If the calf had scours, hair loss is common, but usually on the rump and leg areas. Hair loss on the neck could be caused by rubbing against an object. If you have a local veterinarian, you may want to call him or her and ask if any parasite or disease could be causing it. If the calf does not have an elevated temperature and is eating and growing well, you need not be too concerned.
Mike Hutjens, Extension Dairy Specialist, University of Illinois – Illini DairyNET