RESUMEN
In ruminants, superovulatory treatments started at the time of follicular wave emergence result in greater and less variable ovulatory responses and embryo yields compared with the treatments begun in the presence of a large growing antral follicle(s) from the previous waves. The progesterone-oestradiol treatment is routinely used for follicular wave synchronization in cattle. The main objective of this study was to characterize the ovarian responses, hormonal profiles and in vivo embryo production in anoestrous Rideau Arcott ewes (May-June), which were superovulated after pretreatment with medroxyprogesterone acetate (MAP)-releasing intravaginal sponges and a single dose of oestradiol-17beta (E(2)-17beta). Six days after insertion of MAP sponges, eight ewes were given an i.m. injection of 350 microg of E(2)-17beta (E(2)-17beta-treated ewes); 10 ewes were given an i.m. injection of vehicle (control ewes). Multiple-dose Folltropin-V treatment, followed by the bolus injection of GnRH (50 microg i.m.), began 6 days after E(2)-17beta/vehicle injection. Transrectal ovarian ultrasonography revealed that: (i) the interval between E(2)-17beta/vehicle injection and regression of all follicles > or =5 to 3 mm in diameter was shorter (p < 0.01; 2.6 +/- 0.4 vs 4.8 +/- 0.6 days respectively); and (ii) the interval between injection and emergence of the next follicular wave was longer (p < 0.05; 5.4 +/- 0.3 vs 1.2 +/- 0.4 days, respectively) in E(2)-17beta-treated than in control ewes. During the 6 days after injection, the mean FSH peak concentration and basal FSH concentration were lower (p < 0.01) in E(2)-17beta-treated ewes. The mean ovulation rate and the number of recovered embryos did not differ (p > 0.05) between the two groups of ewes. However, the number of luteinized unovulated follicles per ewe, and the variability in the number of luteal structures and overall embryo yield were less (p < 0.05) in E(2)-17beta-treated compared with control ewes. In conclusion, the MAP-E(2)-17beta pretreatment significantly reduced the variability in ovarian responses and embryo yields, without affecting the embryo production in superovulated anoestrous ewes.