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Оглавление9 Prediction of Ovulation
Patrick M. McCue
Equine Reproduction Laboratory, Colorado State University, USA
Introduction
A decision on when to breed a mare by live cover or artificial insemination is usually dependent on an accurate prediction of impending ovulation. Prediction of the time of ovulation may be accomplished by interpretation and integration of multiple factors for an individual mare. A majority of factors used to predict ovulation are based on normal physiological events in mares that have not been administered an ovulation induction agent.
Technique
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).
It may be difficult to predict the interval to the next fertile ovulation in a mare with a large diestrous follicle ( ≥35 mm) at the time of PGF administration. Mares that ovulate a large diestrous follicle within 2 days after PGF will usually not express behavioral estrus, will not develop uterine edema, and the ovulation is generally not fertile. Mares that ovulate a large diestrous follicle more than 2 days after PGF will usually come into heat, develop uterine edema, and the ovulation is considered to be fertile. The third possibility is that the large diestrous follicle will regress and a different follicle will develop and eventually ovulate at a time interval related to the diameter of the next dominant follicle at the time of PGF administration.
Table 9.2 Interval from prostaglandin F2α (PGF) administration to spontaneous ovulation based on follicle diameter at the time of treatment.
| Follicle Diameter at PGF | Interval to Ovulation (days) |
|---|---|
| 10 mm | 10.4 ± 1.5 days |
| 20 mm | 9.2 ± 1.6 days |
| 25 mm | 8.2 ± 1.6 days |
| 30 mm | 7.1 ± 2.1 days |
| ≥35 mm | Possible outcomes10% ovulate dominant follicle within 2 days80% ovulate dominant follicle 3 or more days after PGF; 68% of these ovulate within 3–6 days and 32% of these ovulate 7 or more days later10% regress the dominant follicle and develop another follicle that ovulates 10–12 days after PGF |
Interval from hCG, deslorelin, or histrelin administration. The duration from administration of hCG, deslorelin acetate, or histrelin to subsequent ovulation is reasonably consistent, provided that the guidelines for administration are followed. Administration of hCG will typically result in ovulation in 24–48 hours, with an average of 36 hours. Administration of deslorelin or histrelin will usually result in ovulation in 36–42 hours, with an average of 40 hours. Administration of an ovulation induction agent at an appropriate time during estrus will typically advance the ovulation by 1–2 days prior to the day of the expected spontaneous ovulation.
Uterine edema pattern during ultrasound examination. Endometrial edema develops in response to the presence of estradiol and the absence of progesterone. In most mares, a predictable pattern of edema development and regression occurs during each heat period (Figure 9.1, see Chapter 8). Estradiol‐17β concentrations and the amount of edema both peak 1–2 days prior to ovulation. Ovulation typically occurs when edema is in a stage of decline or is absent.
Thickness of the follicular wall. Ultrasound examination of the developing dominant follicle will reveal a relatively thin follicular wall during early to mid‐estrus. Final maturation of the follicle is associated with an increased thickness of the follicular wall due to an increase in vascularity around the follicle and pre‐ovulatory luteinization of granulosa cells. In many cases a hypoechoic ring surrounds a more hyperechoic follicular wall. In contrast, the wall of a diestrus or early estrus follicle is not distinguishable from the surrounding ovarian tissue.
Shape of the follicle. The pre‐ovulatory follicle is typically round or slightly oval as viewed during a two‐dimensional ultrasound evaluation. A “cone” or “point” may develop as the follicle tunnels toward the ovulation fossa in the hours preceding ovulation.
Vascularity with Doppler ultrasound. Examination of the dominant follicle by Doppler ultrasound will reveal an increase in vascularity surrounding the follicle between 36 and 12 hours prior to ovulation followed by a decrease in color Doppler signals during the last 4 hours prior to ovulation (see Chapter 76).
Degree of cervical relaxation. The physical characteristics of the cervix change throughout the estrous cycle due to the varied influence of estrogen and progesterone. These characteristics can be visualized on speculum examination and tone can be detected by manual palpation per rectum (see Chapter 10). A mare in heat, in the presence of estradiol 17β and an absence of progesterone will have a cervix that is relaxed and draped onto the floor of the vagina, that is pink in color and moist. In contrast, a mare in diestrus will have a cervix that is high on the cranial vaginal wall and is closed tight, dry, and pale in color.Figure 9.1 Pattern of endometrial edema relative to ovulation.
Peri‐ovulatory ovarian pain on palpation. Many mares will exhibit discomfort when the site of a fresh ovulation is palpated per rectum. Some mares will also be sensitive to palpation of the ovary as the time of ovulation approaches.
Further Reading
1 Ginther OJ. 1979. Reproductive Biology of the Mare: Basic and Applied Aspects. Cross Plains, WI: Equiservices.
2 McCue PM, Scoggin CF, Lindholm ARG. 2011. Estrus. In: McKinnon AO, Squires EL, Vaala WE, Varner DD (eds). Equine Reproduction, 2nd edn. Ames, IA: Wiley Blackwell, pp. 1716–27.
