Control of time of ovulation in heifers treated with progesterone and gonadotrophin-releasing hormone.

The time of ovulation was determined in twenty-nine heifers by laparoscopy following treatment with progesterone. In addition, twenty-five of the animals received 100 mug Gn-RH 30 hr after the end of the progesterone treatment. Only eight of the heifers treated with Gn-RH ovulated. This contrasted with results obtained following examination of ovaries after slaughter in similar animals treated in identical fashion. Heifers which were treated with progesterone for 12 days and then slaughtered had not begun to ovulate 65 hr after the end of treatment, but five of eleven animals had ovulated 4 hr later. When Gn-RH was given, ovulation had started 60 hr after the end of progesterpme treatment, 60% of heifers had ovulated at 65 hr and 90% at 69 hr. Treatment with Gn-RH can synchronize and hasten the time of ovulation in heifers given progesterone. Possible reasons for the discrepancy between the results obtained following laparoscopy and slaughter are discussed.