OVARIAN AND ENDOCRINE RESPONSES OF MERINO EWES TO TREATMENT WITH PMSG AND OR FSH-P

An experiment was conducted to investigate the effects of different superovulatory regimes, including pregnant mare serum gonadotrophin (PMSG) and follicle-stimulating hormone (FSH-P) on steroidogenesis, oestrus, ovulation and premature luteal regression in sheep. Thirty mature Merino ewes were treated in the breeding season with intravaginal progestagen pessaries and 1200 IU PMSG (Group 1), 15 mg FSH-P (Group 2) or a combination of 700 IU PMSG and 11 mg FSH-P (PMSG/FSH-P; Group 3) to induce superovulation. After treatment with FSH-P alone, the incidence of oestrus was lower than after treatment with PMSG alone (6/10 vs. 10/10, P < 0.05) and the incidence of ovulation was lower than after treatment including PMSG (7/10 vs. 20/20, P < 0.05). However, there were no differences in the mean time to onset of oestrus (24.7 +/- 2.3 h after pessary withdrawal) or the time to onset of ovulation (48-54 h after pessary withdrawal) between the treatment groups. The total ovarian response (corpora lutea + large follicles) was higher (P < 0.05) following stimulation with PMSG alone (17.1 +/- 1.9) than with FSH-P (8.3 +/- 2.5). However, the numbers of corpora lutea present on Day 6 (Day 0 = pessary withdrawal) were not affected by the exogenous gonadotrophin treatment (PMSG, 11.4 +/- 1.8; FSH-P, 7.2 +/- 2.5; PMSG/FSH-P, 11.1 +/- 2.1; not significant). The mean pre-ovulatory peripheral plasma peak level of oestradiol-17-beta (E2) per follicle was higher following treatment with PMSG than with either of the other two regimes (PMSG, 23.2 +/- 0.8 pg ml-1; FSH-P, 10.8 +/- 0.6 pg ml-1; PMSG/FSH-P, 13.9 +/-0.4 pg ml-1; P < 0.05). Moreover, E2 peaked earlier in the ewes treated with PMSG than with FSH-P or PMSG/FSH-P (7.6 +/- 2.1 h vs. 33.7 +/- 1.5 h and 24.8 +/- 0.5 h respectively; P < 0.01). There were no differences detected in the peak levels of luteinizing hormone (LH) (8.6 +/- 1.1 ng ml-1) or the mean time to LH peak (32 +/- 1.4 h) between the ewes treated with the different exogenous gonadotrophins. Three ewes (one treated with PMSG alone and two with PMSG/FSH) had premature luteal regression by Day 6. In each case, a secondary peak of E2 secretion was observed 36-44 h after pessary withdrawal. These ewes each displayed a second oestrus on Days 5-6 and a mild superovulatory response at this oestrus. It is concluded that in sheep PMSG is highly steroidogenic in comparison with FSH-P. A combination of moderate doses of PMSG and FSH-P is a suitable superovulation regime for Merino ewes as it induces a consistently high superovulatory response without the secretion of excessive E2 levels. Perturbation of the periovulatory endocrine events may result in premature luteal regression.