chapter  2
Reproductive system _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
Pages 140

Eventually, a follicle progresses through to maturity and ovulates, thereby initiating the ovulatory phase. Plasma FSH levels rise early in the transitional period, causing follicular development, but steadily fall approximately 15-20 days prior to the first ovulation. Plasma luteinizing concentrations are initially low, meaning ovulation will not occur, but they increase slowly until a few days immediately prior to the first true estrus, with a peak around ovulation. Developing follicles in the ovary produce estrogen, but in the transitional mare their levels are not high enough to trigger rises in GnRH and LH, which will lead to the first ovulation. Eventually, a follicle does develop sufficiently to trigger these changes and the mare will show estrous behavior and ovulate. Once the first ovulation has occurred, the mare usually continues to ovulate regularly throughout the remainder of the ovulatory season. The sometimes prolonged transitional phase before a mare starts to ovulate regularly each spring can cause problems for breeders who are keen to breed early foals. This is particularly acute in the northern hemisphere Thoroughbred industry because of the imposed breeding season, which starts in February, at which time only approximately 30% of mares are naturally cycling. This contrasts with the physiologic breeding season for the mare, which starts around March to April in the northern hemisphere (August to midSeptember in the southern hemisphere) and runs through to the early autumn. Unnecessary, repeated breeding during this transitional phase can be minimized by careful management and this will avoid contaminating mares and over-using stallions.