Preovulatory progestagen treatment in mares fails to delay ovulation

The major objective of this study was to determine whether short-term preovulatory progestagen treatment of mares could effectively delay ovulation. Secondary objectives were to determine the effect such supplementation had on signs of estrus, follicular growth, postovulatory luteal function and pregnancy rate. Thirteen cyclic mares of different breeds were used in this study during the natural breeding season. Once mares were confirmed in estrus with a follicle of 35 mm in diameter, they were assigned in random order to receive no treatment (control), placement of a progesterone-impregnated controlled intravaginal drug releasing device (CIDR) for 2 days, or oral altrenogest treatment (0.044 mg/kg/d) for 2 days. Transrectal ultrasonography and teasing with a vigorous stallion were performed daily. Mares were inseminated every 48 h after the end of experimental treatment (progestagen groups) or beginning when the follicular diameter was 35 mm (control group) with fresh extended semen of a single fertile stallion. Each mare was followed for 3-5 cycles, allowing each treatment to be applied one or two times. Neither CIDR nor altrenogest treatment delayed ovulation. Treatment had no effect on follicular growth rate or the size of the ovulatory follicle immediately preceding ovulation. Both forms of progestagen treatment effectively abolished estrous behavior within 24h. Estrous response to the stallion returned to the control level after cessation of treatment. Similarly, a reduction in endometrial edema was detected during progestagen treatment, which returned to normal after cessation of treatment. Altrenogest treatment tended to reduce the chance of pregnancy (P=0.09) compared to the control group. The use of progestagens to delay ovulation in mares lacks efficacy and may threaten successful establishment of pregnancy.