Embryo transfer into ovariectomized
steroid-treated mares was used as a model to evaluate various
progestin/
estradiol treatments and to determine the level of
progesterone necessary for the maintenance of pregnancy in mares. Once a donor mare was in estrus and had a >/=35 mm follicle, an ovariectomized recipient was selected and assigned to one of three groups: 1) 1 mg
estradiol (E(2)) was injected subcutaneously daily until the donor mare ovulated; on the day of the donor mare's ovulation, daily
intramuscular injections of 300 mg
progesterone (P4) were commenced and continued until the end of the experiment (Day 35); 2) E(2) and P4 treatments were identical except E(2) was continued daily until Day 20; and 3) The same E(2) treatment as Group 1, 0.044 mg
altrenogest per kilogram
body weight were administered daily until Day 35. Embryos were recovered 7 d after the donor mare's ovulation and were transferred via surgical flank incision. Twenty additional embryos (controls) were transferred into intact recipients that ovulated 1 d before to 3 d after the donor. Pregnancy rates did not differ (P>0.05) among groups at Days 14 or 35. Pregnancy rates at Day 35 for mares administered
injectable P4 (70%) were identical to those given
altrenogest. Overall, pregnancy rates for ovariectomized-
progestin treated recipients (28 of 40, 70%) were similar (>0.05) to that of intact mares (16 of 20, 80%). Dose of P4 was decreased in Groups 1 and 2 to 200 mg (Days 35 to 39), 100 mg (Days 40 to 44), 50 mg (Days 45 to 49) and 0 mg (>/=Day 50). Blood samples were collected once on Days 34, 35, 39, 40, 44, 45, 49 and 50 and assayed for P4. Dose of
altrenogest was decreased to 0.022, 0.011, 0.0055 and 0 mg per kilogram
body weight at Days 35 to 39, 40 to 44, 45 to 49 and >/=50. Number of mares in Groups 1 and 2 that lost their pregnancy while given 200, 100, 50 or 0 mg P4 was 0, 2, 8 and 4, respectively. Doses of 0.022, 0.011, 0.0055 and 0 mg
altrenogest per kilogram
body weight resulted in 0, 6, 4 and 3 mares aborting.
Fetal death did not occur until concentrations of P4 decreased below 2.56 ng/ml 24 h after injection.