170 Pregnancy Rates of Noncycling Recipient Mares Treated With a New Formulation of Long-Acting Progesterone

Abstract

The mare being a seasonal polyestrous animal is a limiting factor for maintenance of embryo transfer programs during the entire year.At the beginning and end of the breeding season, the percentage of recipients showing normal estrous cycles may be low compared to donors. Exogenous progesterone has been administered to acyclic mares to allow them to be used as recipients. Noncyclic recipient mares treated once a week with long-acting progesterone (P4) have been widely used in Brazil in equine embryo transfer (ET) programs. P4LA was developed for weekly administration of 1.5 g of P4 to acyclic recipients until approximately 120 days of pregnancy. The aim of the present experiment was to evaluate the pregnancy rates of noncyclic recipients treated with a new formulation of long-acting P4 developed in our laboratory that allows a longer interval between treatments (14 days). Noncyclic mares (n = 36) were used as recipients between August and September of 2008 (transition between winter and spring in Brazil). Noncyclic mares were treated with 5 mg of estradiol benzoate during 2 consecutive days followed by the application of 10 mL (i.m.) of P4LA containing 300 mg mL-1 of P4, for a total P4 dose of 3 g. Embryos were transferred between 6 to 12 days after P4LA injection. The same dose of P4LA was repeated on the day of ET and every 14 days until 110 days after pregnancy diagnosis. One hundred fifty-five cyclic recipients were used as a control group, with embryos transferred 4 to 8 days after ovulation. No differences were observed (P > 0.05) when comparing pregnancy rates from acyclic recipients treated with P4LA (82%, 56/68) or cyclic recipients (88%; 137/155) on Day 15. The rate of embryonic loss at 50 days was similar (P > 0.05) for noncycling (11%) and cycling recipient mares (10%). The results obtained in the present study demonstrated that the new formulation of P4LA containing 300 mg mL-1 of P4 and administered every 14 days was effective in maintaining pregnancy in noncyclic recipient mares, allowing a larger treatment interval and longer window of time to transfer embryos.