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Abstract

Thirty-one cycling heifers were superovulated and subsequently ovanectomized to determine if preovulatory follicles could be induced to ovulate by the administration of cloprostenol, a potent agonist of prostaglandin F2α. Superovulation was induced, during the mid-luteal phase, with a total dose of 20 mg porcine pituitary extract of follicle stimulating hormone (FSH) given twice daily over 4 days in decreasing doses; a luteolytic dose of cloprostenol was administered on the third day. Two experiments were carried out in which ovariectomies (OVX) were performed at 64 h following cloprostenol injection. Owing to the variable interval between the preovulatory peak of luteinizing hormone (LH) and OVX observed in Experiment I, the preovulatory LH surge was induced in Experiment II by 100 μg of a synthetic analogue of gonadotropin releasing hormone (GnRH) given 36 h after the luteolysin. To test the hypothesis that cloprostenol is able to induce, enhance or synchronize ovulation, two different doses of this hormone, 1.25 or 2.5 mg, were injected at 4 and 2 h or at 2 and 1 h prior to OVX. Plasma progesterone (P4) and LH concentrations and follicular fluid P4: estradiol-17β (E2) ratios in the non-ovulated follicles were utilized to identify animals that had a typical response to the superovulatory treatment and, at OVX, total number of follicles 8 mm or more in diameter, and number of ovulations were recorded. When data from both experiments were considered, it was observed that: (1) GnRH injection did not affect ovulation rate; (2) cloprostenol injections induced more follicles (P < 0.01) to ovulate, 59.3% vs. 20.1%, SE = 9.8, in cloprostenol-treated vs. controls, respectively, at the time tested; (3) cloprostenol dose had a positive linear effect (P < 0.01) on ovulation rate. In conclusion, the data suggest that prostaglandin F2α or its analogues could be used to induce a more synchronous ovulation of follicles in superovulated heifers.