The science is clear - magnesium deficiency is very rare. There is really only one circumstance when we may add TINY amounts of magnesium.

That is if there is a flush of usually Spring grass on rich, well fertilised pasture. This grass is often VERY high in potassium. Though there will be plenty of magnesium in the grass the horse has to pee a lot to get rid of the potassium. Because it struggles to control magnesium it can lose too much. This is an affect that rarely lasts more than a few days and we would normally add just 0.1 - 0.4 grams of magnesium a day.

Important note. If this works for you it doesn't mean your horse will always benefit from added magnesium. If you keep adding magnesium the horse will overdose. So only use magnesium on fresh pastures for a few days at a time. Once the grass dries off you should not add magnesium at all!

This so rare in Australia that we don't even sell a magnesium supplement here.