Mares respond to PGF2a products with CL regression, but only after the CL has formed and the mare is in diestrus (i.e., day 5 after ovulation). For mares, day 0 is ovulation rather than onset of estrus, as in other species. Duration of estrus is variable, and ovulation does not occur until the last one or two days of estrus; therefore, it is not practical to determine the timing of PGF2a administration based on the onset of estrus. Treatment of mares with responsive CL generally results in estrus in two to four days and ovulation in eight to 12 days.
The orally active progestin, altrenogest (Regumate®), is effective for regulating the estrous cycle of mares. Mares having regular cycles should be fed altrenogest daily for 15 consecutive days (see  in Fig. 1). Altren-ogest may be administered by adding it to a concentrate mix or by direct placement on the posterior of the mare's tongue using a syringe. Estrus is expected an average of five days after the last altrenogest treatment and ovulation occurs near the end of estrus.
Either luteolytic or progestin methods can be combined with human chorionic gonadotropin (hCG) or with a gonadotropin hormone releasing hormone (GnRH) implant (Ovuplant®) to control timing of ovulation. , ] These products are generally administered when a follicle sufficiently large (30 to 35 mm) is detected. Ovulation is expected 24 to 48 h later and mating or insemination is recommended 12 to 24 h before ovulation.
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