THERIOGENOLOGY


post-partum period. Administration of 150 to 300 mg of progesterone with or without addition of 10 to 20 mg of estradiol-17 to post-partum mares have been used in an attempt to delay the first ovulation, enhance uterine involution, synchronize estrus, and/or enhance pregnancy rates.16,17,69–75 In one study, administration of progesterone plus estradiol therapy initiated within 12 hours after foaling and continued for 6 days delayed the first ovulation of the year (day 15.62.6 days vs 10.32.4 days) and increased pregnancy rates (58.5% vs 53.0%) over that of untreated mares.72 The combination of pro- gesterone and estradiol initiated immediately after foaling results in suppression of the normal peripar- turient surge of follicle stimulating hormone and luteinizing hormone, and subsequently delays the development of the first follicular wave postpar- tum.76 Administration of altrenogest (0.044 mg/ kg) orally once per day for 8 days beginning the day after foaling was reported to delay the first post- partum ovulation to 18.2 days and increased preg- nancy rates over that of untreated mares.48 Ecbolic agents have been administered to post-


foaling mares to promote uterine contractions, elim- inate fluid and debris, and/or decrease the size of the uterus in an attempt to enhance foal-heat concep- tion rates.25,77–80 Most studies report that admin- istration of oxytocin or prostaglandins had no effect on increasing the rate of uterine involution or in- creasing pregnancy rate. One study noted an in- crease in foal-heat pregnancy rate when a synthetic prostaglandin analogue was administered twice daily for approximately 10 days beginning on the day of foaling.77 Lavage of the uterus of the post-partum mare has


also been used in an attempt to enhance the rate of uterine involution, remove placental debris, re- duce the degree of inflammation, reduce bacterial numbers, and/or improve foal-heat pregnancy rate.6,8,57,58,81,82 However, controlled clinical trials determined that there is no advantage in performing a routine uterine lavage on post-partum mares that had a normal foaling, no retained placenta, or no prolonged or abnormal lochial discharge.6,57,81,83 A therapeutic uterine lavage may be beneficial in mares that experience a retained placenta, pro- longed lochial discharge or presence of an abnormal volume or adverse character of uterine fluid in the post-partum period.51 Adherence to the principles of antibiotic steward-


ship dictate that use of antimicrobial agents should be justified.84 The routine use of antibiotics in the post-partum mare is controversial and most often not indicated. As noted previously, many post- partum mares have a positive growth on microbial culture and white blood cells are present on uterine cytology, are not treated with antibiotics, and have normal fertility. Systemic antibiotics may be indi- cated in mares with retained placenta, metritis-sep- ticemia, and/or severe trauma to the reproductive tract.34 Intrauterine infusion of antibiotics, if used


142 2020  Vol. 66  AAEP PROCEEDINGS


at all, should follow a uterine lavage to eliminate inflammatory debris which may otherwise bind and inactivate the antibiotics. However, there have been clinical studies that report an increase in post- partum pregnancy rate following intrauterine infu- sion of antibiotics along with either infusion of autologous plasma85 or systemic administration of oxytocin.86


Management Strategy


One management plan to optimize post-partum con- ception rates in mares with an uncomplicated foal- ing is to perform an initial reproductive examination on the mare 6 to 8 days after foaling. The goal of this examination is to evaluate the reproductive tract for trauma associated with foaling, deter- mine the degree of follicular development, note the amount of uterine edema and identify the pres- ence or absence of fluid in the uterine lumen. A subsequent ultrasound examination is per- formed on day 9 or 10. Mares that ovulate before day 10 are not bred, but are administered a dose of prostaglandins 5 to 6 days after ovulation to lyse the corpus luteum and bring them back into heat early. Mares that still have a large follicle 10 days post foaling are bred using standard tech- niques (live cover or artificial insemination). The use of an ovulation-inducing agent such as human chorionic gonadotropin or a gonadotropin-releasing hormone agonist is discouraged until at least day 9 or 10, since ovulations early in the post-partum pe- riod are associated with a lower pregnancy rate and higher embryonic loss rate.


Alternative Breeding Options


If an owner does not want to breed a mare on the foal heat, but also does not want to wait until the 30-day heat, an alternative strategy is to administer a dose of prostaglandins 5 to 6 days after the foal-heat ovulation to short cycle the mare.22,32,33 This strat- egy results in higher per cycle and per season con- ception rates, a lower pregnancy loss rate and ultimately a higher foaling rate than mares first bred on the foal heat. However, it does result in a slightly longer foaling-to-conception interval (34.9 days vs 25.6 days).32 In lieu of multiple ultrasound examinations to determine the day of the foal-heat ovulation, an alternative plan would be to administer a dose of prostaglandins 17 to 20 days after foaling in an attempt to short cycle the mare, assuming that most mares will have ovulated by 12 days post foaling. The obvious risk is not knowing if or when a mare actually ovulated in the early post-partum period.


5. Summary


Factors that lead to consideration of foal-heat breed- ing include the long equine gestation length, photo- period-dependent physiologic breeding season, limited-duration imposed breeding season, designa- tion of January 1 as the official birth date of foals


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