Hormone supplementation protocol using estradiol benzoate and long-acting progesterone is efficient in maintaining pregnancy of anovulatory recipient mares during autumn transitional phase

The present research sought to determine whether the administration of estradiol benzoate and long-acting progesterone to anovulatory recipient mares could maintain the pregnancy after embryo transfer during the autumn transitional phase. Recipient mares (n = 40) received the hormonal supplementation (treated group) whereas the other 36 served as a control. The control group consisted of mares having typical estrous cycles with ovulations, development of a viable corpus luteum and received one transferred embryo 5 days after ovulation. Hormonal administrations in the treated group started 8 days before the embryo transfer. During the first 3 days, the mares received estradiol benzoate (5 mg the first day, 3 mg the second day and 2 mg the third day). At Day 5 subsequent to ovulation, the mares received one administration of 1500 mg long-acting progesterone, and the same treatments occurred at the day of embryo transfer. Afterwards, treated mares also received 1500 mg long-acting progesterone every 7 days until 120 days of gestation. For both control and treated groups, the recipient mares were classified as acceptable, marginally acceptable or unacceptable for embryo transfer, and the embryo quality was also determined. The pregnancy diagnosis in recipient mares was made at Days 13, 30 and 60 of pregnancy. While the pregnancy rate was greater (P < 0.05) in the treated than in the control group, the recipient classification did not influence pregnancy rates. In conclusion, pregnancy in anovulatory recipient mares during the autumn transitional phase can be achieved when estradiol benzoate and progesterone are administered.