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

Dietary requirements for magnesium and other vitamins and minerals are established the National Research Council’s (NRC) Nutrient 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: [1]

  • 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
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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%. [1]

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.

Magnesium Content

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) [2]

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. [1]

Macromineral Ratios

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. [4][5]

It remains unclear whether these deficits affect post-surgical survival or whether magnesium supplementation is warranted in these cases. [6]

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).

Hypomagnesemia is also seen in synchronous diaphragmatic flutter (thumps) and blister beetle toxicosis. [7]

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. [6]

Signs of Deficiency

In horses, severe magnesium deficiency can result in: [1]

  • Irregular heart beat (ventricular arrhythmias)
  • Muscle tremors
  • Poor coordination
  • Seizures
  • Abnormal calcium deposits in soft tissues

In other species, additional symptoms have been observed, including: [6]

  • 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. [3]

Magnesium Supplementation

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. [8]

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. [9][10]

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. [11]

Oral magnesium supplementation has been shown to be effective for a variety of situations including:

Types of Magnesium Supplements

Magnesium supplements are available in several chemical forms. The most common ones supplemented in equine diets are:

Inorganic sources:

Organic sources:

  • 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