BACKGROUND: Early resumption of ovarian activity after parturition could positively influence the fertility of dairy cows. Ovulation of the first postpartum follicular wave and formation of corpus luteum that secretes progesterone can sub-stantially benefit the resumption of ovarian cyclicity in cows. In this regard, the presence of a dominant follicle in the ovary before day ۱۰ postpartum could enhance ovulation rate in the first postpartum follicular wave. Pregnant mare serum gon-adotropin (PMSG) is one of the hormones with the potential to improve the growth of follicles. OBJECTIVES: Accordingly, the present study was conducted to evaluate the effect of
PMSG injection six days after parturition on reproductive parameters in dairy cows. METHODS: The subjects were randomly assigned to the two groups of control (N=۷۴) and treatment (N=۷۹). Cows in the control group received no treatment, while animals in the treatment group received ۵۰۰ IU
PMSG on day six postpartum. In both experimental groups, cows were subjected to Presynch-Ovsynch protocol ۳۰-۳۵ days postpartum. The subjects that experienced estrus during the Presynch-Ovsynch protocol were inseminated. On the other hand, the cows that were not in estrus during this period were subjected to fixed-time insemination at the end of the Presynch-Ovsynch protocol. Pregnancy was diagnosed ۴۵ days after artificial insemination using the rectal examination. RESULTS: In the control group, ۶۶.۲۲% of cows were inseminated based on estrus detection and ۳۳.۷۸% of cows were inseminated at fixed times. In the intervention group, ۶۸.۳۵% of cows were inseminated based on estrus detection and ۳۱.۶۵% of cows were inseminated at fixed times. The first-service conception rate (FSCR) was ۲۷.۰۳% and ۴۸.۱% in the control and treatment groups, respectively (p =۰.۰۱). CONCLUSIONS: The present study showed that the injection of ۵۰۰ IU
PMSG six days after parturition could enhance FSCR in dairy cows.