Feeding Magnesium Oxide

We are often contacted by customers asking what the difference is between Magnesium Oxide and 90% magnesium oxide/calmag. We hope this helps:

The magnesium oxide we sell is pharmaceutical grade and is 99.2% pure. As it is a fine powder, it is more easily absorbed than the 90% magnesium oxide. Magnesium oxide is one of the most easily absorbed forms of magnesium, and is £14.99 for 900g. Due to the higher purity, only 2g per 100kg bodyweight is required.

As a second option, we now sell 90% Magnesium oxide, which is a high purity calmag. It cost £5.99 for 900g and should be fed at 3g per 100kg bodyweight, so is a cheaper option.

They are both a source of magnesium oxide, but vary in their purity, from 90% magnesium oxide to 99.2% magnesium oxide. This gives a differance in costs, with the daily cost being higher, as you use the higher purity.

For example, for a 500kg horse:

90% magnesium oxide fed at 3g per 100kg, would be 15g daily, which costs 9.9p daily

Magnesium oxide (99.2%) fed at 2g per 100kg, would be 10g daily, which costs 14.4p daily


The diet of many horses is low in magnesium due to grazing land been grazed by horses for many years without fertilisers being added. Supplementing magnesium can also help break down fatty deposits.

Magnesum is the main ingredient in most commercial horse calmers. Lack of magnesium can cause nervous and excitable behaviour. In these cases, supplementing magnesium can have a calming effect. However, magnesium is NOT a sedative. horses can show nervous/excitable behaviour for many different reasons including breed, age, exercise, turnout, training, sex, feed, etc. However, IF a lack of magnesium is having an effect on their behaviour, a magnesium supplement can have a dramatic effect.

Magnesium is an important nutrient and can help the growth of strong hooves. It has also been shown to help reduce the build-up of fatty deposits.