Heat synchronization controls when groups of normally cycling females come into heat. Prostaglandin injections cause premature regression of the corpus luteum during days 6-17 of the estrus cycle, resetting the females to the follicular stage. Choosing a synchronization system depends on factors like calving times, financial resources, labor, facilities, and pasture/feed availability. The CIDR insert suspends the estrus cycle regardless of stage by simulating progesterone, while high percentages of cycling females are important for success.