Magnesium for Horses: Deficiency, Excess & Balance

Why Magnesium Matters in Horse Nutrition

Magnesium is one of the most important minerals in your horse's diet, yet it rarely gets the attention it deserves. It plays a role in over 300 enzymatic reactions in the body, influencing everything from muscle function and nerve transmission to bone density and energy metabolism.

Despite its importance, magnesium is frequently either overlooked or over-supplemented. Both scenarios can cause real problems for your horse. In this article, we'll take a thorough look at what magnesium does, how to recognise deficiency and excess, and — most importantly — how to get the balance right.

What Does Magnesium Do in a Horse's Body?

Magnesium is the second most abundant intracellular cation in the horse's body. That's a fancy way of saying there's a lot of it inside your horse's cells, and it's doing critical work. Here's a summary of its key roles:

Muscle Function

Magnesium helps muscles relax after contraction. Without adequate magnesium, muscles can become twitchy, tight, or prone to cramping. This is why magnesium is so commonly associated with calming products — tense muscles often contribute to a horse that feels "on edge."

Nerve Transmission

Magnesium acts as a natural regulator of nerve impulses. It modulates the release of neurotransmitters and helps prevent excessive nerve excitability. A horse that is genuinely low in magnesium may appear more reactive, spooky, or anxious than usual.

Bone Health

Approximately 60% of the magnesium in your horse's body is stored in bone. It works alongside calcium and phosphorus to maintain skeletal integrity. Without enough magnesium, bone mineralisation can be compromised.

Energy Metabolism

Magnesium is essential for the production and utilisation of ATP — the energy currency of cells. Horses in hard work have higher magnesium demands because their muscles are burning through more energy.

Insulin Regulation

Research has highlighted a connection between magnesium status and insulin sensitivity. Horses with metabolic conditions such as Equine Metabolic Syndrome (EMS) or insulin dysregulation may benefit from adequate magnesium levels, though supplementation should always be guided by dietary analysis rather than guesswork.

How Much Magnesium Does a Horse Need?

The National Research Council (NRC, 2007) provides general guidelines for magnesium requirements in horses:

• Maintenance (500 kg horse): approximately 7.5–10 g per day
• Moderate work: approximately 11–13 g per day
• Heavy work: approximately 13–15 g per day
• Lactating mares: approximately 11–15 g per day, depending on stage of lactation
• Growing horses: requirements vary by age, growth rate, and body weight

These are estimates, and individual requirements can vary based on forage quality, breed, metabolism, sweat losses, and overall diet composition.

Most of a horse's magnesium intake comes from forage — grass and hay. Grass pasture typically contains between 0.15% and 0.35% magnesium on a dry matter basis, though this can vary enormously depending on soil type, season, and fertilisation practices. Hay values are equally variable.

Signs of Magnesium Deficiency in Horses

True clinical magnesium deficiency (hypomagnesaemia) is relatively uncommon in horses compared to cattle, but subclinical or marginal deficiency is thought to be more widespread than many owners realise. Here are the signs to watch for:

Muscular Signs

• Muscle tension, stiffness, or tightness
• Unexplained muscle tremors or twitching
• Reluctance to move freely or shortened stride
• Poor recovery after exercise

Behavioural Signs

• Increased anxiety, spookiness, or reactivity
• Difficulty relaxing or settling
• Hypersensitivity to touch or sound

Other Signs

• Poor coat quality
• Irregular heartbeat (in severe cases)
• Reduced performance
• In extreme cases, tetany — a serious condition involving sustained muscle contraction

A Word of Caution

Many of these symptoms overlap with dozens of other conditions. Anxiety could be caused by pain, ulcers, poor saddle fit, training issues, or hormonal changes. Muscle stiffness might indicate tying-up, PSSM, or simple overwork. Never assume magnesium deficiency based on symptoms alone. Blood testing for magnesium has its limitations too — serum magnesium only reflects about 1% of total body magnesium, so a "normal" blood result doesn't necessarily mean your horse's magnesium status is adequate.

The most reliable approach is to analyse your horse's full diet to see whether daily magnesium intake actually meets requirements.

What Causes Magnesium Deficiency?

Several factors can contribute to inadequate magnesium levels:

Low Magnesium in Forage

Soils deficient in magnesium will produce forage that is low in this mineral. Sandy soils and heavily leached soils are particularly prone to low magnesium content. Additionally, high potassium fertilisation of pastures can interfere with magnesium uptake by plants.

High Potassium Intake

Excess potassium in the diet — common in lush spring and autumn grass — can inhibit magnesium absorption in the horse's gut. This is one reason why grass-affected behaviour often coincides with low magnesium status.

Excessive Calcium Intake

High calcium-to-magnesium ratios in the diet can also reduce magnesium absorption. Lucerne (alfalfa) is high in calcium, and diets heavily based on lucerne without balancing minerals can skew this ratio.

Increased Demand

Horses in heavy work, lactating mares, and young growing horses all have elevated magnesium requirements. Sweat contains magnesium, so horses that sweat heavily — whether from exercise or hot climates — lose more than sedentary horses.

Stress

Physiological and psychological stress can increase magnesium excretion. Horses dealing with travel stress, competition, herd changes, or illness may have higher than normal requirements.

Magnesium Excess: Can You Give Too Much?

Yes, you absolutely can — and this is an underappreciated risk. Because magnesium is widely marketed as a calming supplement, many horse owners add it liberally without checking whether their horse actually needs it.

What Happens with Too Much Magnesium?

• Loose droppings or diarrhoea: Magnesium has an osmotic effect in the gut. Excess magnesium oxide, in particular, can draw water into the intestine and cause soft or watery faeces.
• Reduced calcium and phosphorus absorption: Excessive magnesium can interfere with the absorption of other important minerals, potentially creating secondary deficiencies.
• Lethargy or dullness: Ironically, while a moderate amount of magnesium may help a genuinely deficient horse feel calmer, too much can cause an over-sedated, flat, or lethargic demeanour.
• Urinary issues: The kidneys excrete excess magnesium, but chronic over-supplementation puts unnecessary strain on renal function.

The Upper Safe Limit

The NRC suggests that the maximum tolerable concentration of magnesium in the total diet is around 0.8% of dry matter. For a 500 kg horse eating approximately 10 kg of dry matter per day, that translates to roughly 80 g — well above typical requirements. However, problems can occur at levels well below this theoretical maximum, especially when combined with mineral imbalances.

Common Magnesium Supplements for Horses

If dietary analysis reveals a genuine magnesium shortfall, supplementation may be appropriate. Here are the most common forms:

Magnesium Oxide (MgO)

The most widely used equine magnesium supplement. It has a high magnesium content by weight (around 60%) and is relatively inexpensive. However, its bioavailability is moderate, and in high doses it can cause loose droppings.

Magnesium Carbonate

Another commonly used form with reasonable bioavailability. It's gentler on the gut than magnesium oxide for some horses.

Magnesium Sulphate (Epsom Salts)

Often used as a short-term laxative rather than a long-term supplement. It has poor palatability and is not ideal for ongoing magnesium supplementation.

Organic Forms (Magnesium Chelate, Citrate, Aspartate)

These are generally considered more bioavailable than inorganic forms. They are also typically more expensive. Magnesium citrate and magnesium aspartate are popular choices for horses that don't tolerate magnesium oxide well.

Which Form Is Best?

For most horses, magnesium oxide is perfectly adequate and cost-effective. If your horse develops loose droppings or refuses to eat it, switching to an organic form may help. The "best" supplement is the one that meets your horse's actual requirement without causing adverse effects.

Getting the Balance Right

Magnesium doesn't work in isolation. Its absorption and function are influenced by its relationship with other minerals, particularly:

Calcium-to-Magnesium Ratio

The ideal calcium-to-magnesium ratio in the horse's diet is generally considered to be between 2:1 and 3:1. Diets that stray significantly outside this range can impair the absorption and utilisation of both minerals.

Potassium

As mentioned, high potassium competes with magnesium for absorption. Spring and autumn pastures in the UK, Ireland, and many temperate climates are notoriously high in potassium, which can push horses into marginal magnesium status even when total dietary magnesium appears adequate on paper.

Phosphorus

Magnesium and phosphorus interact in bone metabolism. An imbalance between these minerals can affect skeletal health, particularly in growing horses.

The Takeaway

Supplementing magnesium without understanding the broader mineral profile of your horse's diet is a bit like adjusting one instrument in an orchestra without listening to the whole ensemble. You might fix one problem and create another.

This is exactly why a full dietary analysis is so valuable. Rather than guessing, you can see precisely where the gaps and excesses lie.

Practical Tips for Horse Owners

1. Get your forage tested. Knowing the mineral content of your hay or haylage is the single most useful step you can take. It removes the guesswork.

1. Don't supplement blindly. Magnesium is not a harmless "just in case" supplement. Only add it if there's a demonstrated need.

1. Consider the whole diet. Hard feed, balancers, and other supplements may already contain magnesium. Add it all up before reaching for another tub.

1. Watch the ratios. Pay attention to calcium, potassium, and phosphorus alongside magnesium.

1. Monitor your horse's response. If you start supplementing magnesium and see loose droppings, lethargy, or no improvement in symptoms, reassess.

1. Seek professional advice. If your horse has a metabolic condition or complex health issue, work with your vet and an equine nutritionist.

Frequently Asked Questions

Does magnesium really calm horses?

If a horse is genuinely deficient in magnesium, correcting that deficiency can reduce muscle tension and nervous irritability — which may present as a "calmer" horse. However, magnesium is not a sedative and will not calm a horse that is anxious for other reasons (pain, training, environment).

Can I test my horse's magnesium levels?

Serum magnesium testing is available but has significant limitations. It reflects only a tiny fraction of total body magnesium. Red blood cell (RBC) magnesium testing is considered slightly more useful but is not widely available in equine practice. Dietary analysis remains the most practical tool.

How quickly does magnesium supplementation work?

If your horse is genuinely deficient, you may see changes within a few days to two weeks. If you see no improvement after three to four weeks of appropriate supplementation, magnesium was likely not the issue.

Is it safe to feed magnesium long-term?

At appropriate levels — yes. The key word is "appropriate." Long-term over-supplementation is not advisable.

Conclusion

Magnesium is a vital mineral that supports your horse's muscles, nerves, bones, and metabolism. Deficiency can cause real problems, but so can excess. The path to getting it right is straightforward: understand your forage, calculate your horse's total daily intake, and supplement only when there's a genuine gap.

Avoid the temptation to add magnesium (or any mineral) based on symptoms alone. Instead, take the time to build an accurate picture of what your horse is actually eating — and what's missing. That's the foundation of good nutrition.