Hi Susan, My experience is the same as Kaaren Jordan's, once the horses have enough of the minerals they slow down. Also if you buy the Icelandic minerals (not advertised but they make them) they don't eat them as rapidly as it is a slightly different formula. I sent them hair from a variety of Icelandics and they made the formula.
I would suggest a 25# bag should last you a while. Robyn Hood
