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 tran