I would have to agree with mike ,
It is that time of year , when lice seem to be most evident , especially since it is shedding season as well , and teh cows are doing a bit of rubbing and scratching anyway to get rid of hair. Or, they could be bored , and be rubbing for something to do .. but skin critters would be my most likely suspicion.
I agree , Ivomec ,or its new generation derivitives , I like the cyticeden(I have spelled it wrong I am sure ) it is bright purple and I can tell which cows i have hit , plus you can use it on prego cows ....If you cannot get close enough to run a dose down their topline .. here is a simple trick ... take one of the plastic scoops that come in laundry detergent and duct tape it onto the end of a long pole ..... pour your dose in that and reach out over your beasts and apply ...
Now .. if its ring worm , and your vet can tell you .... dont ignore it , I would have to disagree that the sun will clear it up,if you can actually wash your beasts use a solution of 1:10 clorine bleach every few days till you see evidence of hair growth .. or the vet can give you an antifungal cream ,or check your farm supply store for ringworm shampoos ....wear rubber gloves
Ring worm is highly contagious to other stock as well as people too ..
its a good idea to rinse down bunkers , feed tubs , water tanks with the bleach solution ....
paula