Commercial feeds might contain supplemental Mg, such as magnesium oxide or magnesium sulfate, because some factors (such as other minerals) have the potential to negatively affect how much Mg is available for absorption. | Photo: Erica Larson/The Horse

Magnesium (Mg) is a macromineral, meaning a horse requires larger quantities (in excess of a gram per day) compared to trace (or micro-) minerals, which they need in minute amounts.

Many of the historical medicinal references about Mg circle back to the “mineral water well” a a cattle farmer in Epsom, England, discovered around 1618. The mineral water contained large amounts of magnesium sulfate, giving it a bitter taste, and it became famous for alleviating or even curing an array of ailments. Later, many extracts or preparations from this water (including Epsom salts) were manufactured and sold.

Although scientists knew Mg was present in living organisms as early as 1880, chemist Richard Willstätter discovered that Mg played a functional role in chlorophyll in plants (previously researchers believe Mg to have a mere nonfunctional presence). This discovery, together with some of his other work on plant structures, led to a Nobel Prize in 1915