LITMIR.BIZ

       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.

      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.