A family member was walking across a field on footpath with his well behaved welsh border collie on a lead, the land was part of a stately home. He was attacked by a herd of cows, the dog got away but he almost died, chest crushed etc, and was in hospital for weeks.
Years ago I too was surrounded by cows when walking by dog, on lead, on footpath. Luckily the farmer came along and recued us. Cows see the dog as a threat, especially when they have young.
I would never risk going into a field of cows or bull, with a dog, whatever the signs said.