Why is this mare not exhibiting normal estrous cycles and what can I do? (Proceedings)
A. Mares that fail to cycle
By definition, the complete reproductive cycle includes a period of estrus, concluding with ovulation and a period of diestrus when progesterone levels are elevated. Observing periodic estrous behavior does not prove cyclicity. Evidence that ovulation is occurring can be from sequential data from ovarian palpation, presence of a corpus luteum as shown by ultrasonography or elevation of serum progesterone levels. Presumptive evidence of ovulation includes firm uterine and cervical tone and presence of follicular activity on the ovaries.Seasonal effects on the reproductive cycle are a common cause for failure to cycle or abnormal cycles in the winter and spring. In the winter the content and secretion of the GnRH from the hypothalamus is drastically reduced. Shortly after the winter solstice GnRH secretion begins to increase through mechanisms not completely understood. Follicular Stimulating Hormone increases presumably in response to the increase in GnRH and follicles begin to develop on the ovaries. During this time little if any LH is secreted as the gene for LH synthesis is essentially turned off. The pattern of follicular growth during vernal transition is relatively predictable, with increased size and numbers of follicles. Of major importance, the first several follicles that form in vernal transition do not ovulate, although they may reach normal preovulatory size (>30 mm). Mares make on average 3.7 ±.9 follicles that reach a size of 30 mm or greater that do not ovulate during the transitional phase. These follicles are not steroidogenically competent and do not produce estrogen. This leads to reproductive inefficiency as it is difficult to know whether a given follicle is competent and whether it will ovulate. Monitoring development of a follicle over time may be useful in determining the eventual status and outcome of a transitional follicle because the growth rate of follicles destined to regress is considerably slower than that of the follicle which eventually ovulates. Shedding of the long winter hair coat in spring is a rough indicator of impending ovulation as shedding is closely associated with reproductive renewal. While it is not known what factors contribute to the development of the first competent follicle, it is clear that this follicle, destined to be the first to ovulate in the year, is steroidogenically competent. The first ovulatory follicle of the year is accompanied by a surge of plasma estradiol that is followed by a surge in LH.
The transitional period can be accompanied by long periods of erratic ovarian and estrous behavior. Ovaries of mares in late transition usually have several follicles of varying sizes and maturity. The associated prolonged estrous behavior can last from 1 week to 4 months or more. Clients unfamiliar with the seasonality of the reproductive cycle of the mare will typically assume that the mares have ovulated and may breed the mare during this time.