The man, named as Ian Rook, from Clanfield, near Petersfield, Hampshire, was killed on Friday as he attempted to move the animal between two groups of cattle.
William White, south-east regional director for the NFU, described the dead man as an experienced farmer and an "all-round good bloke".
The farmer died on Friday after it is believed the bull turned on him after being startled.
A passer-by ran to the nearby farm house to call for emergency assistance. But when they returned to the scene, the farmer had died from his injuries.
"The bull must have been spooked," White said. He said he knew the farmer through his work as a treasurer and county chairman for the NFU.
"He was an experienced farmer. He was an extremely nice man who did a lot for the NFU. He was genuinely an all-round good bloke," he said.
Rook leaves a wife and two sons, the NFU regional director added.
White said the accident highlighted the risk of working with large livestock.
Geoff Wilson, landlord at the nearby Rising Sun, said he had been told by villagers that the farmer had been "tossed up in the air" by the bull.
The pub landlord also believed the animal remained on the loose for a couple of hours after the attack before being controlled.
It comes just two weeks after a man was killed and his wife left critically injured after they were charged by a bull in Nottinghamshire.