Symptoms of too much magnesium in horses: Dosage Guideline

“Too much of anything is worthless” is a saying true to its core for Magnesium supplements’ dosage in horses. As with any other mineral, magnesium has a sweet spot, a range that favors the body. Horse owners may face the problem of an excess of magnesium or magnesium overload in horses, which leads to dangerous consequences. Knowing the symptoms of too much magnesium in horses can help us to tackle magnesium toxicity.

Why is Magnesium Important for Horses?

If we are to discuss the “too much magnesium” problem at hand, one sensible approach would be to ask why is magnesium required in horse nutrition.

Magnesium, being a macromolecule, acts as an essential component in various physiological activities; majorly related to muscular activities.

Some of these are:-

●     Magnesium is the central component for muscle contraction along with calcium.

●     Magnesium aids in nerve signaling for contraction of muscles.

●     Magnesium is a mediator of various enzymatic reactions.

●     Magnesium helps prevent laminitis in horses.

We have covered the benefits of magnesium for horses here.

A responsible horse owner or breeder can determine the magnesium deficiency in horses by visualizing the deflection from the well-being of their animal.

Magnesium deficiency signs include nervousness, fatigue, tremors, cramps, pain, anxiety, exercise intolerance, etc. These signs serve as red flags to a horse’s normal physiology.

How Much Magnesium to Give a Horse?

Once we have gathered knowledge about the necessity of the mineral, we should apply our wisdom on how to provide for the same.

Booming population and urbanization have led to fewer grasslands, forcing animals to feed either on a composed diet or newly bedded pastures. Often these fast-growing grasses are low on magnesium.

NRC recommends - a 500 kg healthy adult horse needs the prescribed amount of 7.5 grams of magnesium per day in the diet. Some texts also recommend 10g/day, which depends on the diet and lifestyle of the horse.

Half of this required magnesium is more often found in pasture feeding, premix feed, and hay. The other half is to be fulfilled by adding magnesium supplements to the diet. Therefore, we fulfill the requirement of animals by adding 5g of magnesium as a supplement. To meet the horse’s magnesium demand, we add supplements like magnesium oxide (the most economical option), magnesium citrate, magnesium chloride, magnesium sulfate, etc., to their diet as powders and pellets.

Excess of Magnesium in Horses

Magnesium is no doubt an essential dietary supplementation in horses, but it also makes animals prone to magnesium toxicity. 

The ideal blood magnesium level according to NRC is 2.2-2.7 mg/dl, and roughly 7.5-10g of magnesium is to be supplemented to a 500kg horse daily.

When the magnesium level in the body reaches 0.8% of the total diet, it equates roughly to 80-90g of magnesium in the total diet of the horse.

All these figures serve no purpose if one cannot identify the signs of overdosing, because “You can’t heal something if you don’t know its cause”.

Symptoms of Too Much Magnesium in Horses

There are certain indicators of magnesium toxicity or excess magnesium in horses:

1. Diarrhea- Scientists say the kidney of a healthy animal flushes excess magnesium out of the healthy animal's body. Still, diarrhea remains one of the first and most confusing signs of magnesium toxicity, especially while using magnesium sulfate (Note: it can be associated with other health conditions).

2. Muscle Fatigue- The horse will show signs of weakness. It will feel difficulty in standing or exercising and may try to sit around.

3. Muscle tremors- occur in horses because of neuro-muscular dysfunction.

4. Lack of Interest in Performing Tasks- Horses will be continuously in a state of uneasiness and hence, would disobey exercise.

5. Inappetence- The horse will lose interest in food and water because of the increased discomfort.

6. Cardiac Dysfunction- Horses can suffer from cardiac arrhythmia because of the excess magnesium. This can be fatal if not treated.

7. Rapid Breathing. The horse will show an abrupt increase in breathing rate with the increase in heart rate.

Treatment of Magnesium overload

Once we know the status of the horse, the first reasonable step would be to reevaluate the dietary supplementation, check for excessive magnesium consumption, and consult a veterinarian immediately.

Eliminating the supplements from horses’s diet for a particular period can yield excellent results.

Your vet would proceed with the diagnosis of magnesium toxicity and to counter it with the administration of calcium gluconate intravenously. The Basic idea behind this is that calcium competes with magnesium for absorption.

Another sensible approach would be the incorporation of diuretics and fluid therapy.


Q1. Do horses need magnesium supplementations?

Ans. Magnesium supplementation is a must in horses. It has various uses, ranging from muscle physiology to electrolyte balance.

Q2. How much magnesium should I give to my horse?

Ans. NRC recommends 7.5 grams of magnesium intake per day for horses with an average weight of 500kg; This may increase for larger horses (12-15g per day).

Q3. How much magnesium is too much magnesium?

Ans. Maximum tolerance level of magnesium is 0.8% in total diet for a 500kg horse.

Q4. What is the most economical supplement of magnesium in horses?

Ans. There are various magnesium supplements available in the market, such as magnesium citrate, magnesium chloride, magnesium sulfate, etc., but the most economical magnesium supplement is magnesium oxide.


Magnesium is a pivotal macronutrient of a horse’s biology. It helps maintain electrolyte balance, muscular contractions, enzymatic reactions, and nervine functions. It also plays a role in cardiac functioning to a certain extent.

Supplementation of magnesium is more of a necessity than choice and it’s widely practiced among the horse's owner community. This fairly deals with the ‘lack of magnesium’ problem but may also serve as a cause for the ‘more of magnesium’ problem.

Signs of magnesium toxicity in horses may include muscle weakness, lethargy, low blood pressure, cardiac arrhythmias, dyspnea, muscle twitches, etc.

The toxicity of magnesium is as much a problem as the deficiency of magnesium. Consulting a veterinarian in such conditions is the right choice to make.


Dr Akanksha Agnihotri is another animal lover like you cum veterinarian by profession. She has earned her B.V.Sc & AH from COVS & AH, Jabalpur, India. She has a knack for befriending animals/pets and treating them compassionately. She believes...