Bulls are most likely to develop problems early in the mating season then later on; in order to minimise this adequate bull to cow ratio should be followed. Under reasonable paddock conditions, a rising 2 year old bull should handle 40 empty cows on his own. An older bull can probably handle slightly more. Also try to use your lowest birth weight, highest calving ease direct and shortest gestation length bull over heifers to reduce chances of dystocia.
Yearling bulls can be used but don't over work them; 25-30 females is a good work load for him.