Magnesium in horses is often only associated with nervousness (calming effect) and tension (muscle relaxation).
In reality, magnesium is a vital macromineral involved in over 300 metabolic processes.

Magnesium is essential for the function of many enzymes, primarily in muscle and nerve tissues. It plays a crucial role in energy production and muscle tone and is involved in the formation of bone and tendon tissue. It also significantly contributes to calming the nervous system by inhibiting nerve excitation—thereby helping reduce stress levels.

A magnesium deficiency can lead to a wide range of symptoms, including not only muscular tension and nervousness but also colic, anxiety, cardiac issues, and labored breathing.

It’s well worth paying closer attention to this mineral—understanding both your horse’s needs and the possible signs of magnesium deficiency.

Top Questions in Advance

Magnesium needs – rarely covered by forage alone

The typical magnesium requirement of a horse is about 10–13 g/day. However, in many situations, demand increases significantly to 18–20 g/day—for example in lactating mares, sport horses under heavy work, and growing or young horses in training. The requirement also rises with age, as older horses can no longer utilize magnesium from feed as efficiently.

Magnesium content in hay

High-quality hay may contain up to 1.7 g of magnesium per kg. On average, however, it contains only around 0.8 g/kg. The actual amount depends on the quality and must be determined individually through forage analysis.

At a daily hay intake of 12 kg, that translates to around 9.6 g magnesium. With better hay, perhaps 12 g. Only top-quality hay would provide as much as 20.4 g/kg.

This means that average hay just about covers the magnesium requirement—but only for horses at rest and without additional stress.

Why magnesium is so important

Magnesium is not a “nice-to-have” – it is a key structural and regulatory mineral in the horse’s metabolism. As an essential mineral, it is indispensable for numerous vital functions. It ensures that:

• Enzymes can utilize ATP to provide energy within cells.

ATP (adenosine triphosphate), often referred to as the “energy currency” of the cell, stores energy in its phosphate bonds. However, to activate ATP for muscle contraction, cellular building, or nerve impulse transmission, it must bind to a magnesium ion (Mg²⁺).

• Muscles and nerves function smoothly thanks to its role as a natural calcium antagonist.

In simple terms: calcium triggers muscle contraction and nerve stimulation. Magnesium counters these effects, enabling relaxation and calmness to follow.

• Bones and cartilage remain healthy, as magnesium supports their structure, stability, and shock-absorbing function. 

Around 60% of magnesium in the body is stored in the bones.

• Stress reactions are regulated, as magnesium modulates cortisol release and reduces nerve cell overactivity.

Symptoms – How magnesium deficiency manifests in horses

Because magnesium is involved in so many bodily functions, its deficiency can cause a wide array of symptoms.

Most common signs

The most common signs include unwillingness to work, fatigue, and a general decrease in performance, especially in sport horses. Tense muscles, muscle tremors, or even cramps indicate a disturbed neuromuscular signal transmission—magnesium is essential here to ensure proper muscle relaxation. Swollen legs may point to circulatory disorders, as magnesium also regulates vascular tone.

Nerves and organs

A deficiency may also cause nervousness, irritability, and lack of composure—particularly in young, sensitive, or stressed horses. Irregular heartbeat and labored breathing (heaves) may occur and often improve quickly with targeted magnesium supplementation.

Metabolism

Magnesium also plays an important role in metabolism, which makes it relevant for horses with EMS, Cushing’s, or laminitis: it improves insulin sensitivity, stabilizes blood sugar levels, and can help modulate inflammatory processes.

Digestion

Other unexplained symptoms

Even skin issues, like in horses with sweet itch, can worsen with magnesium deficiency—often accompanied by increased zinc demand.

The broad range of symptoms shows that magnesium deficiency doesn't just impact individual organs but can disrupt the entire system.

That’s why it’s crucial to keep an eye on magnesium—especially when multiple symptoms occur at once.

Increased magnesium demand in specific life stages

The magnesium requirement of a horse is not static, but increases significantly during certain life stages or under specific stressors. This is particularly evident in sport horses, which lose magnesium continuously through muscle activity and increased sweating – as sweat contains considerable amounts of electrolytes, including magnesium.

Less well known, however, is the fact that stress situations such as changes of stable, transportation, social hierarchy conflicts, or even foaling in mares massively increase the magnesium requirement. During stress, the body releases increased amounts of adrenaline and noradrenaline – these stress hormones promote intensified fat metabolism (lipolysis). This process produces free fatty acids, which irreversibly bind to magnesium ions. The magnesium that becomes bound in this way is no longer available to the organism for its actual functions – such as muscle and nerve control – and is ultimately excreted.

Young horses in growth, horses in training (e.g., during the backing process), heavily worked competition horses, and pregnant or lactating mares also have an increased requirement, since cell division, muscle development, or hormonal changes are particularly active in these cases – all of which require magnesium-dependent enzyme systems.

The magnesium requirement of older, weather-sensitive horses should also not be underestimated: as horses age, their ability to absorb magnesium from feed decreases – especially when dental condition is poor, digestion is impaired, or liver and kidney function is suboptimal. Studies indicate that older horses are often latently under-supplied, even when the ration appears “sufficient” on paper.

Conclusion: Whether due to sport, stress, pregnancy, or age – there are numerous situations in which the magnesium requirement exceeds the normal level. An adjusted intake is crucial in such phases to prevent deficiency symptoms and their consequences such as muscle problems, nervousness, or metabolic disturbances.

Interaction with Other Nutrients – The Interplay Within the Mineral Balance

The importance of magnesium in the horse’s mineral balance goes far beyond its individual function. Especially in combination with other minerals like calcium and phosphorus, it becomes clear how essential a balanced diet is for the horse’s overall health.

Magnesium and calcium are closely interconnected: while calcium is responsible for muscle contraction and bone strength, magnesium enables muscle relaxation and regulates nerve function. A deficiency in magnesium can therefore not only lead to muscle issues but also disrupt the calcium and phosphorus balance.

For both bone stability and muscular performance, the correct ratio of these minerals is critical. An excess of calcium – for example, due to an unbalanced diet – can further increase the horse’s magnesium requirement and thus raise the risk of magnesium deficiency. Vitamin D also plays an important role, as it regulates the absorption of calcium and phosphorus in the body and thereby indirectly influences magnesium needs.

A balanced diet that supplies all essential minerals and vitamins in the right quantities and proper ratio is therefore key to meeting the horse’s magnesium requirements. Only under these conditions can magnesium’s many metabolic functions be optimally supported and deficiency symptoms be prevented. Horse owners should not only consider the magnesium content of the feed, but also monitor the overall mineral supply to ensure long-term health and performance of their horse.

Preventing Magnesium Deficiency – The Right Dosage

Ideally, you should have current laboratory analyses of your hay. This allows you to easily calculate how much magnesium is already being supplied through forage and then supplement the remainder with a high-quality mineral feed or a standalone magnesium supplement.

Without a forage analysis, you’ll need to estimate the magnesium content in the hay. To be on the safe side, assume slightly lower values—mild over-supplementation with magnesium is not harmful to horses. With average-quality hay, you can expect around 10 g/day of magnesium from forage. Additional magnesium may come from concentrate feed (e.g., oats contain about 1.3 g of magnesium per kg). Muesli or other compound feeds should include information on magnesium content.

Whether your horse actually had a magnesium deficiency can be identified quite easily. With a good magnesium supplement, you’ll often notice positive effects within just a few days.

More on This Topic

On our topic page Feeding Horses with Gastric Issues, you’ll find plenty of additional information about proper equine nutrition.

Horses with gastric problems need extra support. It’s important to know which types of feed are suitable for your horse – and which ones are better avoided.