The ultimate goal of insemination is to provide semen in a time frame that coordinates the availability of capacitated spermatozoa with the arrival of the transported oocyte within the mare’s oviduct. It is also the responsibility of the veterinarian to ensure the optimum intrauterine environment that supports the developing embryo. With these objectives in mind, preparation of the mare begins well in advance of the actual anticipated breeding date.
Too often, the fertility of the mare is somewhat overlooked and she is selected as a broodmare candidate simply due to her availability. Truly, her potential fertility needs to be as carefully evaluated as the fertility of the stallion. A breeding soundness examination (BSE) should be performed at the beginning of the breeding season that is based on her age and parity. The BSE may range from a rectal and ultrasonographic examination of the reproductive tract to a cytology, culture and/or biopsy of the endometrium to a videoendoscopic examination of the endometrium. Inclusion of an ultrasound examination at every rectal exam enhances the continual education of the veterinarian and aids in detection of many changes that are not palpable, thereby increasing pregnancy rates.
In young maiden mares, especially those that have never raced, a rectal and ultrasonographic exam may constitutea a sufficiently adequate prebreeding examination. In this author’s opinion, in the foaling mare a cytologic evaluation of the endometrium should be included. This test is a simple stall-side procedure that provides immediate information on the status of the lining of the endometrium. If significant numbers of polymorphonuclear leukocytes (PMNs) are present on the smear, the mare has endometritis and its etiology needs to be investigated before proceeding with insemination. Bacteria and yeast forms also may be detected with a cytologic exam.
During the estrous phase of the heat cycle, an ultrasound examination of the reproductive tract usually supports the rectal palpation findings of a dominant, growing follicle, in addition to the classic “spokewheel” pattern of endometrial edema, which may be subjectively quantified. The ultrasound examination also may demonstrate the presence of echogenic particles within the follicle in addition to increasing echogenicity of its wall — both indicative parameters of ovulation within the next 24 hours. Although the duration of heat in the mare may be variable, most mares ovulate near the end of this estrous phase. Interestingly, the edema is less apparent on ultrasound just before ovulation. Because the timing of insemination with respect to ovulation is so critical, the presence or absence of this edema can be a powerful tool used to optimize pregnancy rates through the control of ovulation timing. A more detailed discussion is provided in site.
A number of agents that shorten the interval to ovulation in the mare have been investigated. The most effective agents possess luteinizing hormone (LH) activity with varying degrees of follicle stimulating hormone (FSH) activity. Human chorionic gonadotropin (hCG) with its potent LH-like activity, is, at the time, the least expensive and perhaps the most popular agent used for the induction of ovulation. It has been reported to be effective at doses ranging from 1000 to 5000 IU given intramuscularly, intravenously, or subcutaneously. The author tends to base the dosage on size of the mare; very large breeds receive larger doses and breeds such as the Miniature Horse or small ponies receive the minimum dose of hCG.
The use of hCG in the mare is somewhat controversial. First, with its repeated use, antibody development has been documented in several studies; however, the clinical impression of many practitioners is not in agreement with these studies. Secondly, mare owners often complain that their mares experience pain associated with administration of some hCG products. Finally, the reliability of hCG in its ability to hasten the interval to ovulation, especially in the older or compromised mare, has been questioned. Regardless of its potential disadvantages, hCG remains a popular, inexpensive and effective means of inducing ovulation in the majority of mares.
Synthetic gonadotropin-releasing hormone (GnRH) analogs (deslorelin acetate and buserelin) also have proved effective in inducing ovulation. Although the use of these latter agents may delay slightly the mare’s return to estrus if she fails to conceive, in this author’s experience these GnRH analogs are more reliable in inducing ovulation, especially in mares more prone to ovulation failure. These include older mares, mares in vernal transition, and mares concomitantly treated with prostaglandin inhibiting agents such as many of the antiinflammatory drugs.
Ovulation-inducing agents are far more reliable and effective if given at the appropriate time during estrus. If endometrial folds are apparent on the ultrasound examination, and a dominant softening follicle is present (usually >30 mm in diameter) hCG, deslorelin acetate, and buserelin are expected to hasten ovulation on average of 36, 41 to 48, and 24 to 48 hours, respectively, after administration of the induction agent. Samper (see readings list) reported that 98% of mares with maximal endometrial edema given hCG or deslorelin would consistently ovulate with 48 hours of administration.