Why some mares respond to hormones and others don't (Proceedings)

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Why some mares respond to hormones and others don't (Proceedings)

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Aug 01, 2009

The use of ovulatory inducing agents is a key tool in the breeding management of mares. In order to maximize the efficiency of ovulatory inducing hormones it is imperative to have a clear understanding of the physiology and the dynamics of follicular growth during the mare's estrous cycle. The estrus period is characterized by receptivity to the stallion, cervical relaxation, presence of a large or dominant follicle and endometrial edema. The duration of behavioral heat is on the average 5 to 7 days but can vary widely between mares. The diestrus period is characterized by lack of stallion receptivity, presence of a corpus luteum, a tight cervix and lack of endometrial edema (circulating progesterone levels > 2 ng/ml). Although the normal mare diestrus lasts approximately 15 days, mares can, and often do, develop large preovulatory size follicles during diestrus.

Monitoring of the estrous cycle

Ultrasonographic examination of the reproductive tract of the mare has become standard practice. Monitoring and determination of the stage of the estrous cycle, early pregnancy diagnosis, diagnosis of pathological reproductive conditions and monitoring responses to uterine therapies are some of the uses of ultrasonographic technology in equine reproduction.

In order to implement therapies to hasten equine ovulation, the presence, detection and interpretation of endometrial edema is critical. Normal mares under the influence of estrogen will have edematous hyperplasia of the endometrial folds. The endometrial edema increases progressively during the first few days of estrus and decrease as the mare approaches ovulation (Samper et al. 1997). Alterations of this normal pattern such as the presence of endometrial edema after ovulation or during diestrus could be an indication of endometrial inflammation or an underlying problem.

Maximal pregnancy rates are obtained when mares are bred within 48 hours prior to ovulation by natural mating, 12-24 hours prior with cooled shipped semen and less than 12 hours with frozen semen. In order to breed mares at these "ideal" times, it is imperative that estrus be detected and ovulation be predicted accurately.

Induction of estrus

In the normal cycling mare, estrus is induced exclusively by terminating the luteal phase with an injection of prostaglandin. The authors use an intramuscular injection of 7.5 mg of prostaglandin F 2 alpha for this purpose as early as 5 days post ovulation. Onset of estrus and ovulation after a prostaglandin treatment usually occurs in 3-4 and 8-10 days respectively. If there is a large size follicle on the ovaries at the time of treatment, ovulation is possible as early as 72 hours after treatment without overt signs of heat (Samper et al 1993).