Effect of exogenous progesterone on pregnancy rates after surgical embryo transfer in mares

A completely randomized experimental design was used to investigate the effect of supplemental progesterone on pregnancy rates of recipient mares. Every other recipient mare received daily 200 mg progesterone in oil beginning the day of surgical embryo transfer and lasting until either Day 120 of pregnancy or until pregnancy failure was confirmed by ultrasound. Progesterone supplementation did not affect pregnancy rate (P > 0.05). Overall, embryos that did not result in pregnancy were of greater mean diameter than embryos that resulted in pregnancy (P < 0.05). Pregnancy rates tended (P < 0.1) to be greater in recipients that were detected to be ovulating the same day or prior to that of the donor and that had been supplemented with progesterone (75 %) as opposed to untreated control mares of the same synchrony group (40 %). Progesterone supplementation did not affect the incidence of embryonic loss; however, there was a slightly higher loss of pregnancies between Day 15 and 30 in treated versus untreated recipients. There was no effect (P > 0.05) of treatment on pregnancy rate for embryos recovered from fertile versus subfertile donor mares. However, overall, there tended (P < 0.1) to be fewer pregnancies with embryos recovered from subfertile (50 %) as compared to fertile donors (75 %). It was concluded that supplemental progesterone at the dosage and frequency described was not beneficial in improving pregnancy rates in cyclic recipient mares after surgical embryo transfer.