Induction of estrus and ovulation: why some mares respond and others do not

The two most common procedures for breeding management of mares involve induction of luteolysis and induction of ovulation. Although both of these events are usually achieved, physiologic conditions affect the timing of the response. In a diestrus mare treated with prostaglandin F(2alpha) (PGF), or a PGF analogue, it is well documented that, on average, the interval from treatment to the onset of estrus is 3-4 days, whereas ovulation occurs 8-10 days after treatment. However, the diameter of the ovulatory follicle, as well as its status at the time of PGF treatment, determines the intervals from treatment to onset of estrus and to ovulation; these intervals can range from 48h to 12 days. Ovulation is routinely induced with human chorionic gonadotropin (hCG), recombinant LH (rLH), or the GnRH analogue Deslorelin. On average, ovulation occurs approximately 36h after treatment, but the effectiveness of any of these treatments can be affected by the stage of the estrus cycle, follicle size and maturity.