As spring is here, the number of flies and external paraites have increased. It could be Biting flies, but if that were the case, then I would expect for you to see it on the other animals. I know that with young calves, the mother cows will lick the hind end of the calf while it is nursing to stimulate it to excrete waste. This licking compiled with warmer weather conditions can cause the hair to fall out. If the affected area is in other regions such as the "twist" (area between back legs underneath the tail), and around the ears, there might be a more serious problem than an overzealous mother. I would suspect that it could possibly be a fungus or other external parasite. Although fungus is very contagious, some fungal infections will not appear on older or otherwise more immune animals, although it can and more than likely is still present. I would recommend treating the calves with iodine on the affected areas, and using some sort of external parasite/fungal medication on the older animals.
JR Cattle Co.