Hyfeed - Health Food for Horses
Health Food for Horses

Magnesium - The Miracle Macro Mineral

Shannon Godwin BaAppSc GTDL          


Magnesium (Mg) is an essential macro element that is required for cellular energy-dependent reactions involving adenosine triphosphate and for the regulation of calcium channel function.  Approximately 60% of magnesium is found in the skeleton and the remainder is distributed in the soft tissues and fluids and is crucial for the equine’s well-being.  Magnesium is the most common enzyme activator in over 300 different biochemical reactions. It plays a fundamental role in many functions of the cell, including energy transfer, storage and use; carbohydrate, protein and fat metabolism; maintenance of normal cell membrane function; and the regulation of parathyroid hormone (PTH) secretion.  Systemically, magnesium lowers blood pressure and alters peripheral vascular resistance.    Therefore, magnesium is a key element in cellular biochemistry and function.

Calcium and magnesium work in conjunction with each other, each requiring the other for balance.  Muscle action primarily relies on two minerals and they are calcium and magnesium.  The diagram below shows how calcium is required to make a muscle contract and magnesium is required to make the muscle relax.  When a muscle cell is stimulated, the cell membrane opens, letting calcium in and raising the calcium level in the cell setting off a reaction and causing the muscle to contract.  When the contraction is finished, the magnesium inside the cell helps to push the calcium back out of the cell releasing the contraction.    As the diagram depicts there is a relationship between calcium and magnesium and they balance each other out. 

Diagram from the Visual Textbook of Nutritional Medicine by Igor Tabrizian


When there is not enough magnesium in the cell, calcium can leak back in causing a stimulatory effect and the muscle cannot completely relax.  This can put the horses’ body in a continually stressed state. Low Mg levels increase the release of acetylcholine at nerve endings, which can induce tetany.   This leads to nerve endings being hypersensitive and therefore exacerbating pain and noise.  Magnesium is required for proper nerve and muscle function. 

Calcium requires magnesium in order to integrate in the body however, magnesium uptake is inhibited from high calcium and phosphorus diets, oxalates in tropical grasses and high potassium intakes.  To maintain the correct levels in the blood, the body will borrow magnesium from soft tissue and the bone to make up for the shortfall.  The major losses of magnesium from the body are from sweat (140mg/L), urine (100mg/L) and milk (45-90mg/L).  Less than 1% of the total body magnesium is contained in the extracellular fluid, therefore serum Mg concentration may not adequately reflect the total body Mg stores.  Low Mg levels have been associated with insulin resistance and there has been some interest in the usefulness of Mg supplementation to horses with insulin resistance with either equine metabolic syndrome or pituitary pars intermedia dysfunction.  A number of anecdotal reports from veterinarians indicate that Mg supplementation in addition to previously attempted dietary modifications to horses with equine metabolic syndrome has been beneficial in reducing neck crestyness and the frequency of episodes of laminitis.

Horses with a deficiency in Mg are likely to have a poor tolerance to work, they are quick to fatigue and are prone to tying up.  Lactic acid is more readily built up and these horses often have behavioural issues due to muscle cramping.  Following are some signs that may indicate magnesium deficiency in horses:

  • Very tight, sore back not related to activity, fitness level or saddle fit.
  • Physiotherapy, chiropractic adjustments, massage and other body work do not have lasting effects.
  • Horse appears to never really relax.
  • History of tying up.
  • Shows signs of irritability when brushed, rugged or being palpated especially over the back on either side of the spine.
  • Muscle tremors or all over trembling not related to temperature.
  • Does not tolerate work well.
  • Requires long periods of lunging before being able to focus on work.
  • Bucks shortly after workout begins, seems fine at first then bucks or balks.
  • Is described as hypersensitive to touch or “thin skinned”.
  • Demonstrates repetitive movement, head bobbing, weaving, pacing etc.

Severe Hypomagnesemia (deficiency of magnesium in the blood) results in neuromuscular disturbances, but the following signs are rarely document in horses.

  • Muscle weakness
  • Ataxia
  • Collapse
  • Seizures/Tetany
  • Tachydysrhythmias (abnormal heart rhythm)
  • Hyperpnea (increase in depth of breathing)
  • Refractory hypokalemia/hypocalcemia

The causes of hypomagnesemia are reduced intake (poor nutrition), reduced absorption (eg, chronic diarrhoea), redistribution and increased excretion (stress, excessive physical exertion and lactation).  Many performance horses can become deficient as the season progresses as they are using the available magnesium more rapidly due to stress, travel and competition.  These horses can often become difficult to work with, so riders tend to increase exercise in an effort to manage behaviour.  This increase in physical exertion only ads 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. 

According to Merck Veterinary Manual, Ralston (2011), the daily magnesium requirement for maintenance has been estimated at 0.015 g/kg body weight based on limited studies. Working horses are estimated to require 0.019 to 0.03 g/kg body weight for light to strenuous exercise, respectively, due to sweat losses.  Magnesium is often the most neglected mineral in horse feeds.  Lush, young grass is typically deficient in magnesium and grasses and cereal crops have lower magnesium levels than clovers and Lucerne.  Kohnke (1999) has listed the approximate magnesium levels in common feed sources as follows:  Lucerne 3.0g/kg, Grains 1.1-1.4g/kg, Canola Meal 5g/kg, Soyabean Meal 2.7g/kg, Milk Powder 10g/Kg, Magnesium Oxide 10.6g/20mL, Magnesium Carbonate 5.6g/20mL, Magnesium Sulfate (Epsom Salts) 2g/20mL, Dolomite average 2g/20mL. 

Magnesium toxicity is rare because excess is naturally excreted.  If supplementing with Mg it should be split between morning and evening feedings to increase absorption and decrease its occasional laxative effects (especially if using magnesium sulphate).  It is very difficult to combat a true magnesium deficiency without the use of supplementation.  Magnesium supplements can come in the form of oral supplements, injectables and trans dermal applications (delivered through the skin).  The most popular source is by oral supplementation of magnesium oxide due to its availability and being economical to use.  Trans dermal application can be therapeutic prior to intence exercise and also post work out to assist the body to recover and relax muscles. 

Although rare, severe hypomagnasemic tetany has been reported in lactating mares, transported and stressed horses.  In contrast, subclinical hypomagnesemia can be more common than what is expected.  Being familiar with deficiency signs and symptoms is important for owners and as toxicity is extremely rare it a safe mineral to feed.  Veterinary advice should be sought for horses with reduced kidney function prior to supplementation.  To ensure good nutritional health a good source of roughage should comprise at least 50% of the total equine ration by weight and good quality drinking water should be available at all times.  


Elin RJ. Magnesium metabolism in health and disease.  Dis Mon 1988;34(4):161-218

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

Jean-Pierre Lavoie, Kenneth Hinchcliff, 2011.  Blackwell's Five-Minute Veterinary Consult.  John Wiley & Sons, USA

Ralston, Sarah., Nutritional Requirements of Horses.  The Merck Veterinary Manual.p., July 2011. Web. 10 July 2015.

Stewart, Allison.  Magnesium Disorders in Horses.  Vet Clinic Equine, Elsevier inc. 27 April 2011. Web 10 July 2015.

Tabrizian, Dr Igor., 2004.  Visual Textbook of Nutritional Medicine, NRS Publications, Greenwood.