The farmer is under a duty to control his livestock either by keeping them fenced in a field or keeping them under control when on the road.
If the farmer is saying that he is not liable because they are animals, he is wrong. Using his logic, they could damage cars and run riot and he would escape liability.
It would be easy to resolve by simply having one person at the back herding the cattle and another person preventing them straying off the road.
The farmer is liable in negligence for any damage caused by animals which are under his control.
Can I clarify anything for you?
There is no legal obligation to have a gate. In fact, on some modern open plan properties, there is a restrictive covenant actually precluding gates and fences.