Effects of elevated concentrations of prostaglandin F2α on pregnancy rates in progestogen supplemented cattle

An experiment was performed to determine the effect of elevated prostaglandin F-2 alpha (PGF(2 alpha)) on pregnancy rates of progestogen-treated bred cows in the presence or absence of luteal tissue. Ninety-one beef cows were bred (Day 0) and assigned randomly to receive either 3 mL saline (CON), 15 mg PGF(2 alpha) or 15 mg PGF(2 alpha) + lutectomy (P + L) administered intramuscularly (i.m.) at 8 h intervals on either Days 5-8, 10-13, or 15-18 postbreeding. Lutectomies were performed by transrectal digital pressure before initiation of treatment on Day 5, 10 or 15 for the respective treatment groups. All cows were fed 4 mg/day of melengesterol acetate from two days prior to initiation of treatment until Day 30 postbreeding. Mean concentrations of 13,14-dihydro-15-keto-PGF(2 alpha) (PGFM) were increased in cows administered PGF(2 alpha) and P + L treatments (398 +/- 23 and 413 +/- 22 pg/ml, respectively; p < 0.01) compared to the CON group (80 +/- 29 pg/ml) regardless of treatment group. Mean concentrations of oxytocin (OT) were increased in cows given PGF(2 alpha) on Day 10 and 15 (p less than or equal to 0.0001) and tended to be in creased on d 5 when compared to CON and P + L treatment groups on Day 5. Pregnancy rates were reduced (p less than or equal to 0.03) in the PGE(2 alpha) treatment group (23%) and by Day 5-8 compared to CON (72%). Lutectomy tended to improve pregnancy rate in P + L(5-8; 55%) compared to PGF(2 alpha) (5-8; p = 0.1). Pregnancy rates tended (p less than or equal to 0.07) to increase in the PGF(2 alpha) treatment groups on Days 5-8 treatment (23%, 50%, and 60% for Days 5-8, 10-13, and 15-18, respectively). The later the treatments were initiated pregnancy rates did not differ between treatments given on Days 10-13 and 15-18. in conclusion, the most susceptible period of embryonic growth to the negative effects of PGF(2 alpha) was during morula to blastocyst development. Removal of luteal tissue diminishes the negative effects of PGF(2 alpha) through interruption of the luteal oxytocin-uterine PGF(2 alpha) feedback loop.