How to manage the anovulatory mare during breeding season

Introduction

The female of the equine species is stimulated to cycle with increasing exposure to day length. Consequently, mares are considered seasonally polyestrus which means that they have a long period of regular cyclicity also known as the breeding season where mares have inter estrus intervals of 15-16 days and inter-ovulatory intervals of 20-21 days. In the northen hemisphere the breeding season extends from late March or early April until early October. In the majority of the mares this period of regular cyclicity is marked by variable transitional phase on both the early (spring) and late (fall) in which mares might show behavioral estrus but fail to have regular ovulatory cycles. The start of the ovulatory season in mares is determined by the first ovulation of the year, except in mares that foal early in the year (January, February) and have not been exposed to artificial light. These mares often will have an initial ovulation and revert to anestrus for another few weeks. In between the late fall transition and the early spring transition most mares have a period of anestrus where follicular activity is suppressed. Since the breeding season is limited in the northern and southern hemispheres, mares that fail to cycle during this period pose an economic burden to the breeding operation.

Conditions that interfere with regular cyclicity

a) Inter-estrus and inter-ovulatory intervals for mares that are bred to carry their own pregnancies inter-estrus intervals (16 days) and inter-ovulatory intervals (19-22 days) or are key indices for brood mare management. Prolonged luteal phases are presumably caused by a block of the endometrial release of prostaglandin at around 15 days. On the other hand, short luteal phases are likely caused by an early release of uterine prostaglandin causing a premature lysis of the CL. Both of these conditions are not normal and should be investigated further either with a culture and/or preferably by evaluating a uterine biopsy to insure that the uterine health is normal. It is imperative that veterinarians have an exact day of ovulation from the previous cycle in order to accurately evaluate those indices. Pregnancy and possible early embryonic death after day 14 must also be ruled out in cases of prolonged luteal phases.

b) Anovulatory follicles- Follicles that fail to ovulate are a frustrating condition for veterinarians especially when mares are being bred by artificial insemination. Anovulatory follicles either bleed into the antrum creating a localized hemorrhage or keep growing after the expected day of ovulation. In either case these follicles fail to release an oocyte and consequently the mare is not able to become pregnant. It is important that veterinarians recognize the failure ton evacuate follicular fluid and ovulation since often the organized anovulatory hemorrhagic follicle (AHF) could be similar to a corpus luteum or a hemorrhagic CL Unfortunately there are no markers or indicators that would reliably help predict the formation of an AHF. However, in the author’s experience there are certain mare conditions that increase the risk of developing AHF. Some of these conditions include:

1- Failure to ovulate within 48 hrs after an ovulatory agent is administered- Timely administration of ovulatory inducing agents is imperative for good mare breeding management and efficient stallion use. However mares that are treated with the appropriate dose when a dominant follicle(s) is present and the mare has marked uterine edema and fail to ovulate within 48 hrs of administration are at a high risk of developing AHF’s.

2- Mares with insulin resistance or other endocinological issues- These mares although not completely acyclic, characteristically will have multiple small to medium follicles for several weeks and fail to ovulate on a regular basis. For these mares it is recommended to place them on constant pergolide therapy.

3- Delayed follicular growth due to endometritis- As discussed above mares with uterine infections can have a long period of poor follicular growth and in extreme cases anestrus. This can be often seen in post-partum mares that have foaled under poor hygienic conditions.

4- Chronic stress/pain- Social stress in a herd situation or the stress from pain due to osteoarthritic conditions, laminitis or social stress have the potential to interfere with follicular growth and ovulation and consequently the development of AHF’s.

Conditions that result in acyclicity

Permanent lack of cyclicity although not common in general is due to either permanent conditions such as chromosomal or genetic or to acquired conditions that have conditions that have been failed to be diagnosed.

A clinician evaluating a mare for lack of regular cycles or anestrus mut insure that there is no iatrogenic cause for the condition such as exogenous use of progesterone, progestogens, estrogens or anaboloic steroids.

Chromosomal problems most commonly the 63 XO condition also known as Turners syndrome. These mares will have a smaller body, small genitalia and underdeveloped uterus and ovaries. However with more recent techniques and the sequencing of the equine genome, new conditions of permanent a cyclicity have been surfacing in the face of a normal 64 XX karyotype. These mares in contrast to the 63XO mares, have normal body size, and genitalia but have very small ovaries with follicles that do not grow past 5-10 mm and have an inability to respond to exogenous gonadotropin stimulation.

Breeds such as Arabians and ponies are well known for their longevity. In addition with mares from other breeds with proper husbandry and veterinary care can also reach advanced age. In addition mares that are in regular and prolonged Transvaginal Oocyte Aspiration programs are likely to go through “menopause” due to lack of follicles for recruitment.

As described above, mares that foal under poor hygienic conditions are at high risk of having uterine contaminants that could affect cyclicity. In some of these mares one could argue that they have a period of lactational anestrus. However in my experience primary lactational anestrus as seen in other species is a rare condition in the horse, unless the mares are under a severe malnutrition and with very low body conditions scores while nursing a foal. Mares without a foal at foot but under a constant plane of poor nutrition are also at a high risk of falling into a prolonged period of acyclicity.

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