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The objective of any successful synchronization program is to manipulate the estrous cycle of normally cycling females so that a large percentage will exhibit estrus with normal fertility at a pre-determined time. Synchronization programs have typically been most successful with heifers due to a lack of interference from extraneous factors. Since minimum age and weight are the primary prerequisites for a heifer of a particular breed to reach puberty, a nutritional program can be designed to allow the majority of heifers to reach their appropriate target weight by 13-14 months of age. Provided there are no health problems, or extreme environmental effects, this allows for a large percentage of these heifer to be cycling by the time the manager wants to breed them. Consequently, synchronization programs utilizing prostaglandins, progestogens or combinations of the two have been relatively successful at the conclusion of appropriate heifer development programs.
Several factors complicate this process with the lactating cow. First, cows don't all calve at the same time. Therefore we have differences in the number of days postpartum. Lactation and the suckling stimulus tend to prolong the postpartum period to first estrus. Body condition and/or nutrition will play a role and often be confounded by the age of the female. In addition, winter environments in the northern Great Plains and intermountian range country, can have a major impact on condition and energy requirements.
It should be emphasized in these introductory comments that successful estrous synchronization, whether it be with heifers or postpartum cows, requires a high level of management in order to be successful. A total herd health program and absence of calving difficulty along with a minimum postpartum anestrous period would be additional factors required to identify appropriate candidates for a synchronization program.
The objective of synchronization efforts should also be considered when selecting the synchronization approach and the most appropriate system. Producers will want to meet their objectives with the most economically feasible system. In a majority of cases, synchronization is being utilized in order to enhance the genetic contribution through the use of superior sires by artificial insemination. In other situations a producer might merely be attempting to group breeding/calving dates to enhance management. In extensively managed operations where producers utilize public land grazing or have management restrictions necessitated by the grazing environment, the producer might simply want to get as many cows bred as possible prior to the time these restrictions are applied. In some cases this may mean utilizing natural service sires.