Magnesium oxide. It’s one of those ingredients people love to disagree over, should we use it, shouldn’t we use it, is it bioavailable or not for horses??
Part of this contention is due to the fact that magnesium oxide is a created ingredient… it is cooked in a process called calcination, that reacts magnesite with oxygen to create magnesium oxide.
Like all things, it can be made well, or it can be made poorly, and this then impacts on whether it will be bioavailable or not. So it’s not like limestone (calcium carbonate) that is mined for example and is generally always the same.
So when reading about magnesium oxide for horses, keep in mind that most studies do not report the conditions under which it was manufactured OR its final particle size, both of which have a huge impact on final bioavailability.
I still believe that quality magnesium oxide is a very useful source of magnesium for horses, but then I am very fussy when it comes to where it comes from and how it is produced!
We have information on various sources of magnesium for anyone who would like to read some more here: https://feedxl.com/36-which-form-of-magnesium-is-best/
And for anyone who loves details, this is a now old but still informative paper on this subject (Beede et al 1992). It is a Dairy paper but talks in detail about factors affecting magnesium oxide bioavailability. https://dairy.ifas.ufl.edu/dpc/1992/Beede.pdf