My horses are supplemented with a standard mineral mix combining Zinc, Copper and Magnesium compounds that help supplement the feed here, as it tends to be deficient in these minerals.
The copper and zinc require a smaller amount of volume to add to the feed and so can be packaged and delivered easily and adds less bad taste for the horses to adapt to in their feed.
I'm looking for natural sources of magnesium so I can reduce the supplementation in the form of a powder.
Whole oat would be the only source I can think of at the moment.
In case of mineral deficits I'd always consider changing the hay if you feed one. If you can, analyse it and compare with other hays. Might be that you can cover up mineral deficits with other hay as the quality differs a lot.
Maybe you can also change the weed and gras-combination on the meadow. I am not sure what is best for magnesium as I never experienced a magnesium deficit in horses myself. There sure are tables in books and the internet that can be used to find out which weeds are best to sew.
Are you feeding the copper to prevent a zinc-related copper-deficit?
As far as I learned a deficit must not always mean that a horse does not eat enough of the thing that is lacking. It might be that another factor is suppressing the intake of some mineral, just as the zinc does.
Sorry for my english, I could explain this a lot better in my mothertongue.