EFFECT OF PROSTAGLANDIN-F2-ALPHA TREATMENT BEFORE NORGESTOMET AND ESTRADIOL VALERATE TREATMENT ON REGRESSION, FORMATION, AND FUNCTION OF CORPORA-LUTEA IN BEEF HEIFERS

Two experiments were conducted to determine if corpus luteum regressio n, formation, and function were associated with the decreased calving rate observed in beef females administered PGF(2 alpha) 5 days before Syncro-Mate B (SMB) treatment. Experiment 1 included 31 beef heifers 1 1 to 13 months old and experiment 2 included 31 beef heifers 19 to 21 months old. Heifers were randomly assigned to 1 of 2 groups (control a nd PGF(2 alpha) 5 days before SMB treatment). Heifers were bled 10 day s before PGF(2 alpha) treatment, immediately before PGF(2 alpha) and S MB treatments, at the time of implant removal, and twice weekly after implant removal. Heifers in experiment 2 were observed twice daily for estrus for 5 days after PGF(2 alpha) treatment and for 3 days after n orgestomet implant removal. Based on the blood samples collected befor e SMB treatment, 15 heifers in experiment 1 and every heifer in experi ment 2 were with estrous cycles. All heifers in experiment 1 had proge sterone concentrations <0.5 ng/ml 2 days after implant removal. Howeve r, progesterone concentrations during the luteal phase in control heif ers with estrous cycles were higher (P < 0.05) than in PGF(2 alpha) tr eated heifers with estrous cycles and in heifers previously without es trous cycles. In experiment 2, based on the occurrence of estrus and p rogesterone concentrations, heifers were also classified as metestrus or diestrus at the time of SMB treatment. The data were analyzed as a 2 X 2 factorial with treatment (control or PGF(2 alpha)) and stage of the cycle (metestrus and diestrus) as main effects. More metestrus hei fers (40%) had progesterone concentrations > 1.0 ng/ml 2 days after im plant removal than diestrus heifers (0%), In addition, progesterone co ncentrations during the luteal phase in metestrus heifers were lower ( P < 0.05) than in diestrus heifers. PGF(2 alpha) treatment had no effe ct (P > 0.25) on the number of heifers with > 1.0 ng/ml progesterone 2 days after implant removal and progesterone concentrations during the luteal phase. There was no treatment by stage of the estrous cycle in teractions. In summary, the administration of PGF(2 alpha) 5 days befo re SMB decreased the calving rate by causing more heifers to be metest rus at SMB treatment. Fewer metestrus heifers (than diestrus heifers) were synchronized (with < 1.0 ng/ml of progesterone 2 days after impla nt removal) to SMB treatment and those synchronized had lower progeste rone concentrations during the luteal phase. (C) 1997 Elsevier Science B.V.