EFFECT OF PROSTAGLANDIN F-2-ALPHA-RELEASING 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 F-2 alpha (5 mg Alfaprostol; PGF(2 alpha)) and then gonadotropin releasing horm one (100 mu g Cystorelin 30 h after PGF(2 alpha); GnRH). The objective was to determine if PGF(2 alpha) would cause a release of LH in the a bsence of progesterone and affect the GnRH-induced LH release and ovul ation (Experiment 1). LH concentrations increased (P < 0.05) after PGF (2 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 o vulation response in previously anestrous cows were greater (P < 0.05) when PGF(2 alpha) was administered 30 h earlier. In Experiment 2, 49 beef cows received PGF(2 alpha) (5 mg Alfaprostol) and GnRH (100 mu g Cystorelin) 30 h later to determine if the profile of the preovulatory LH surge was associated with the occurrence of subnormal luteal phase s 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 r esponse curve and a greater (P < 0.05) mean GnRH-induced LH peak ampli tude than cows that had subnormal luteal phases. In summary, results s uggest that PGF(2 alpha) may exert a fertility effect by causing a LH release independent of progesterone withdrawal; administration of PGF( 2 alpha) 30 h before GnRH elevated the GnRH-induced LH release and ovu lation response. In addition, cows with subnormal luteal phases had Gn RH-induced LH surges of less area and peak amplitude than cows with no rmal luteal phases. (C) 1997 Elsevier Science B.V.