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A study was conducted to test the hypothesis that injecting Gonadotropin-releasing hormone (GnRH) concurrently with Prostaglandin F2 alpha (PGF 2 α  ) followed by an injection of human Chorionic Gonadotropin (hCG), would advance follicular growth to ovulation in Holstein heifers bearing a corpus luteum (CL). After manual examination of the CL, group 1 (PGF;  n  = 12) received an injection of PGF 2 α   (25 mg, im). Group 2 (PGF + GnRH;  n  = 13) received an injection of GnRH (100  μ g, im) immediately after an injection of PGF 2 α  . Group 3 (PGF + GnRH + hCG;  n  = 12) received concurrent injections of PGF 2 α   and GnRH followed with hCG (1500 IU, im) two days later. Follicular size and day of ovulation were monitored by daily ultrasonographic examination from days 1 to 10. Blood was collected on days-7, 0 (PGF 2 α   administration), 2, and 7. Progesterone was not different ( P  > .05) on days-7, 0, and 2 between the experimental groups. However, it was higher ( P  < .005) in the PGF + GnRH + hCG group on day 7 compared to PGF + GnRH heifers, but not significantly higher than the PGF. Additionally, heifers in the PGF + GnRH + hCG group ovulated earlier ( P  < .05) than heifers in the PGF + GnRH and the PGF group. This data indicates that hCG advances follicular growth to ovulation in spite of high levels of progesterone when injected 48 h after concurrent treatments of GnRH and PGF 2 α   on heifers bearing a CL.

Roles of hCG in Advancing Follicular Growth to Ovulation after Concurrent Injections of and GnRH in Postpubertal Holstein Heifers Bearing a CL