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 regression, formation, and function were associated with the decreased calving rate observed in beef females administered PGF2 alpha 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 PGF2 alpha 5 days before SMB treatment). Heifers were bled 10 days before PGF2 alpha treatment, immediately before PGF2 alpha 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 PGF2 alpha 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 PGF2 alpha 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 x 2 factorial with treatment (control or PGF2 alpha) 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. PGF2 alpha 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 PGF2 alpha 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.

Effect of interval from melengestrol acetate to prostaglandin F2 alpha on timed and synchronized pregnancy rates of beef heifers and cows.

The objective of this experiment was to determine the optimal interval from the last day of melengestrol acetate (MGA) feeding to prostaglandin F2 alpha (PGF2 alpha) treatment on pregnancy rates of beef heifers and cows. All females (149 heifers and 399 postpartum cows) were fed MGA (.5 mg) daily for 14 d and then administered PGF2 alpha (25 mg Lutalyse) 13, 15, and 17 d (Groups 1, 2, and 3, respectively) after the last day of MGA feeding. Females not in estrus the first 52 h after PGF2 alpha treatment were artificially inseminated 72 h after PGF2 alpha treatment. Females in estrus 0 to 52 h and 78 h to 6 d after PGF2 alpha treatment were inseminated at estrus. Blood sera (collected immediately before and 3 d after PGF2 alpha treatment) were assayed for progesterone concentrations. Pregnancy was determined 44 to 47 d after the 72-h AI by rectal examination. The intervals from MGA feeding to PGF2 alpha that had the highest 72-h AI pregnancy rates were 17 d for heifers (43%) and 15 d for cows (43%). Heifers with a 17-d interval had a higher (P < .05) 72-h AI pregnancy rate than heifers with 13-d and 15-d intervals, and cows with a 15-d interval had a higher (P < .05) 72-h AI pregnancy rate than cows with a 17-d interval. The 4-d synchronized pregnancy rates (the 72-h AI and the succeeding 3 d inseminations) for both heifers (44%) and cows (53%) were not different (P > .05) among groups. Fewer (P < .05) cows with a 17-d interval from MGA to PGF2 alpha had corpora lutea regression (by 72 h) after PGF2 alpha treatment than cows with 13-d and 15-d intervals. We interpret the results to indicate that the interval from MGA to PGF2 alpha treatment may influence 72-h AI pregnancy rates, that optimal intervals are 17 d for heifers and 15 d for cows, and that pregnancy rates are improved by insemination for 3 d after the 72-h AI.