Ovarian and endocrine responses of merino ewes following treatment with PMSG and GnRH or PMSG antiserum

Abstract

Forty-eight mature merino ewes were treated in the breeding season with intravaginal progestagen pessaries and 1200 IU pregnant mare serum gonadotrophin (PMSG). The ewes were randomly allocated into four treatment groups. Group 1 served as controls; Groups 2 and 3 received an i.v. injection of 100 μg of synthetic gonadotrophin releasing hormone (GnRH) 12 or 24 h after pessary withdrawal (PW), respectively; and, Group 4 received a s.c. injection of 2 ml of ovine PMSG antiserum (A/S) 24 h after PW. Injecting GnRH at 12 h, advanced the time to onset of ovulation, but reduced the total follicular response (corpora lutea plus large follicles) and numbers of corpora lutea. The total follicular response was similar in Groups 1, 3 and 4, but the numbers of corpora lutea were higher in ewes injected with GnRH or PMSG A/S 24 h after PW compared with control animals. The mean peripheral plasma peak levels of oestradiol-17β (E 2) per follicle and progesterone per corpus luteum were reduced following treatment with GnRH or PMSG A/S. Moreover, treatment with GnRH advanced the time to and magnitude of the luteinising hormone peak. It was concluded that treatment with GnRH or PMSG A/S at the appropriate time may improve the superovulatory response of ewes to PMSG by increasing the proportion of developing follicles which ovulate. These treatments may also have beneficial effects on the fertility of superovulated ewes by limiting peri-ovulatory oestrogen secretion.