As the gents have said, dogs are a serious no-no and might even get you killed if there's calves around.
Assuming the above is a given, cows - especially young ones - are very inquisite and will want to come and see what you are doing in their field. Face up to them, make no sudden movements and simply walk calmly past. Easier said than done, I know, but I speak from experience having to cross several pastures every Sunday where I live.
If you run, the beasties will thinks it's a great game and chase after you! To be on the safe side check out the field before you enter - is there a fence you can dive over if needs be? And carry a stick as a last resort!
