Magnesium (Mg) is an essential mineral that horses require for proper nerve and muscle function. Magnesium functions as an electrolyte, plays a role in protein synthesis and is involved in over 300 metabolic processes in the horse’s body. It is especially important for growing and heavily exercised horses. 
Horses that are not getting enough magnesium in their diet may be excessively irritable, nervous, or they may chronically tie-up. If your horse experiences thumps (synchronous diaphragmatic flutter) they likely are low in magnesium.
Magnesium deficiency is more likely to occur in the springtime when fast growing grasses do not accumulate much of this macromineral. Although severe deficiency is rare in horses, suboptimal levels can affect their mood and performance.
Magnesium toxicity is unlikely in horses receiving supplementation because excess amounts are efficiently excreted in urine. Providing too much, particularly in the form of magnesium sulfate, might cause temporary diarrhea. Magnesium oxide is the recommended form for supplementation.
Why Horses Need Magnesium
Magnesium is a macromineral that horses need for optimal muscle function and nerve transmission. It is involved in energy metabolism, enzyme activity and muscle tissue recovery following exercise.
In horses, 60% of magnesium can be found in the bones and the other 40% can be found in extracellular fluids and soft tissues. It is especially important to maintain optimal levels of this mineral in horses that are growing or being heavily exercised.
Horses may not be getting enough magnesium when levels are low in their feed. This is most likely to occur in the spring but can also happen at other times of year if pastures are fertilized to encourage fast growth.
Horses that exercise frequently may have a greater need for magnesium because it is lost in sweat. Young foals also have a higher need to support growth.
Horses with colic might benefit from supplementation. Low magnesium levels are seen in 78% of horses with enterocolitis.
Approximately 40% of horses that have colic will have endotoxemia (presence of bacteria from inside the intestine in the blood) which triggers an immune response and causes magnesium levels to drop. 
More studies need to be done to assess whether administering magnesium in these cases would be beneficial.
Health Benefits of Magnesium for Horses
Below are the top 10 reasons why horses need adequate magnesium in their diet:
- It works closely with calcium to maintain proper muscle function. When muscles receive nerve signals telling them to contract, calcium is released from special compartments within cells and moves onto muscle fibres causing them to contract. Magnesium stops the contraction by pushing calcium back into these compartments.
- By enabling muscle relaxation, it can support recovery and help to ease muscle pain and cramping in animals following heavy work.
- This macromineral can have a performance boosting effect for equine athletes, supporting enhanced oxygen delivery to muscles, muscle strength, and metabolic processes that result in protein synthesis.
- Magnesium helps nerve cells transmit signals to each other and to muscles by regulating ion balance across cell membranes.
- It is known to have a calming effect on horses, helping to ease muscle tremors and nervousness. It is frequently fed as a mood supplement to horses showing signs of excitability or abnormal behaviour.
- It can improve tolerance to stress and resistance to illness and injury. Stress causes magnesium to be depleted in the body faster. Ensuring optimal levels of magnesium intake can help to speed up return to homeostasis following exposure to an external stressor.
- It can contribute to normal fat distribution in horses by minimizing cresty neck or fatty pockets.
- Magnesium helps cells respond to insulin. Supplementation with this mineral might improve insulin sensitivity, particularly in overweight horses.
- It helps prevent laminitis in horses especially in those that are more prone to laminitis in the spring. Horses with laminitis are often found to have low levels of this mineral in their blood.
- Optimal levels of magnesium help with absorption of calcium from the diet. This can support healthy bones and overall well-being in your horse.
Many horses require magnesium supplementation in their diet to support optimal well-being, mood and performance.
Mad Barn’s Omneity Premix is a fully balanced equine mineral and vitamin supplement that provides comprehensive nutritional coverage for your horse’s needs. It contains 2.6% magnesium in the form of highly absorbable magnesium oxide, sufficient to meet the needs of most horses.
Signs of Magnesium Deficiency
The good news is that chronic magnesium deficiency is unlikely to occur in horses. In scientific experiments, the only way to induce chronic deficiency is to feed artificially low magnesium diets to young growing animals. 
However, mild to moderate deficiency can occur in horses and may affect their mood and performance. Horses that do not get adequate amounts of this mineral from their feed might only show signs of deficiency during stress or competition.
Most signs of magnesium deficiency are related to disordered neural or muscular function including: 
- Unable to relax or focus
- Muscle tremors, spasm, twitching, flinching skin, trembling
- Muscle pain or cramps
- Not tolerant of long periods of work
- Highly sensitive to sound or movement
- Hypersensitive skin
- Irritable moods
- Resistance to training
- Irregular heartbeat
- Teeth grinding
- Increased perspiration and anhidrosis
In more serious cases, the following rare symptoms may be observed:
- Chronic Exertional Rhabdomylosis (tying up), especially tight, sore backs
- Irregular heartbeat
- Increased respiratory rate or laboured breathing
- Metabolic dysfunction, abnormal fat distribution and weight gain
- Muscle convulsions
Factors Leading to Magnesium Deficiency
As previously mentioned, quickly growing grasses are typically low in magnesium. Such grass is also likely low in sodium and high in potassium which can exacerbate a deficiency. High potassium can slow the absorption of magnesium. Sodium typically enhances absorption but is low in these circumstances.
Other nutritional factors might affect how magnesium is absorbed and assimilated in the body. High amounts of fibre, oxalates, phosphates, and fat in the feed might decrease absorption in horses. Feed grown in acidic or clay soils might be low in magnesium throughout the year.
Diet composition might influence magnesium levels in horses because digestibility varies by feedstuff. Alfalfa has the highest magnesium digestibility at 50% while grain has the lowest at 38%. High grain diets might make horses more susceptible to being deficient in this essential mineral.
The best way to find out if your horse is likely to be magnesium deficient is to analyze their diet.
A blood test will likely not tell you if your horse has suboptimal levels because blood magnesium levels are maintained in a narrow range.
How Much Magnesium Should You Feed Your Horse?
Magnesium is a macromineral, meaning it is required in larger amounts compared to other minerals.
According to the National Research Council (NRC) , the daily requirement for horses is estimated at 20 mg per kg of body weight. For a 500 kg horse, the magnesium requirement would be 10 g per day.
A horse’s intake should be increased 1.5 to 2 times if they are undergoing moderate to intense exercise. For a 500 kg horse, the requirement during training and competition will be 15 to 20 g per day. This will help compensate for the magnesium that is lost in sweat and help muscles recover after exercise.
During lactation, mares require more of this mineral to keep up with how much is excreted into milk. Lactating mares require 15 to 30 mg of magnesium per kg of feed. Low levels are most likely to occur in horses that produce a lot of milk, especially if they are being transported long distances without feed. 
Before feeding your horse additional magnesium, it is recommended to have their diet reviewed by a qualified nutritionist. You can submit your horse’s diet for analysis to determine whether any nutrient levels need to be adjusted.
Rather than feeding your horse more magnesium in isolation, it is important to look at their diet as a whole. Minerals need to be fed in balanced ratios to ensure optimal health.
Mad Barn’s Omneity Equine Mineral and Vitamin contains 5.4 grams of magnesium in a typical serving size for a 500 kg horse. In addition to magnesium, Omneity provides all other essential minerals and vitamins that your horse needs at levels scientifically formulated to bring the majority of equine diets into balance.
In horses, magnesium absorption occurs in the small intestine with very little occurring in the hindgut. The majority of this mineral is absorbed by passive diffusion, moving through cell membranes following a concentration gradient.
This means that when magnesium is high within the intestine during feeding, it slowly moves towards the bloodstream where its concentration is lower. With higher dietary levels of this mineral, more will be absorbed. A minor portion of absorption occurs by active transport in which magnesium is pumped across the cell membrane through special transporters.
These transporters also move calcium across cell membranes. For this reason, calcium and magnesium are said to compete with each other for absorption. Dietary ratios of two-parts calcium to one-part magnesium are optimal to have proper absorption of both minerals.
Absorption of this compound from forages and feeds is around 40-60%. In comparison, common inorganic sources like magnesium oxide, citrate and sulfate have absorption rates around 70% in foals. 
Best Magnesium Sources for Horses
There are several factors to consider when determining which magnesium supplement is best for your horse. Absorption rate is one factor, which is higher with inorganic forms (~70%) than common feedstuffs (40-60%).
Some supplements are less concentrated meaning you need to feed larger amounts to provide the same amount of mineral.
For example, to provide 10 g of elemental magnesium, you would need to feed approximately 17 grams of magnesium oxide compared to 186g of magnesium gluconate. This has cost implications.
Bulk magnesium oxide powder is a popular choice for supplementation in horses because it is readily absorbed and highly concentrated. It is also one of the cheapest sources of magnesium when calculated on a daily basis.
This mineral can also be provided by other means including transdermal and injectable forms. Transdermal magnesium, delivered by absorption through the skin, can access muscles rapidly and help calm a nervous horse before competition or help their muscles recover post-exercise.
How to Feed Supplemental Magnesium
Mineral levels in your horse’s diet should be adjusted in consultation with a qualified equine nutritionist. You can submit your horse’s diet for analysis online and one of our nutritionists will be happy to provide a complementary evaluation.
To provide calcium and magnesium in an optimal two to one ratio you need to know the nutrient levels already present in your feed. If your hay contains 0.15% magnesium and 0.8% calcium, you can calculate how much 15 lbs of hay provides:
15 lbs of hay x 454g/lb x 0.008 = 54.5 g calcium
In this case, an additional 17 g of magnesium is required to achieve an optimal balanced 2:1 ratio of calcium to magnesium.
If you are using magnesium oxide which has a concentration of 60% magnesium, you would add 28g of magnesium oxide to the horse’s diet.
It is recommended that you divide dosages of supplemental magnesium oxide over two meals per day to improve palatability. This minimizes the risk of digestive upsets like diarrhea.
Some horses may take time to adjust to the chalky texture. You should slowly increase the dose over a few days to ensure tolerability.
Risk of Toxicity and Side Effects
The risk of toxicity is very low when feeding supplemental magnesium to your horse. Excess magnesium is rapidly excreted in the urine, meaning that it will not accumulate to dangerous levels in the horse’s body.
In general, it is more important to maintain a proper calcium to magnesium ratio than to be concerned about providing excess magnesium.
Horses that have kidney issues might not be able to excrete magnesium well and could accumulate excess magnesium in the blood. Horses with kidney issues should only be given supplements under supervision by a veterinarian.
Administering high dosages can cause diarrhea, especially when magnesium sulfate (epsom salt) is used. Epsom salts should be avoided except on the advice from veterinarians to treat intestinal compaction.
In two cases of acute toxicity due to high doses of epsom salts, horses experienced symptoms including agitation, muscle tremors, accelerated heart rate and rapid breathing within a few hours. These symptoms were successfully remedied by providing intravenous calcium. 
Before altering your horse’s feeding program to add more magnesium, we recommend consulting with a professional equine nutritionist. You can submit your horse’s diet for analysis one of our nutritionists will be happy to provide a complementary review.
- Stewart, Allison. Magnesium Disorders in Horses . Vet Clin Equine. 2011.
- Harrington, DD. Pathological features of magnesium deficiency in young horses fed purified rations. Am J Vet Res 1974.
- Toribio, RE. Magnesium and disease. . In: Reed SM, Bayley WM, Sellon DC, editors. Equine internal medicine. 3rd edition. St Louis (MO): Saunders; 2010. p. 1291â€“5.
- Harrington D, Walsh J. Equine magnesium supplements: Evaluation of magnesium oxide, magnesium sulphate and magnesium carbonate in foals fed purified diets . Equine Vet J 1980.
- Henniger RW, Horst J. Magnesium Toxicosis in Two Horses . J Am Vet Med Assoc 1997.