Thanks, GG and Wombat! That was very helpful about calcium and boron. It's pretty obvious what side effect I get when I take too much magnesium , but how would I know when I get enough selenium? Is there a side effect or improvement I should be looking for?
Hopefully not too OT, but my horse has been on a mineral-balanced diet for several months with improvement in his health -- I tested his pasture and hay for nutrients and balanced against those values. There are a couple Yahoo groups about insulin resistance in horses, etc. I will be adding borax to his diet and see if that helps his arthritis.