Basically no. He would have every right to graze his cattle in a field crossed by a right of way as long as they were all cows and calves or that any bulls among them were from beef breeds such as Herefords or Welsh Blacks etc. It is illegal to keep bulls from dairy breeds such as Jersey or Fresian in a field crossed by a right of way. Either way, proceed with caution whenever you pass a bull and take particular care with cows when there are calves present.