So who should 'know better' the farmer or the helicopter pilot? The pilot knows the dynamics of his machine. Why should the farmer know?
The airspace above the field may be free for all but just how low is safe? Is Trabant or Landrover height safe?
In my days as a helicopter pilot you flew low where you had properly recced the area. No one should drive at a helicopter. That is just daft. But the pilot has to take responsibilty for where he flies his aircraft.