Effect of prostaglandin F2α 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 regression, formation, and function were associated with the decreased calving rate observed in beef females administered PGF2α 5 days before Syncro-Mate B (SMB) treatment. Experiment 1 included 31 beef heifers 11 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 and PGF2α 5 days before SMB treatment). Heifers were bled 10 days before PGF2α treatment, immediately before PGF2α and SMB 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 PGF2α treatment and for 3 days after norgestomet implant removal. Based on the blood samples collected before SMB treatment, 15 heifers in experiment 1 and every heifer in experiment 2 were with estrous cycles. All heifers in experiment 1 had progesterone concentrations < 0.5 ng/ml 2 days after implant removal. However, progesterone concentrations during the luteal phase in control heifers with estrous cycles were higher (P < 0.05) than in PGF2α treated heifers with estrous cycles and in heifers previously without estrous cycles. In experiment 2, based on the occurrence of estrus and progesterone concentrations, heifers were also classified as metestrus or diestrus at the time of SMB treatment. The data were analyzed as a 2 × 2 factorial with treatment (control or PGF2α) and stage of the cycle (metestrus and diestrus) as main effects. More metestrus heifers (40%) had progesterone concentrations > 1.0 ng/ml 2 days after implant removal than diestrus heifers (0%). In addition, progesterone concentrations during the luteal phase in metestrus heifers were lower (P < 0.05) than in diestrus heifers. PGF2α treatment had no effect (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 interactions. In summary, the administration of PGF2α 5 days before SMB decreased the calving rate by causing more heifers to be metestrus at SMB treatment. Fewer metestrus heifers (than diestrus heifers) were synchronized (with < 1.0 ng/ml of progesterone 2 days after implant removal) to SMB treatment and those synchronized had lower progesterone concentrations during the luteal phase.