Effects of elevated concentrations of prostaglandin F₂α on early embryonic survival in cows supplemented with exogenous progestogen

An experiment was performed to determine the stage of embryonic development detrimentally affected by elevated prostaglandin F_2α (PGF_2α) in the presence or absence of luteal oxytocin. Ninety-one beef cows had their estrous cycles synchronized and were bred by natural service and artificial insemination (Day 0). Cows were randomly assigned to receive either 3 ml saline (CON), 15 mg PGF_2α (PGF) or 15 mg PGF_2α + lutectomy (P+L) administered i.m. at 8 h intervals on either Days 5-8, 10-13 or 15-18 (3 x 3 factorial). Lutectomies were performed by transrectal digital pressure before initiation of treatment on Day 5, 10 or 15 for Days 5-8, 10-13 and 15-18, respectively. All cows were fed 4 mg/d of melengesterol acetate (MGA) from two days prior to initiation of treatment until Day 30. Lutectomies were performed on nine CON cows to verify the ability of MGA to sustain pregnancy. Seven of these nine animals were diagnosed pregnant by transrectal ultrasonography at Day 30. All cows were bled by jugular venipuncture at 0600 and 2200 h of their respective treatments for determination of Progesterone (P₄), estradiol-17β (E₂), 13,14-dihydro-15- keto-PGF_2α (PGFM). Additionally, cows were bled at 30 min following initiation of treatment for determination of oxytocin (OT) concentrations. Concentrations of P₃ were reduced (p < 0.05) in all cows administered PGF and P+L by 2200 h on the first day of treatment. Mean concentrations ,of PGFM were increased in cows administered PGF and P+L treatments (456 ± 35 398 ± 35 pg/ml, respectively; p < 0.01) compared to CON cows (84 ± 58 pg/ml) regardless of daygroup. Mean concentrations of OT were significantly increased in cows administered PGF when compared to CON and P+L in the Day 5-8 (P < .07) and in the Day 10-13 and 15-18 groups (P = .0001). Pregnancy rates were reduced (p = 0.03) in the PGF(5-8) group (3/13, 23%) compared to CON (5/7, 72%). Lutectomy tended to improve pregnancy rate in P+L(5- 8; 6/11, 55%) compared to PGF(5-8; p = 0.1). Pregnancy rates tended (p = 0.07) to increase across daygroups in the PGF treatment (3/13, 23%; 5/10, 50%; and 6/10, 60% for Days 5-8, 10-13 and 15-18, respectively). Pregnancy rates did not differ between CON, PGF and P+L groups for Days 10-13 and 15-18. In conclusion, the most susceptible period of embryonic growth to the negative effects of PGF_2α appears to be during morula to blastocyst development. Removal of the luteal source of oxytocin diminishes the negative effects of PGF_2α administration on the bovine embryo during early development, possibly through interruption of the luteal. oxytocin-uterine PGF_2α feedback loop.