i had bother few years back mine was sheep doing well on a pretty bare field moved them to a field with 3 inches of nice grass and they melted . after re fluking and worming had them blood tested came back selenium and cobalt deficient .i tryed a few drenches which as has been said are a bit variable . then i started giving them rockies sc .the sc being high selenium and cobalt with other trace elements and obviously salt. never had any probs since .the rockies are there all the time they dont go through alot .