The Importance of Magnesium For Horses

Magnesium (Mg) is an essential macro mineral closely associated with calcium and phosphorus and constitutes approximately 0.05 percent of the body mass.  The skeleton stores sixty percent of body magnesium and about 30 percent can be found in muscle.  Magnesium is also an important ion in the blood, and it plays a role as an activator of over 300 different biochemical reactions and participates in muscle contractions.


Magnesium is absorbed from the small intestine in the horse, with very little thought to be absorbed from the large intestine.  Many commonly used feedstuffs contain 0.1-0.3 percent magnesium and absorption from these feeds appears to be 40-60 percent (NRC, 2007).  According to Kohnke (1999), there are several dietary interactions related to the absorption of magnesium with reduced uptake being associated with high calcium and phosphorus in diets, oxalates in tropical grasses and high potassium intakes. The level of magnesium in forage varies between different species, soil type and climate but legumes tend to have a higher concentration of magnesium than grasses.   The magnesium concentration of a plant is greater in the stems than leaves and the availability of forage magnesium appears to increase with maturity and preservation, however the high use of potassium and nitrogen fertiliser results in a reduced magnesium content in grasses.  Inorganic supplemental sources such as magnesium oxide, magnesium sulphate and magnesium carbonate appear to have a higher absorption rate (70%) than the magnesium found in natural sources (Harrington D.D., 1980).  

Magnesium is Required for Correct Nerve and Muscle Function 

Muscle action primarily relies on two minerals being calcium and magnesium.  Calcium is required to make a muscle contract and conversely magnesium is required to make the muscle relax.  When a muscle cell is stimulated the cell membrane opens letting calcium in and increasing the calcium level in the cell, setting off a reaction and resulting in the contraction of the muscle.  Once the contraction is finished the magnesium inside the cell helps to push the calcium out, releasing the contraction.  When there isn’t enough magnesium inside the cell calcium can leak back in causing a stimulatory effect that doesn’t allow the muscle to completely relax, putting the horses’ body in a persistently stressed state.  Insufficient magnesium levels increase the release of acetylcholine at the nerve endings and can induce tetany, leading to nerve endings being hypersensitive and therefore exacerbating pain and noise.


The causes of magnesium deficiency are reduced intake (poor nutrition), reduced absorption (e.g., chronic diarrhoea, mineral interactions) and increased excretion (stress, excessive physical exertion, and lactation).  The NRC guidelines (2007) identify the clinical signs of magnesium deficiency in horses as being: nervousness; muscle tremors; ataxia (incoordination); hyperpnea (deep breathing); potential for collapse; and death. 

Horses with a deficiency in magnesium are likely to have a poor tolerance to exercise they are quick to fatigue and are prone to tying up.  These horses more readily build up lactic acid and often have behavioural issues to do with muscle cramping.  Many performance horses can become deficient as the season progresses as they are using the available magnesium more rapidly due to intensity of exercise, stress, and travel.  These horses can often become difficult to work with, so riders tend to increase exercise in an attempt to manage behaviour.  Unfortunately, the physical exertion only adds to the shortfall due to more sweating and muscle cramping, resulting in fatigue, soreness, and negative association with work.  Consequently, behaviour gets worse with more work and exposure to stress, not better.  

Growing, lactating, and exercising animals have a higher requirement of dietary magnesium and hypomagnesemia is more likely to occur in high-producing mares, especially if transported long distances without feed.  Substantial amounts of magnesium can also be lost in sweat; therefore the magnesium intake should be increased 1.5 to 2 times for maintenance horses undergoing moderate to intense exercise (Stewart, 2011).

Magnesium and pastures

Grass tetany or hypomagnesaemia is a well-documented nervous disorder of ruminants that occurs when the magnesium concentration in the cerebrospinal fluid (CSF – fluid surrounding the brain and spinal cord) falls below a critical level.  Animals that suffer from grass tetany are usually high producing animals that are grazing lush green pastures.  Lush, young, rapidly growing pastures have high potassium and low magnesium levels.  The low magnesium levels in blood serum and CSF leads to hyper-excitability, muscle spasms, convulsions, respiratory distress, collapse, and death.  

Humans can also suffer from a deficiency in magnesium and can display some of the following symptoms:  poor heart health; weakness; muscle cramps; tremors; anxiety; hallucinations; dizziness; fatigue; poor memory; difficulty swallowing; seizures; and aggression.

The NRC guidelines (2007) indicate that pastures that are prone to magnesium deficiency, tetany and death in ruminants do not affect horses similarly.  However, many horse owners have noticed that during times of the year when pasture is young, lush, very green, and actively growing the behaviour exhibited by their horses undergoes a dramatic change.  Horses that are normally calm and easy-going suddenly become “spooky”, behave erratically, show signs of muscle tenseness, incoordination, soreness or twitching and sometimes can even display dangerously aggressive behaviours.

Magnesium deficiencies are most likely to occur in Spring, during periods of strong grass growth and in winter when temperate pastures are heavily fertilised.  Grass in both circumstances is likely to be low in magnesium and sodium and likely high in nitrogen and potassium.  This leads to multiple issues as high potassium levels can inhibit the absorption of what little magnesium there is, while sodium, which is low in these situations is known to help uptake.

Despite the lack of documented evidence of pasture-related magnesium deficiency in horses, the symptoms that some horse owners are reporting are like symptoms that other animal species exhibit during conditions of magnesium deficiency.  Dr. Nerida Richards, outlines in her article “Is Pasture Affecting Your Horse’s Behaviour?” numerous scientific reasons why a lack of magnesium could be contributing to abnormal behaviour in horses: 

  1. Magnesium is essential for the functioning of an enzyme called acetylcholine esterase and during periods of magnesium deficiency this enzyme does not function as it should and builds up in the motor end plates causing neuromuscular excitability.
  2. Magnesium is required by adenyl cyclase, an enzyme involved in the action of the parathyroid hormone (PTH).  The compromised function of the PTH is thought to be the main cause of hypocalcaemia (low blood calcium levels) which is commonly observed in Mg deficient animal species including ruminants, humans, and rats.
  3. Magnesium is essential for turning vitamin B1 (thiamine) into the biologically active thiamine pyrophosphate, so in effect a magnesium deficiency will also cause a vitamin B1 deficiency which can cause confusion, disorientation, hallucinations, aggression, stiffness, and muscle twitching.
  4. Magnesium deficiency affects nitric oxide production by motor neurons.  In situations of low extracellular nitric oxide but high intracellular nitric oxide in humans, aggression can result.

Magnesium Supplementation

According to the NRC guidelines (2007), an intake of 20mg of magnesium per kilogram of bodyweight per day is necessary to maintain normal blood serum levels.  Therefore, a 500kg horse in light to moderate exercise requires an intake of 10g per day to maintain blood levels at the minimum value reported.  

Magnesium is often the main ingredient in calming supplements and new research has found evidence indicating that magnesium could have a calming effect on horses.    A recent study by a group of researchers in Australia (Dodd J.A., 2015) showed that magnesium supplementation significantly slowed horses’ reaction speed responses.  It was discovered that adding 10 grams of magnesium to a roughage (clover/ryegrass hay) diet reduced the average reaction speed response by more than one third in the trial Standardbred geldings.  Without the supplement the horses’ average response time was 5.3 metres per second and with the supplement it slowed to 3.1 metres per second.   

There also appears to be a connection between lack of cellular magnesium and metabolic problems such as insulin resistance (IR).  Magnesium deficiency has been linked with IR and metabolic syndrome in humans and rodents but has yet to be scientifically confirmed in the horse.  There are, however, anecdotal reports from veterinarians that magnesium supplementation in addition to previously attempted dietary modifications to horses with equine metabolic syndrome has been beneficial in reducing neck crestyness (neck fat) and the frequency of laminitis episodes.  However, there are no published reports or experimental substantiation of such claims (Stewart, 2021).


Magnesium is an important macro mineral for horse health and performance.  Although the research into the benefits of magnesium supplementation has been focused around human and ruminant animals, the anecdotal evidence suggests that supplementation is worthwhile in horses.  Further research into equine metabolic issues associated with grazing lush, green pastures may prove valuable in managing horses in the future.

Providing horses with a balanced diet is important for overall health and vitality and MegaMin Equine Supplements help provide crucial vitamins and minerals to replace nutrient shortfalls. As mentioned previously in this article, calcium and magnesium have very important interactions within the body and should be fed in balance within the diet, ideally the two minerals should be kept within a ratio of 2.5:1 to 3:1, calcium to magnesium.  MegaMin Bone Defender is a premium macro mineral top up supplement formulated to promote strong, healthy bones.  MegaMin Bone Defender provides extra calcium, phosphorus, magnesium, and vitamin D in balanced levels and may assist horses that have higher requirements for these nutrients such as working, growing, pregnant and lactating animals or those that are grazing high oxalate pastures.

Dodd J.A., D. G. (2015). Magnesium aspartate supplementation and reaction speed response in horses. Journal of Equine Veterinary Science 35, 401-402.

Galland, L. (1991-1992). Magnesium, Stress and Neiuropsychiatric Disorders. Bethesda: US National Library of Medicine National Institues of Health.

Harrington D.D., a. W. (1980). Equine magnesium supplements: evaluation of magnesium sulphate and magnesium carbonate in foals fed purified diets. Equine Veterinary Journal , 12:32-33.

Kohnke, J. (1999). Feeding Horses in Australia: A Guide for Horse Owners and Managers. Barton ACT: Rural Industries Research and Development Corporation.

Magnesium: The Mineral Superhero. (2021, May 19). Retrieved from Performance Equine Nutrition:

N, R. (2011). Is Pasture Affecting Your Horse’s Behaviour? Equilize Horse Nutrition Pty Ltd.

NRC. (2007). Nutrient Requirements of Horses Sixth Revised Edition. Washington DC USA: Nation Research Council of the National Academies.

Stewart, A. (2011, May 19). Magnesium Disorders in Horses. Vet Clin North Am Equine Pract, 27(1): 149-63. Retrieved from Performance Equine Nutrition:

Tabrizian, I. (2004). Visual Textbook of Nutritional Medicine. Greenwood: NRS Publications.