The tragedy occurred as the man led his cows and two bulls from the barn of his farm in the Bichwil-Oberwil region, cantonal police said.
The older of the two bulls, three and a half years old and weighing 900 pounds, wandered behind the barn.
When the farmer tried to round him up the bull attacked him.
A neighbouring farmer came to the aid of the fallen man before medical attendants from the air rescue service arrived at the scene by helicopter.
Despite resuscitation attempts by rescue personnel the man could not be saved, police said.
A hunter meanwhile shot the bull.
German-language newspaper Blick has posted a video on its website showing the dead animal being carted away in a trailer pulled by a tractor.