My advice is to stay out of the field. They are probably just inquisitive and have no intention of harming you. Why not do your metal detecting in an empty field, I'm sure if they are milking cows they will be moved to fresh pasture in a few days, so go back then and do it. A crowd of young cattle can be intimidating so the best advice is stay out, speak to the landowner, I'm sure he will allow you to detect on an alternative field.