Usefulness of Zeranol in Synchronizing Breeding in Gilts

Purdue University 2002 Swine Research Report. In the pig, administration of high doses of estrogen (5 mg/d) on days 11-15 of the estrous cycle results in pseudopregnancy, i.e., maintenance of the corpora lutea following ovulation (Frank et al., 1977). In addition, Zavy et al. (1988) demonstrated that the administration of prostaglandin-F2a (PGF2a) to pseudopregnant gilts resulted in regression of corpora lutea (CL) and the synchronization of estrus. More recently, Pusateri et al. (1996) reported that much lower doses of estradiol-17b could induce long-term pseudopregnancy in gilts. Zeranol has been reported to possess about 14% of the affinity of estradiol-17b for the estrogen receptor (Katzenellenbogen et al.,1979). It was of interest to examine whether treatment of gilts with implants containing zeranol could induce pseudopregnancy in gilts. Zeranol is the active component of Ralgro, a growth promoting implant for cattle and sheep. Inducement of pseudopregnancy by zeranol followed by regression of induced CL by PGF2a could provide a useful tool for swine producers to synchronize breeding in their gilt replacement pool.