Haven't a clue- but it does seem to be true. It is very noticable in British Alpine's as they are black. If they are low in copper one symptom is that their coats will not be jet black, but dark brown instead. Generally at the edges, so down the legs etc. If you keep a mixed herd, and you have copper licks up, some of breeds spend far more time at them than the others. Oddly enough, its our BT's that spend most time at the copper licks, not the BA's. But the white one's, Saanen's, British Saanen's etc seem to need less.
Beth