Combination of estradiol cypionate and altrenogest to control ovulation timing in mares

Controlling ovulation timing in mares is used primarily for convenience when frozen or transported semen is used, or to provide recipients for an embryo transfer program. Luteal or progestational phase termination with prostaglandin F 2α or its analogs, results in too variable a time to ovulation to be of practical use (3 - 15 days). Progestogens alone, although capable of inhibiting ovulation, have no control on follicular development. Time from withdrawal of progestogen to ovulation is too variable to be useful in timed insemination with frozen semen or embryo reception. A combination of progesterone and estrogen (P & E) to inhibit both follicle stimulating hormone (FSH) and luteinizing hormone (LH) synthesis and release results in more precise control of follicular development and ovulation. This protocol requires daily intramuscular (IM) injections and use of compounded products. Estradiol-17 β is reported to be the best product, because estradiol cypionate (ECP) and estradiol benzoate are too slowly metabolized and cause delayed or erratic return to estrus. Anecdotal reports dispute this claim. Goal of this study was to determine the usefulness of a combination of commercially available products and minimize the number of IM injections to control ovulation in the mare. Procedures were approved by Oregon State University Animal Care and Use Committee. Ten mares of variable age (17 - 23 years, mean 19.7 years) and size (515 - 626 kg, mean 570 kg) were given altrenogest liquid (Regumate® , Merck Animal Health, Madison, NJ) 0.044 mg/kg orally, once daily, for 10 days and estradiol cypionate (Depo Estradiol® , Pfizer, New York, NY) 0.011 mg/kg IM on day 1 and day 5. Study was performed during normal breeding season for the latitude. None of the mares were examined prior to treatment to determine stage of estrous cycle. Mares were evaluated by transrectal ultrasonography of reproductive tract, had serum progesterone concentrations measured, and received cloprostenol (Estrumate® , Merck Animal Health, Madison, NJ) 0.5 µg/kg IM on day 10. Reproductive examinations were performed daily or on alternate days, from days 15 or 16, to detection of ovulation. Nine out of 10 mares had progesterone concentrations < 1 ng/ml at the end of 10 day study periods. Largest follicle at cloprostenol treatment was 38 mm (< 20 - 38 mm) and second largest was 27 mm. Eight mares ovulated 7.25 - 7.625 days after discontinuation of altrenogest. Even a mare with 38 mm follicle did not ovulate until day 17, despite having < 0.2 ng/ml progesterone at cloprostenol treatment. Two mares did not ovulate within 14 days of discontinuation of treatment. It is concluded that ovulation timing can be controlled in mares using commercial compounds readily available to the practicing veterinarian, although fertility of those ovulations remains to be determined.

Keywords: Mare, ovulation, ECP, altrenogest

This manuscript was originally published in the journal Clinical Theriogenology Vol 12(3) Sept 2020.  Clinical Theriogenology is the official journal of the Society for Theriogenology (SFT) and the American College of Theriogenologists (ACT).  This content has been reproduced on the IVIS website with the explicit permission of the SFT/ACT.