PHYSIOLOGIC OVULATION FAILURE

The spring and fall transition periods are characterized by waves of follicular development and regression without ovulation.³ The spring transition or resurgence phase may last 2 to 3 months. Dominant follicles of a follicular wave may reach preovulatory size (i.e., >35 mm in diameter) late in the spring transition period and yet still regress without ovulating. Ovulation of a dominant follicle marks the end of the spring transition period. A majority of mares subsequently ovulate at approximately 21-day intervals. In the fall, mares may exhibit waves of follicular growth and regression for several weeks after the last ovulation of the year. Follicular activity gradually decreases in association with the diminishing photoperiodic stimulation, and the ovaries of most mares become inactive during the winter months. Large anovulatory follicles may form during the fall transition period and have historically been referred to as “autumn follicles.”^4,5 Autumn follicles are classically described as being hemorrhagic, with a liquid to gelatinous consistency.³

Postpartum mares may also exhibit physiologic ovulation failure. A majority of mares develop follicles and ovulate within the first 2 weeks postpartum and continue to cycle thereafter. Alternatively, a foal heat ovulation may be followed by a variable period of anestrus or anovulation until the mare resumes normal cyclic activity. Finally, some mares may have no significant follicular development or may exhibit moderate to substantial follicular development without ovulation during the immediate postpartum period. Mares in the latter groups may remain anestrous or anovulatory for weeks or months before cyclic ovarian activity is initiated.

A majority of mares that do not cycle after giving birth are mares that foal early in the year. Consequently, it may be difficult to distinguish between postpartum anestrus due to a short ambient photoperiod versus anestrus due to the effects of lactation. In general, failure to ovulate postpartum is more likely to be attributed to seasonal effects than lactation effects.^6,7 However, some mares that do not exhibit significant follicular development in the first 40 to 60 days after foaling or become anestrous following a foal heat ovulation will have rapid follicular development and associated estrus soon after the foal is weaned.^3,8 The incidence of lactation-associated anestrus in mares has been reported to be 21% to 74%.⁷ In contrast, other investigators have reported that suckling had no effect on postpartum ovarian activity.⁹

Poor body condition in late gestation and the early postpartum period may also contribute to poor reproductive performance. The effects of inadequate nutrition and poor body condition may be manifested in delayed return to reproductive cyclicity postpartum, reduced pregnancy rates, and increased embryo loss rates.¹⁰ The phenomenon of “lactational anestrus” may, in fact, represent the combined effects of season, body condition, and lactation.³ Maintenance of late-term pregnant mares due to foal between January and March (Northern Hemisphere) under a stimulatory artificial photoperiod for the last 2 to 3 months of pregnancy may be beneficial. Pregnant mares housed under lights have been reported to foal approximately 10 days earlier than mares not maintained under lights,¹¹ are more likely to have a foal heat ovulation and continued estrous cycles,⁷ and ovulate earlier in the postpartum period.¹²

PATHOLOGIC OVULATION FAILURE

Ovulation failure occasionally occurs during the physiologic breeding season. Anovulatory follicles may be large (5 to 15 cm in diameter), persist for up to 2 months, and result in a prolonged period of behavioral anestrus and a long interovulatory interval.¹³ Specific causes of ovulation failure in the mare are not known but have been suggested to be insufficient pituitary gonadotropin stimulation to induce ovulation,¹⁴ insufficient estrogen production from the follicle itself,¹⁵ or hemorrhage into the lumen of the preovulatory follicle.³ Mares may develop anovulatory follicles without prior exposure to exogenous hormones.²

Anovulatory follicles were reported in a recent study to occur in approximately 8.2% of equine estrous cycles.² The incidence of anovulatory follicles increases with age. Mares 16 to 20 years old were noted to form anovulatory follicles during 13.1% of estrous cycles during the physiologic breeding season. A high percentage (43.5%) of mares that developed anovulatory follicles experienced subsequent estrous cycles with anovulatory follicle formation during the same breeding season. The interval between actual ovulations for mares that developed anovulatory follicles was noted to be 38.5 days.

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