Understanding Magnesium Purity for Horses

Magnesium is one of the most consistently deficient minerals we see in forage. In fact, nearly 100% of the grass, hay, and haylage samples we analyse are deficient in magnesium for the adult horse.

Despite this, magnesium is absolutely critical for:

• Muscle function and relaxation
• Nerve signalling
• Energy metabolism
• Supporting calm behaviour and resilience to stress

Because of this, many horse owners turn to magnesium supplements. However, once you start looking at different products, terms like purity, elemental composition, and trace elements can quickly become confusing.

This guide explains what these terms really mean, and more importantly, how to choose the right form of magnesium for your horse

Purity of Magnesium for Horses

Purity, also referred to as assay or potency, is the simplest concept to understand. It tells you how much of the product is actually the mineral you want. For example:

• 100g of a product that is 50% pure contains 50g of magnesium and 50g of impurities
• 100g of a product that is 99% pure contains 99g of magnesium and only 1g of impurities

This sounds straightforward, but purity alone does not tell you how much usable magnesium your horse is actually receiving.

Elemental Magnesium – What Really Matters

This is where things become more important. Magnesium is never fed as a pure element; it is always attached to something else (oxygen, chloride, citrate, etc.). This affects how much actual magnesium is delivered per gram.

Example: Magnesium Oxide (MgO)

• Magnesium (Mg) = 24.3
• Oxygen (O) = 16
• Total molecular weight = 40.3

Elemental magnesium content: 24.3 ÷ 40.3 = 60.3% elemental magnesium. This means: 100g of magnesium oxide provides 60g of actual magnesium.

Why This Matters

To deliver the same amount of magnesium, you would need 3–5 times more citrate or glycinate. You may need up to 5 times more chloride. This creates a very practical problem. Horses simply won’t eat large enough quantities of these lower-density powders.

This is why, from a formulation and feeding perspective, Magnesium Oxide is the most effective and practical choice for horses. It delivers high levels of magnesium in small volumes. It’s easier to include in daily bucket feed. You get a more consistent intake.

Trace Elements and Hidden Impurities

Even when a product is labelled “99% pure”, the remaining 1% matters. These impurities are often measured in parts per million (ppm): 1 ppm = 0.0001%

That sounds insignificant, but a 0.1% impurity = 1000 ppm. This can represent very high levels of unwanted minerals, particularly iron. Excess iron:

• Interferes with copper and zinc absorption
• Contributes to metabolic imbalance
• Adds unnecessary burden to the horse

In lower-quality magnesium sources, we often see:

• Elevated iron levels
• Heavy metal contamination
• Poor refinement processes

Why Purity of Magnesium Source Matters

Not all magnesium oxide is created equal, and this is an area that is often overlooked. A high-quality magnesium source should deliver a consistently high level of elemental magnesium, typically around 60%, while also maintaining very low levels of trace contaminants.

Just as importantly, it should be supported by a verified Certificate of Analysis (CoA) so you can clearly see what you are feeding.

In contrast, cheaper magnesium products are usually driven by cost rather than quality. This often leads to less refined materials, higher levels of heavy metals such as iron, and a product that can negatively impact the overall mineral balance of the diet.

What may seem like a saving at first can quickly become a problem when unwanted contaminants are introduced.

Magnesium in Forageplus Balancers

At Forageplus, magnesium is never added as a generic ingredient or included without purpose. Every formulation is based on real data from thousands of forage analyses carried out each year, allowing us to identify consistent deficiencies and imbalances in UK forage.

From this foundation, we adjust formulations to reflect the individual needs of the horse, taking into account age, workload, forage type, and metabolic demand. This ensures magnesium is supplied at the correct and effective level, rather than being under- or over-supplied.

We also place strong emphasis on ingredient quality. We use high elemental magnesium sources, apply strict control over impurity levels, and ensure magnesium is correctly balanced alongside copper, zinc, and other key minerals to support optimal utilisation.

The result is a balancer that provides your horse with the right amount of magnesium, in a form they can realistically consume, without introducing unnecessary contaminants or disrupting mineral balance.

Not Sure If Your Horse Is Getting Enough Magnesium?

Nearly all forage is deficient for the adult horse, but every horse is different.

The most accurate way to understand your horse’s mineral intake is through forage analysis, or with a forage-focused mineral balancer that is matched to the common deficiencies found within grass, hay and haylage.

At Forageplus, we base every recommendation on real data from thousands of forage samples, helping you avoid guesswork and provide targeted, effective nutrition.

Learn more about minerals horses by exploring our various horse mineral-related articles here.