Review of Techniques for Prediction of Ovulation in the Mare

A combination of growth pattern and size of the dominant follicle, increase in follicular wall thickness, development of an irregular shape, softening of the follicle, and a decrease in endometrial edema is suggestive of impending ovulation. Authors’ address: Colorado State University, Department of Clinical Sciences, Fort Collins, CO 80523; e-mail: Patrick.McCue@ColoState.edu.

1. 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 potential interval to ovulation is accomplished in clinical practice by interpretation and integration of multiple factors on an individual mare (Table 1). The goal of this paper is to review the parameters that can be used in clinical practice to predict impending ovulation in the mare.

2. Reproductive History

An individual mare will often ovulate a follicle of approximately the same diameter each cycle.¹ 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.
 

3. Growth Pattern of Follicle

A dominant follicle will typically increase in diameter by 2.7 to 3 mm per day during estrus, reach a maximum diameter, and subsequently remain the same size for approximately 2 days prior to ovulation.^2-4 The diameter of the follicle may decrease by 2 to 3 mm in the 12 hours preceding ovulation.^3,5 The growth pattern of the dominant follicle is disrupted if a mare receives human chorionic gonadotropin (hCG) or deslorelin acetate to induce ovulation.¹ The ovulation induction agent is generally administered when the mare is in behavioral estrus or has endometrial edema visible on ultrasound and the dominant follicle first reaches a size that is appropriate (often ≥35 mm in diameter for light horse breeds). Ovulation will typically occur 36 to 40 hours following administration of hCG or deslorelin, respectively. The induced follicle often ovulates before it attains its maximum potential diameter.^1,6

4. Diameter of Follicle

The potential diameter of the preovulatory follicle prior to ovulation can often be predicted based on mare size and breed. In addition, mares typically ovulate follicles that are similar in size in consecutive cycles.¹ In general, mares of smaller light horse breeds will ovulate a follicle that is smaller in diameter than mares of larger breeds. Friesian mares are notorious for developing very large follicles (ie, 50 mm or greater) that remain present for several days prior to ovulation (Stout, personal communication). It has been noted that the diameter of a preovulatory follicle is 5 to 8 mm larger early in the ovulatory season than later in the season and is also greater in single-ovulating mares than mares with unilateral or bilateral double ovulations.⁷ It has been suggested that the diameter of the dominant follicle may be the most reliable single criterion in the prediction of ovulation,^3,7 but follicle diameter alone is not useful in predicting ovulation in the 48 hours preceding spontaneous ovulation.⁸

5. Shape of Follicle

A developing dominant follicle is typically spherical in shape during most of the estrous period. It has been reported that 84% to 89% of follicles change shape from spherical to nonspherical (pear-shaped or conical) prior to ovulation.^2-4 A “cone” or “point” or “stigma” may develop as the follicle tunnels toward the ovulation fossa in the hours preceding ovulation. The follicle may become more flattened or irregular in the 3 hours prior to ovulation.

6. Changes in the Follicular Wall

A developing dominant follicle has a relatively thin follicular wall during early to mid-estrus. Follicular wall thickness increases progressively as the interval to ovulation decreases.⁴ The echogenicity is potentially due to an increase in the number of granulosa cells, accumulation of mucosubstances between cells⁹ or preovulatory luteinization of granulosa cells. Unfortunately, the thickening generally occurs too early to be useful in predicting ovulation.¹⁰ An increase in the echogenicity of the follicular wall⁵ and an increase in prominence of an anechoic layer beneath the granulosa¹⁰ have been noted as ovulation approaches. The anechoic layer has been suggested to be due to hypervascularity and edema of the theca interna. A rent or tear in the follicular wall was observed in 77% of mares prior to ovulation.⁵ The duration of ovulation, defined as a rapid decrease in follicular size, has been reported to occur in an average of 42 seconds (range 5–90 seconds).⁵ Serrations of the inner surface and/or outer periphery of the granulosa cell layer, as indicated by an irregular or notched appearance, was noted in 37% of mares 24 hours prior to ovulation and 59% of mares within 12 hours of ovulation.¹¹

7. Tone of Follicle

A developing dominant follicle has a firm tone during the early and middle part of the normal growth phase. The dominant follicle may become noticeably softer as detected by manual palpation within the 12 to 24 hour period prior to ovulation.^3,12,13 In one study, a perceptible softening of the follicle was noted in 28% of mares prior to ovulation, and some follicles softened and then became turgid again prior to ovulation.¹⁴ In another study, it was reported that 90% of follicles were softer at 12 hours before ovulation than they had been at 72 hours prior to ovulation.¹⁴

8. Vascularity with Doppler Ultrasound

Examination of the dominant follicle in color Doppler-mode revealed that the percentage of the follicle with color display and the prominence of the signals increased between 36 and 12 hours prior to ovulation but decreased during the last 4 hours prior to ovulation.¹¹ In addition, a loss of color Doppler signals was evident at the apex of follicles that had lost their spherical shape.

9. Evaluation of Follicular Fluid

Echogenic particles within the follicular fluid, presumed to be clusters of granulosa cells, have been noted in 50% to 54% of equine preovulatory follicles^5,15 but is not common enough or consistent enough to be predictive of impending ovulation.¹⁰

10. Endocrine Markers

Measurement of estrogen concentration, either estradiol 17β, total estrogens, or conjugated estrogens, have been used in an attempt to predict ovulation in mares.^3,16,17 In general, estrogen levels increase in relation to the increase in follicle diameter during estrus, peak approximately 2 days prior to ovulation, and decline near the day of ovulation.^18,19 Unfortunately, the pattern of estrogen secretion is not consistent to be a useful diagnostic marker for prediction of ovulation. Luteinizing hormone concentrations increase gradually during estrus in the mare and reach peak levels near the day of ovulation.^19,20 [...]