Effect of prostaglandin F2 alpha-and gonadotropin releasing hormone-induced luteinizing hormone releases on ovulation and corpus luteum function of beef cows.

Luteinizing hormone (LH) concentrations were measured in suckled beef cows treated during the postpartum period with prostaglandin F2 alpha (5 mg Alfaprostol; PGF2 alpha) and then gonadotropin releasing hormone (100 micrograms Cystorelin 30 h after PGF2 alpha; GnRH). The objective was to determine if PGF2 alpha would cause a release of LH in the absence of progesterone and affect the GnRH-induced LH release and ovulation (Experiment 1). LH concentrations increased (P < 0.05) after PGF2 alpha treatment in both anestrous and cyclic cows but to a greater extent (P < 0.05) in anestrous cows. The GnRH-induced LH release and ovulation response in previously anestrous cows were greater (P < 0.05) when PGF2 alpha was administered 30 h earlier. In Experiment 2, 49 beef cows received PGF2 alpha (5 mg Alfaprostol) and GnRH (100 micrograms Cystorelin) 30 h later to determine if the profile of the preovulatory LH surge was associated with the occurrence of subnormal luteal phases in postpartum beef cows suckling calves. Cows that had normal luteal phases had a greater (P < 0.05) mean area under the GnRH-induced LH response curve and a greater (P < 0.05) mean GnRH-induced LH peak amplitude than cows that had subnormal luteal phases. In summary, results suggest that PGF2 alpha may exert a fertility effect by causing a LH release independent of progesterone withdrawal; administration of PGF2 alpha 30 h before GnRH elevated the GnRH-induced LH release and ovulation response. In addition, cows with subnormal luteal phases had GnRH-induced LH surges of less area and peak amplitude than cows with normal luteal phases.