result in spooky, anxious or reactive behaviours, it is also true that magnesium deficiency is uncommon in the UK.


Like with most micro and macro nutrients, there is a fine line between under and over supplying magnesium, unfortunately the effects of over supplying certain nutrients actually mimic the effect that undersupplying them would have. In this case, over supplying magnesium in the diet can actually have the opposite of the desired calming effect and may exacerbate spooky and reactive behaviours - not quite the result you are hoping for if buying a supplement for ‘calming’ purposes. For this reason we ensure that all of our supplements are entirely magnesium free.


Magnesium supplementation can be beneficial however if your horse is actually deficient in magnesium, but otherwise you could be creating more problems for yourself (and wasting a lot of money) if unnecessarily supplementing magnesium. Whilst very uncommon, it is not impossible for your horse to have a magnesium deficiency.


Signs of magnesium deficiency

(other than spooky/reactive behaviours) in horses include:

  • Muscle pain, spasms or tremors
  • Tying up
  • Tension
  • Decreased appetite
  • Increased heart rate


If you do suspect your horse is deficient in magnesium, it is important to contact a vet or nutritionist before going ahead and providing a magnesium supplement. A vet can carry out a blood test to confirm or rule out a deficiency and a nutritionist can analyse your horse’s diet to see what nutrients it is lacking in.


Steps to take for magnesium in grazing

If you are concerned about the magnesium content of fields used for grazing or hay production, you can get a soil nutrient analysis carried out. This is a relatively cheap procedure and is offered by many labs. The process involves taking a soil sample and sending it off to a lab for processing. Following scientific analysis, the lab will be able to provide a comprehensive breakdown of the soil nutrient content. Many will use the results of this to help make informed decisions regarding fertiliser application. If it is revealed that your land has a low magnesium content, this can be easily corrected using a fertiliser with a high magnesium content.



By no means are we trying to demonise magnesium. It is an essential nutrient that serves many purposes in the body, however like many other elements in your horse’s diet should be provided in moderation. Getting the correct balance of nutrients is essential for a happy and healthy horse. If you are struggling to get this balance right, contact a vet or nutritionist; in most cases they will be more than happy to help.


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