IMPROVING THE PRODUCTIVITY IN EQUINE REPRODUCTION


was recently evaluated as a potential predictor of ovulation in mares using a commercial LH kit.21 LH was first detectable beginning 2.5  1.2 days prior to ovulation and continued to be detectable in all mares out to 2 days after ovulation. Conse- quently, a single positive test was not a reliable indicator of impending ovulation.


11. Uterine Edema Pattern


Endometrial edema develops in response to the presence of estradiol and the absence of progester- one. In most mares, a predictable pattern of edema development and regression occurs during a heat period,22,23 which is generally consistent from cycle to cycle in a given mare.1 The amount of edema peaks approximately 2 days prior to ovulation. Ovulation typically occurs when edema is in a stage of decline or is absent.


12. Degree of Cervical Relaxation


The physical characteristics of the cervix change throughout the estrous cycle based on the relative presence of estrogen and progesterone. A mare in heat, in the presence of estradiol 17and an absence of progesterone will have a cervix that is relaxed and draped onto the floor of the vagina, pink in color, and moist. In contrast, a mare in diestrus will have a cervix that is high on the cranial vaginal wall, closed tight, dry, and pale in color. These characteristics can be visualized on speculum examination and tone can be detected by manual palpation per rectum. The greatest degree of change in cervical color and tone occur near ovulation but are not accurate enough alone to indicate time of ovulation.14


13. Periovulatory Ovarian Pain on Palpation


It has been suggested that some mares exhibit ovar- ian pain associated with follicular development.10,24 It is more common that mares exhibit discomfort when the site of a fresh ovulation is palpated per rectum. Consequently, expression of pain or dis- comfort on transrectal palpation of the ovaries can- not reliably be used as a predictor of impending ovulation.


14. Number of Days in Estrus


The average length of the equine estrous cycle was reported to be 20.6 days,14 and the average duration of estrus was 5.7 days. It was further noted that 46%, 32%, and 12% of mares ovulated within 24 hours, 48 hours, or 72 hours, respectively, prior to the end of estrus, while 10% of mares were out of estrus before ovulation occurred.


15. Interval from Prostaglandin Administration


The average interval from prostaglandin adminis- tration to the next spontaneous ovulation is approx- imately 7 to 12 days.2,25 Assessment of follicle diameter at the time of prostaglandin admini- stration can be used to estimate the interval to sub- sequent ovulation. The interval from PGF admin-


Table 2. Interval to Subsequent Ovulation Based on Follicle Size at Time of PGF Administration


Follicle


Size (mm) 10–14


15–19 20–24 25–29 30–34


Number of Cycles


113 84 88 30 15


Average day to ovulation


9.6  0.2a 8.6  0.2b 7.6  0.2c 6.7  0.4d 4.9  0.4e


Note: Data within column with different superscripts are sig- nificantly different (p  0.05).


istration to subsequent spontaneous ovulation is inversely proportional to the diameter of the largest follicle at the time of treatment. In general, mares with small follicles at the time of prostaglandin ad- ministration take longer to develop a dominant fol- licle and ovulate than mares with a larger follicle (Table 2).


Administration of prostaglandin to a mare with a


large diestrous follicle (35 mm) is associated with one of 3 outcomes: (1) ovulation within 48 hours in the absence of uterine edema (14.5%), (2) ovulation after 48 hours accompanied by uterine edema (75.4%), or (3) regression without ovulation followed by emergence of a new follicular wave (10.1%) (un- published data).


16. Interval from hCG or Deslorelin Administration


Administration of an ovulation inducing agent is common in broodmare practice. Typically a follicle that has reached a minimum critical size that is characteristic for the type or breed of mare (ie, 35 mm in diameter for most light horse breeds) will respond to administration of hCG or deslorelin ace- tate and ovulate within 24 to 48 hours if the mare had been in heat for 2 to 3 days.26 If ovulation occurs prior to 24 hours, it is likely that the follicle was already primed to ovulate even before the agent was administered. hCG and deslorelin acetate will usually induce ovulation in approximately 36 and 40 hours, respectively. If used appropriately, it is gen- erally expected that 85% to 95% of mares should ovulate between 24 and 48 hours after administra- tion of hCG or deslorelin acetate.


17. Other Comments


It has been suggested that a majority of mares ovu- late at night.27 However, several studies have in- dicated that ovulations are equally distributed throughout the day in mares.1,3,28


18. Summary


No single morphologic criterion is consistently use- ful to predict interval to ovulation or impending ovulation. In clinical practice, a combination of growth pattern and size of the dominant follicle, in conjunction with an increase in follicular wall thick- ness, development of an irregular shape, softening of


AAEP PROCEEDINGS  Vol. 60  2014 43


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