Supplementation and iron

[ratherrapid]

I'm puzzling over supplements. We know so much more than we used to.

The immediate problem: all these supplements, everyone of them has tons of iron. e.g. if you feed several different supplements you might be feeding iron in some or all of them. I'm presuming feed has a significant amount of iron absent any supplements at all.

Anybody have a feel as to how much iron supplementation, if any is desirable in terms of miligrams or grams. I am presuming--but really am without any idea--that we should supplement iron occasionally to race horses. BUT, since iron is not excreted in any way and builds up in the system, i have this q.