Exogenous hormone regimens to utilize successfully mares in dioestrus (days 2-14 after ovulation) as embryo transfer recipients.

Two hormone regimens were utilized for recipient mares which were 2-14 days after ovulation at the time of non-surgical embryo transfer. In Exp. I, 20 embryos were transferred non-surgically into recipient mares which had been given 22 mg altrenogest daily starting the day of recipient ovulation. Higher (P less than 0.05) pregnancy rates (50% vs 0%) were obtained in mares which were 2-6 days after ovulation at the time of transfer compared with mares which were 7-12 days after ovulation. In Exp. II, on the day the donor mare ovulated (Day 0), 10 mg PGF-2 alpha were given to the recipient mare at 0 h, 20 mg oestradiol cypionate at 12, 24 and 36 h, 500 mg progesterone in oil at 48 h and then 22 mg altrenogest at 60, 72 and 96 h. Altrenogest (22 mg/day) was continued until Day 28 (fetal heart beat). The pregnancy rate (58% vs 10%) was higher (P less than 0.05) for the 12 recipient mares which were 10-14 days after ovulation than for the 10 mares that were 5-8 days after ovulation when treatment began. Early embryonic loss was detected in 4/8 pregnant recipient mares between Days 21 and 28 of gestation. We suggest that mares which are 2-14 days after ovulation and not in ovulation synchrony with the donor may be used as embryo transfer recipients; mares which are in early dioestrus can be given altrenogest while those in mid- to late dioestrus can be placed on the more complex hormone regimen.