Magnesium supplements for horses come in many forms and are used to support muscle function and calming.
This important macromineral is required in the equine diet to facilitate muscle contraction, maintain ion balance in the blood, and activate enzymes throughout the body.
Most feedstuffs for horses contain sufficient magnesium to meet minimum dietary requirements. Horses in moderate to very heavy work have a higher magnesium requirement and may benefit from supplementation.
If your horse is not getting enough magnesium in their diet, they may show anxious behaviour, muscle tremors and poor coordination.
When choosing a magnesium supplement, look at the concentration and bioavailability of different products. Magnesium oxide is highly concentrated, making it one of the most cost-effective options for horses.
Magnesium Requirements for Horses
The NRC sets out the minimum amount of magnesium to avoid a deficiency. This is not necessarily the optimal amount to feed but rather the amount required to prevent signs of deficiency.
A typical mature, 500 kg (1100 lb) horse at maintenance (not exercising) requires a minimum of 7.5 grams of magnesium daily. Horses exercising, growing or lactating need higher amounts of this mineral.
The following amounts of magnesium are required by a 500 kg (1100 lb) horse in varying levels of work and varying physiological states: 
- Maintenance: 7.5 grams per day
- Light exercise: 9.5 grams per day
- Moderate exercise: 11.5 grams per day
- Heavy or very heavy exercise: 15 grams per day
- Late gestation (9 – 11 months): 8 grams per day
- Lactation (3 months): 11 grams per day
Magnesium in the Equine Diet
A forage-based diet will usually provide sufficient magnesium to meet the NRC requirements of most horses.
Magnesium is typically higher in legume hays – such as alfalfa and clover – compared to grass hays – such as timothy or bermuda grass.
Most feedstuffs such as forages and grains contain 0.1 – 0.3% magnesium. Absorption from these sources has been estimated at around 40 – 60%. 
If your hay has a magnesium content of 0.2% dry matter and you provide 10 kg (22 lb) of hay daily on a dry matter basis, you will supply your horse with 20 grams of magnesium per day.
40 – 60% of the magnesium in forage will be absorbed, providing 8 – 12 grams of elemental magnesium to the body.
The magnesium content of forages is largely dependent on soil characteristics.
Forages that are grown in soil with a low pH (high acidity) are more likely to be low in magnesium. Magnesium absorption in plants is optimized when soil pH is near neutral or slightly alkaline (pH of 7 – 8.5) 
High potassium and/or calcium levels in soil can also interfere with plant magnesium accumulation.
It is always recommended to submit a hay sample for analysis to determine the precise magnesium content of your hay.
Too Much Magnesium
Excess magnesium consumption is likely not a concern as this mineral is readily excreted in the urine.
However, supplementation in horses with kidney issues should be discussed with a veterinarian.
Toxicity has not been described in horses, but the maximum dietary concentration is 0.8% for all horses. 
Potassium, calcium and magnesium are all important electrolytes for transmitting neural signals and muscle contraction.
During muscle contraction, signals from nerves result in potassium being pumped into muscle cells. This triggers the release of calcium from within special structures in the cell, which causes muscle contraction.
Magnesium is involved in muscle relaxation following contraction.
In ruminants, magnesium deficiency interferes with muscle relaxation causing grass tetany, a potentially fatal condition with prolonged muscle twitching and a stiff gait.
Horses are less susceptible to grass tetany, although tetany during long transport has been attributed to hypomagnesemia (low magnesium levels).
It is recommended to provide a diet with a potassium to calcium and magnesium ratio K/(Ca + Mg) of less than 2.2.
This ratio will ensure each macromineral is adequately absorbed in the gut and maintained at appropriate levels within cells and bodily fluids.
Magnesium Deficiency in Horses
Horses with gastrointestinal disease, including horses with colic and small intestinal volvulus, often have low circulating magnesium levels, perhaps caused by issues with regulating magnesium metabolism during illness. 
It remains unclear whether these deficits affect post-surgical survival or whether magnesium supplementation is warranted in these cases. 
Severe magnesium deficiency in horses has primarily been described in horses hospitalized for gastrointestinal issues.
Lactating mares, especially if under-fed for extended periods, and heavily exercising horses are most likely to experience low levels of circulating magnesium (hypomagnesemia).
Low magnesium status often occurs alongside low levels of other electrolytes, including potassium and chloride. This can occur because of low food intake, imbalanced electrolyte infusion or supplementation, and increased loss of electrolytes in the gut. 
Signs of Deficiency
In horses, severe magnesium deficiency can result in: 
- Irregular heart beat (ventricular arrhythmias)
- Muscle tremors
- Poor coordination
- Abnormal calcium deposits in soft tissues
In other species, additional symptoms have been observed, including: 
- High blood pressure
- Platelet aggregation
- Poor respiratory function
- Restlessness and disorientation
- Reduced gut motility (transit of feed through the gastrointestinal tract)
Assessing your Horse’s Magnesium Status
Blood tests of magnesium levels do not accurately measure the whole-body magnesium status in healthy horses.
Only 1% of total body magnesium is in the extracellular fluid (such as plasma), which can be measured with a blood test. Another 50% of magnesium is found within cells, and the remainder is in bone.
This distribution makes it difficult to access the main sites of magnesium storage to determine whether a horse is deficient.
The best method for evaluating magnesium status in horses is via a urine test that measures fractional clearance of magnesium. 
Historically, high doses of intravenous magnesium sulfate were used to calm horses prior to competition. This practice is now banned by the Federation Equestre Internationale (FEI).
Although the precise mechanism is not well understood, high dose magnesium may cause a calming effect by interfering with stimulatory receptors in the central nervous system or by lowering blood pressure. 
Studies on the calming effects of oral magnesium have yielded inconsistent results. In two studies supplementing magnesium aspartate, one showed a decreased reaction time but the other study showed no effect. 
Intravenous infusions have also been shown to decrease symptoms of severe asthma by altering the horse’s breathing patterns. However, this method is not superior to medications such as salbutamol for the treatment of severe asthma in horses. 
Oral magnesium supplementation has been shown to be effective for a variety of situations including:
- Treatment of trigeminal-mediated neuritis (head shaking syndrome) 
- Antacid effect in the stomach 
- Improved glucose tolerance in some horses with equine metabolic syndrome (EMS) 
Types of Magnesium Supplements
Magnesium supplements are available in several chemical forms. The most common ones supplemented in equine diets are:
- Magnesium oxide
- Magnesium sulfate
- Magnesium chloride
- Magnesium aspartate
- Magnesium carbonate
- Magnesium citrate
How to Choose the Best Supplement for your Horse
- Magnesium aspartate
- Magnesium carbonate
- Magnesium citrate
How to Choose the Best Supplement for your Horse
There are several factors to consider when deciding which magnesium supplement to feed your horse. Most importantly, consider the bioavailability and concentration of magnesium in each product.
Less concentrated sources will need to be fed at higher levels to supply the same amount of elemental magnesium. This has cost and palatability implications.
Table 1: Bioavailability, concentration and dosage needed to provide 20 grams of elemental magnesium
|Supplement||Bioavailability||Concentration||Amount for 20 g magnesium|
|Magnesium oxide||70%*||56%||36 g|
|Magnesium sulfate||70%*||20%||100 g|
|Magnesium chloride||50%||12%||167 g|
|Magnesium aspartate||10%||10%||200 g||Magnesium lactate||10%||12%||167 g|
|Magnesium carbonate||72%*||45%||44 g|
|Magnesium citrate||45%||16%||125 g|
Bioavailability measures the percentage of a given substance absorbed into systemic circulation and made available in the body.
Only magnesium oxide, magnesium sulfate and magnesium carbonate have had their bioavailability studied in horses. As shown in Table 1, all three forms have a similar oral bioavailability of 70%. 
The bioavailability of other magnesium compounds is unknown in horses. The above values should be interpreted with caution as they are derived from studies on humans and rodents.
Concentration measures how much elemental magnesium is present within a given serving size.
Magnesium oxide has the highest concentration of elemental magnesium per serving, meaning you can feed much less of this product than others.
You only need to feed 36 grams of magnesium oxide to yield 20 grams of elemental magnesium. In comparison, you would need to feed 125 grams of the less-concentrated magnesium citrate to yield the same dosage.
Powdered magnesium supplements tend to have low palatability when fed on their own. The less you need to feed, the better. Products with lower feeding rates also tend to cost less per day.
For these reasons, magnesium oxide is usually the best choice in terms of cost-effectiveness and your horse’s likelihood of consuming enough.
Benefits of Other Forms
If you are looking to address a specific concern, other sources of magnesium may be more appropriate.
For example, magnesium sulfate (epsom salt) is known to have a laxative effect.  If your horse is dealing with impaction colic, your vet may recommend giving magnesium sulfate either orally or by intravenous infusion.
Magnesium sulfate should not be fed on an ongoing basis and should only be provided under veterinary supervision to minimize gastrointestinal side effects.
There is evidence that magnesium citrate improves headshaking syndrome in horses.  This may be due in part to the citrate component of this molecule and other citrate complexes may be similarly effective.
Most magnesium is absorbed in the small intestine, although some absorption also occurs in the large intestine. 
Absorption occurs through the spaces between cells lining the digestive tract (paracellular route) and through active transport by the cells (transcellular route). 
Supplemental sources have a higher absorption rate (around 70%) than plant sources. Of plant sources, magnesium digestibility is reported to be higher with alfalfa hay (51%) compared to concentrates (31%). 
In general, magnesium absorption is dose-dependent. This means that the more magnesium in the digestive tract, the more will be absorbed. 
Interactions with Other Nutrients
- Fermentible fibres (ie. hemicellulose)
- Non-fermentible fibres (ie. lignin)
The extent to which these interfere with magnesium absorption will depend on their level in the diet.
For example, ponies fed excess phosphorus (0.125 g phosphorus per kg bodyweight compared to recommended the 0.028 g phosphorus) had a magnesium digestibility of 41 – 45%.
The NRC uses a digestibility estimate of 40% to calculate requirements. Therefore, even such high phosphorus levels in the diet are unlikely to result in a magnesium deficiency. 
In other species, the following factors have been shown to increase magnesium absorption: 
- Medium-chain triglycerides
- Indigestible carbohydrates (ie. oligosaccharides, inulin)
Consult with your equine nutritionist to determine whether your horse needs a magnesium supplement. A nutritionist can also help you decide which form of magnesium is most appropriate and how much to feed based on the composition of your horse’s diet.
You can submit your horse’s information online for a free diet analysis by a qualified nutritionist.
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- National Research Council Minerals. NRC. 2007.
- Reid, R.L. and Horvath, D.J. Soil chemistry and mineral problems in farm livestock. A review. Animal Feed Sci Tech. 1980.
- Stewart, A.J. et al. Validation of diagnostic tests for determination of magnesium status in horses with reduced magnesium intake. Am J Vet Res. 2004.
- Garcia-Lopez, J.M. et al. Prevalence and prognostic importance of hypomagnesemia and hypocalcemia in horses that have colic surgery. Am J Vet Res. 2001.
- Stephen, J.O. et al. Small Intestinal Volvulus in 115 Horses: 1988–2000. Vet Surg. 2004.
- Borer, K.E. and Corley, K.T.T. Electrolyte disorders in horses with colic. Part 1: potassium and magnesium. Equine Vet Educ. 2006.
- Stewart, A.J. Magnesium disorders in horses. Vet Clin North Am Equine Pract. 2011.
- Schumacher, S.A. et al. Pharmacokinetics of magnesium and its effects on clinical variables following experimentally induced hypermagnesemia. J Vet Pharmacol Ther. 2020.
- Dodd, J.A. et al. Magnesium aspartate supplementation and reaction speed response in horses. J Equine Vet Sci. 2015.
- Dodd, J.A. et al. Does oral magnesium aspartate supplementation affect reaction speed in horses of different breeds? ICEEP. 2018.
- Tanquerel, L. et al. Effects of magnesium sulfate infusion on clinical signs and lung function of horses with severe asthma. Am J Vet Res. 2018.
- Sheldon, S.A. et al. Effects of magnesium with or without boron on headshaking behavior in horses with trigeminal-mediated headshaking. J Vet Intern Med. 2019.
- Clark, C.K. et al. Effect of aluminum hydroxide/magnesium hydroxide antacid and bismuth subsalicylate on gastric pH in horses. J Am Vet Med Assoc. 1996.
- Winter, J.C. et al. Oral supplementation of magnesium aspartate hydrochloride in horses with Equine Metabolic Syndrome. Pferdeheilkunde. 2016.
- Harrington, D.D. and Walsh, J.J. Equine magnesium supplements: Evaluation of magnesium oxide, magnesium sulphate and magnesium carbonate in foals fed purified diets. Equine Vet J. 1980.
- Guerrera, M.P. et al. Therapeutic Uses of Magnesium. AFP Journal. 2009.
- Stewart, A.J. Magnesium homeostasis and derangements. Equine Fluid Therapy. 2014.
- Blancquaert, L. et al. Predicting and Testing Bioavailability of Magnesium Supplements. Nutrients. 2019.
- Alonso, J.M. et al. Effects of magnesium sulfate and dioctyl sodium sulfosuccinate on fecal hydration, output and systemic hydration in healthy horses. Arq Bras Med Vet Zootec. 2020.
- Schuchardt, J.P. and Hahn, A. Intestinal Absorption and Factors Influencing Bioavailability of Magnesium-An Update. Curr Nutr Food Sci. 2017.
- Ates, M. et al. Dose-Dependent Absorption Profile of Different Magnesium Compounds. Biol Trace Elem Res. 2019.