I usually have some cobalt (which contain iodine) blocks out, and supplement with loose salt and mineral mix... Our area is particularly low in Phosphorus, and everyone thinks I typed the ratio wrong when I say 6% Ca, 12% Phosphorus... we also need a lot of copper (2500mg/kg) and selenium (125mg/kg) around here, your area will be different... the best thing to do is feed the local type out to your cows for a year, and then before the breeding season get a blood sample tested, preferably from one of the heavier milking cows, and that should give you a good idea if you have any problems