Are pony and larger mares similar as recipients for non‐surgical transfer of Day 7 embryos?

Abstract

SummaryNon‐surgical transfer of equine embryos to nine recipient pony mares failed to result in any pregnancies whereas transfer of nine embryos to larger Selle‐Français recipient mares resulted in four pregnancies. Plasma progesterone profiles in the recipient and sham transfer control mares showed the occurrence of early luteolysis (before Day 12 after ovulation) in three of the nine recipient pony mares whereas luteolysis occurred at the normal time (Day 14 to 15) in the control mares and the non‐pregnant Selle‐Français and pony recipient mares. Daily plasma PGFM measurements showed the occurrence of PGF release on the day before luteolysis in all groups, except the recipient pony mares which underwent early luteolysis. In these animals, luteolysis occurred four days after the initial PGF release. PGFM profiles in closely timed serial blood samples taken on the day of transfer showed that cervical transfer induced PGF release from 1 min to 6 h after transfer in both breeds. This release was not directly responsible for non‐pregnancy in the pony recipient mares; in all three pony recipient mares which underwent early luteolysis, this immediate post‐transfer PGF release was followed by an additional release on the day after transfer. This suggests that the uteri of pony mares respond differently to those of larger mares with regard to mechanical stimulation of the cervix and/or bacterial or other types of endometritis. Clearly, pony and Selle‐Français mares are not similar as recipients for non‐surgical transfer of Day 7 equine embryos.