Magnesium is a macromineral, meaning it is required by the horse in grams per day. Microminerals such as copper and zinc are required in milligrams per day. 60% of the magnesium in the horse’s body is found in the bones and 32% in the muscles. Magnesium is also found inside cells, where it has a role in energy metabolism, protein synthesis and other metabolic processes.
Supplementary magnesium on top of dietary requirements is used in horses for brain function (as a calmative) and for hoof health, potentially via a metabolic route, or due to the pro-inflammatory state associated with magnesium depletion.
Currently there is no scientific evidence to support these therapeutic uses of magnesium in horses, but hopefully these uses will be investigated properly in the future.
Forages are relatively rich in magnesium for horses, and grains less so. Most forages will supply above requirements for most horses, but there may be circumstances where supplementation is warranted.
Several different magnesium compounds are available for supplementation, and all are different in their content of magnesium and how well they are absorbed. Magnesium oxide – one of the most commonly used – is not particularly available but is 50% magnesium (10 g compound gives 5 g magnesium), so is still a valid source. Magnesium carbonate is about 31% (10 g compounds gives 3.1 g), magnesium phosphate is 24% magnesium (10 g compound gives 2.4 g), magnesium chloride is about 12% magnesium (10g of compound gives 1.2 g magnesium), and magnesium aspartate is about 8% magnesium (10 g compound gives 0.8 g). Requirements for magnesium take into account poor absorption, so using a poorly absorbed magnesium salt can still be a useful option, providing enough is fed.
Magnesium sulphate – aka epsom salts – is not a particularly useful source of dietary magnesium because it supplies so little at about 9% (10 g gives 0.9 g) and because it acts as a laxative due to its action of drawing water into the gut.
Magnesium for horses can also be supplemented in the form of a chelation, which is a compound made up of the mineral bonded to amino-acids and/or peptides. Amino acids are the building blocks of protein, and peptides are short chains of amino acids. Chelated magnesium is more like the type of magnesium compounds found naturally in feedstuffs, and is more bioavailable than the magnesium salts. Feeding chelates also avoids any detrimental interactions between dietary minerals. Chelated minerals are much more expensive than inorganic forms (the salts) and a combination of the two tends to be used in feed and supplement products.
Further research is required to understand how different sources of magnesium function in the horse, and whether or not there are clear benefits from using one source rather than another. Until then, using a combination of inorganic magnesium salts and chelated magnesium would seem to be a fair compromise.