Use of altrenogest to prepare ovariectomized mares as embryo transfer recipients

Altrenogest was administered to ovariectomized mares to determine if treatment would enable establishment and maintenance of pregnancy after transfer of a 7-d embryo. Three different treatment regimens were used: Group A received 22 mg altrenogest daily starting 5 d before transfer, Group B received 66 mg altrenogest daily starting 6 days before transfer, Group C received 300 mg progesterone in oil intramuscularly daily starting 5 d before transfer. Intact, ovulation-synchronized recipients were used as controls for transfer technique. Pregnancy rates were 1 6 , 2 6 , 2 5 , and 13 19 for Group A, Group B, Group C, and controls, respectively. The pregnancy rate in Group A was significantly different from controls and Group A mares had poor uterine and cervical tone. These results show that ovariectomized mares treated with altrenogest are capable of establishing pregnancy after embryo transfer. Treatment with 22 mg altrenogest appears to be insufficient for optimal pregnancy rates after transfer in ovariectomized recipients.