REVIEW OF ESTROUS SYNCHRONIZATION SYSTEMS : GnRH

Synchronization of estrus contributes to optimizing the use of time, labor, and financial resources by shortening the calving season, in addition to increasing the uniformity of the calf crop. The major limitation of estrus-synchronization programs is their inability to induce a potentially fertile estrus and ovulation in noncycling cattle (i.e., prepubertal heifers and anestrous suckling cattle). Because initial estrus-synchronization programs were not designed for successful treatment of noncycling cattle, their use in cow-calf operations generally has not produced results that would encourage greater A.I. use in beef cattle. Currently, less than 7% of beef cows and an estimated 8 to 10% of beef heifers are A.I.-bred in the U.S. The potential for increasing A.I. in beef cattle is great if a system can resolve successfully the problem of the noncycling female at the beginning of the breeding season. The premise behind synchronizing cows and heifers is to first control the timing of onset of estrus by controlling the length of the estrous cycle. The choice of approaches for controlling cycle length are: 1) to regress or “kill” the corpus luteum (CL) of the animal before the time of natural luteolysis, and thereby shorten the cycle (by administration of a prostaglandin F2α [PGF2α]), or 2) to administer exogenous progestins to delay the time of estrus following natural or induced luteolysis that may extend the length of the estrous cycle. A further approach is to “select” the ovulatory follicle by an injection of GnRH, which should cause premature ovulation of that follicle. Using these concepts, researchers have made tremendous strides in developing numerous system to synchronize the estrous cycle for an A.I. after a detected estrous or a timed-A.I. Table 1 summarizes common products available for use in cattle estrus synchronization systems.