Do high progesterone concentrations decrease pregnancy rates in embryo recipients synchronized with PGF2alpha and eCG?

The objective of this study was to evaluate the effects of equine chorionic gonadotropin (eCG) treatment on the number of induced accessory corpora lutea (CL), plasma progesterone concentrations and pregnancy rate in cross-bred heifers after transfer of frozen-thawed (1.5M ethylene glycol) embryos. All recipients received 500 microg PGF2alpha (dl-cloprostenol, i.m.) at random stages of the estrous cycle (Day 0) and were observed for estrus for 7 days. On Day 14, heifers detected in estrus between 2 and 7 days after PGF2alpha treatment were randomly allocated to four groups ( n=83 per group) and given 0 (control), 200, 400, or 600 IU of eCG. Two days later (Day 16), these recipients were given PGF2alpha and observed for estrus. Six to eight days after detection of estrus, plasma samples were collected to determine progesterone concentration and ultrasonography was performed to observe ovarian structures. Heifers with multiple CL or a single CL >15 mm in diameter received an embryo by direct transfer. Embryos of excellent and good quality were thawed and transferred to the recipients by the same veterinarian. Pregnancy was diagnosed by ultrasonography and confirmed by transrectal palpation 21 and 83 days after embryo transfer (ET), respectively. Plasma progesterone concentrations on the day of transfer (Day 7 of the estrous cycle) were 3.9+/-0.7, 4.2+/-0.4,6.0+/-0.4 and 7.8+/-0.6 ng/ml for groups Control, 200, 400, and 600, respectively (Control versus treated groups P=0.009; 200 versus 400 and 600 groups P=0.0001; and 400 versus 600 P=0.012 ). Conception rates 83 days after ET were 41.9, 50.0, 25.0, and 20.9% for groups Control, 200, 400, and 600, respectively (200 versus 400 and 600 groups P=0.0036 ). In conclusion, an increase in progesterone concentration, induced by eCG treatment, did not improve pregnancy rates in ET recipients. Conversely, there was a decline in conception rates in the animals with the highest plasma progesterone concentrations.