If you place more than one bull with a group of cows, make sure the bulls are compatible. Otherwise, they may spend more time fighting each other than breeding cows. Adding several animals at once will also reduce the amount of fighting as they establish their social order. Depending on their age and other factors, bulls have varying capacities to breed. Size is obviously important and a very young bull will have physical difficulty breeding mature cows.
Younger bulls are also unable to service as many cows as more mature bulls.
As a rough guide, assign 10 to 20 open cows per bull aged 15 to 20 months. These numbers assume the bull is of normal size for his age. Just as heat stress can reduce milk production from cows, it also affects semen production in bulls. Adequate shade and supplemental cooling in very hot weather can significantly improve semen volume and quality. A bull's desire to mate also improves if he's kept cool and comfortable.