LITMIR.BIZ

       Reproductive history. An individual mare will often ovulate a follicle of approximately the same diameter each cycle. Consequently, data from previous cycles can often be used to predict follicle size at ovulation of subsequent cycles. Unfortunately, some mares will ovulate a dominant follicle during one estrous cycle that is of a very different size, either markedly smaller or larger, than follicles of other cycles. Prediction of ovulation is difficult in these mares and more than one breeding may be required so the cycle is not missed.

       Season of year. Follicular growth is often slower and the period of estrus longer during the first cycle of the year (short daylight length). Conversely, follicular growth is usually more rapid and the interval from onset of estrus to ovulation shorter during the middle of the physiological breeding season (long daylight length). Decisions on when to schedule mating or insemination need to be made earlier in the cycle during the middle of the breeding season.

       Follicular growth pattern. A dominant follicle will typically increase in diameter by 3–5 mm per day and occasionally up to 10 mm per day during early to mid‐estrus. The follicle will reach its maximum diameter and subsequently remain approximately the same size for 2 or 3 days prior to ovulation in mares not administered an ovulation‐inducing agent. This pattern is disrupted in mares receiving human chorionic gonadotropin (hCG) or gonadotropin‐releasing hormone (GnRH) agonists, such as deslorelin or histrelin, as a developing dominant follicle may ovulate prior to attaining maximum diameter.

       Diameter of follicle and breed. The maximum diameter of the pre‐ovulatory follicle can often be predicted based on mare size and breed. In general, mares of smaller light breeds will ovulate a follicle that is smaller in diameter than mares of larger breeds (Table 9.1). Friesian mares are notorious for developing very large follicles (i.e., 50 mm or greater) that remain present for several days prior to ovulation. A decision to administer an ovulation‐inducing agent or when to cover or inseminate a mare should be based in part on breed and follicle diameter. For example, it would not be appropriate or effective to attempt to induce ovulation of a 35 mm follicle in a draft mare, but would be appropriate in an Arabian or Quarter Horse mare.

       Softness of follicle. A developing dominant follicle has a firm tone during the early and middle part of the normal growth phase. The dominant follicle usually becomes noticeably softer, as detected by manual palpation, within the 24‐hour period prior to ovulation.Table 9.1 Average diameter of pre‐ovulatory follicles in various mare breeds.BreedFollicle Size at Ovulation (mm)Arabian35–45Quarter Horse35–45Thoroughbred45–55Warmblood45–60Draft50–60

        Number of days in estrus. Mares are usually in estrus for 4–7 days and the interovulatory interval is approximately 21 days. Ovulation typically occurs 3–6 days after the onset of behavioral estrus. Mares are usually in heat for 1–2 days after ovulation, until the concentration of progesterone increases sufficiently to block behavioral estrus.

       Interval from prostaglandin administration. The average interval from prostaglandin F2α (PGF) administration to the next spontaneous ovulation is approximately 5–11 days with an average interval of 8–9 days. It is important to understand that mares will return to estrus and ovulate in a reasonably predictable time period, based on the diameter of the largest follicle at the time of prostaglandin administration. As a consequence, ultrasound examination of the mare and recording follicle diameter at the time of PGF administration can be of great benefit in predicting when the mare should be bred and when ovulation may occur. In general, mares with small follicles at the time of PGF administration take longer to develop a dominant follicle and ovulate than mares with a larger follicle (Table 9.2).