We have run the calves through the chute, take a curry comb and scuff up the ringworm and get as much scaly stuff off then spray iodine right on it. Have also used plain old vaseline on the scuffed up spots and it smothers the stuff. Someone also told me about the diluted bleach. One guy uses hydrogen peroxide....sprayed on the ringworm spots.
We were told by an old vet that the lack of iodine seems to cause it to show up more often.
We use only trace mineral blocks of salt in the mineral feeders with the loose mineral. And with the pinkeye problems also put out the yellow sulfur salt blocks. Another old vet said that he really believes in the sulfur helping. So they have 2 salt blocks and loose trace mineral in the feeders. Cover all our bases and the trace salt blocks aren't that much more than the plain white.