Effects of elevated concentrations of prostaglandin F 2α on pregnancy rates in progestogen supplemented cattle

Abstract

An experiment was performed to determine the effect of elevated prostaglandin F 2α (PGF 2α) 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 2α, or 15 mg PGF 2α + 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 2α (PGFM) were increased in cows administered PGF 2α 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 2α on Day 10 and 15 (p ≤ 0.0001) and tended to be increased on d 5 when compared to CON and P+L treatment groups on Day 5. Pregnancy rates were reduced (p ≤ 0.03) in the PGF 2α 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 2α (5–8; p = 0.1). Pregnancy rates tended (p ≤ 0.07) to increase in the PGF 2α 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 2α was during morula to blastocyst development. Removal of luteal tissue diminishes the negative effects of PGF 2α through interruption of the luteal oxytocin-uterine PGF 2α feedback loop.