Bull-riding may be the most renowned rodeo event, and it even has its own specialized professional tour, the Professional Bull Riders tour. But the sport depends on the aptness of one animal to perform according to expectations: the bull. The bull is encouraged to buck violently in a variety of long-term and short-term strategies and stimuli, all in order to make a better show.
Experienced rodeo bulls buck because they are in the habit of bucking and have never been trained not to buck. They have never been broken with a rider, and are therefore always uncomfortable with a rider on their backs. A horse is only comfortable being ridden after a long and delicate breaking process as the animal becomes accustomed to carrying a person on its back. Bulls in the rodeo have simply never undergone this process, and so are never comfortable carrying a person on their backs.
Bulls are carefully bred by bull breeders who select particularly for aggressive bucking traits. If a bull is a strong rodeo bull, he is more likely to be used for stud purposes, breeding a successively better and better pool of bucking bulls. Just like racehorses are carefully bred for their speed and endurance, bulls are bred for bucking.
When a bull is loaded into the chute before he is ridden, a flank strap is tied around his midsection. Contrary to popular belief, this strap does not restrict or inflict pain on the bull's genitals. Rather, it acts as a mild irritant, an annoyance that the bull will attempt to dislodge with forceful kicks of the hind legs once the ride has begun. This strap is removed immediately after the ride.
The bulls are also naturally adrenaline-filled animals, especially in the noise and excitement of a rodeo. Just as athletes, and indeed the bull-riders themselves, become excited at the moment of the ride, so the bulls will also become nervous and excited.
During the course of the ride, the bull-rider touches the bull's flanks with spurs. This is an added stimulation, and one more annoyance that the bull will try to rid himself of with bucking, twisting and kicking violently. Once again, of course, this annoyance is eliminated immediately upon the bull-rider's leaving the bull's back, which helps to explain why the bulls often calm down immediately after the ride is complete.