Postpartum cows with short-lived corpora lutea produce embryos that arrive at the uterus, but pregnancy rates are low even with exogenous progestogen. Four experiments were conducted to determine whether prostaglandin (PG) F2 alpha, known to cause early luteolysis, could have a direct effect on embryonic loss. Exogenous progestogen was injected s.c. twice daily in each experiment, starting 3 or 4 days after estrus (day of estrus = Day 0). Nonlactating, cycling beef cows were mated and injected i.m. every 8 h on Days 4 through 7 (experiment 1) or 5 through 8 (experiment 3) with either 15 mg PGF2 alpha or 3 ml saline. In experiment 1, cows in a third group received 1 g flunixin meglumine i.m. every 8 h. Ten of 18 PGF2 alpha-treated cows in experiment 3 were luteectomized on Day 5. Pregnancy rates were higher (p < 0.05) in cows given saline or flunixin meglumine (5 of 7) than in cows given PGF2 alpha (1 of 5) in experiment 1, and in cows given saline (6 of 9) or given PGF2 alpha, and luteectomized (8 of 10) than in cows given PGF2 alpha (2 of 8) in experiment 3. Postpartum beef cows, mated at weaning-induced first estrus, received i.m. injections every 8 h on Days 4 through 9 of 3 ml saline or 1 g flunixin meglumine (experiment 2); 14 flunixin meglumine-treated cows were luteectomized on Day 7. Pregnancy rates were higher in cows given flunixin meglumine and luteectomized (7 of 14) than in cows given saline (4 of 15) or flunixin meglumine alone (3 of 15; p < 0.05). In experiment 4, postpartum cows were luteectomized or sham-operated on Day 5. Pregnancy rates (2 of 13 and 2 of 14, respectively) did not differ. Thus, both reduction of endogenous PGF2 alpha and luteectomy were required for embryo survival in postpartum cows with short-lived corpora lutea, whereas luteectomy alone prevented effects of exogenous PGF2 alpha in cycling cows.
Experiments were conducted in beef cows without a primary CL, in which pregnancy had been maintained with exogenous progestogen. In preliminary trials, replacement CL induced ipsilateral to the embryo and after, rather than before, d 36 of pregnancy, maintained more pregnancies after withdrawal of exogenous progestogen (13/13 vs 2/6; P < 0.05). In Exp. 1, in cows with replacement CL induced by treatment with hCG on d 28 of pregnancy, treatment with flunixin meglumine on d 31 through 37 did not increase maintenance of pregnancy. Experiment 2 was conducted to evaluate directly the effects of concentrations of PGF2alpha and estradiol-17beta during d 31 through 35 of pregnancy on maintenance of pregnancy by replacement CL induced between d 28 and 31. In cows that maintained pregnancy while progestogen was provided, maintenance of pregnancy after withdrawal of exogenous progestogen tended to be greater with high (5/5) than with low (2/6; P < 0.10) concentrations of PGF2alpha and greater with low (6/7) than with high (2/6; P = 0.10) concentrations of estradiol-17beta. Secretion of progesterone by replacement CL was greater (P < 0.05) in cows with high than in those with low concentrations of PGF2, during d 31 through 35. Prostaglandin F2alpha may facilitate attachment of the bovine embryo (d 30 to 40) in a manner similar to that reported for implantation in other species. Cows that did not form CL in response to hCG on d 28 to 31 responded well when retreated after d 36. Again, maintenance of pregnancy was greater when replacement CL were induced after (9/9) rather than before d 36 (8/16; P < 0.05).
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