Management of a recipient herd is a daunting and labor-intensive task. This task begins in the fall of the previous year. During this period, it is important to acquire the total number of mares that are needed for the coming breeding season. New mares that may introduce contagious disease into the herd should not be added during the breeding season; the risk is too great among both the nonpregnant and pregnant mares. All individual mares should have a means of permanent identification to avoid any confusion regarding which mare is pregnant with which foal.

One of the most common complaints among mare owners concerns the quality of the recipient mares; therefore, selection of recipients that are reproductively sound is important, as is musculoskeletal soundness, dental health, vision, and behavior. The examination should include evaluation of udder health, because the ability to provide sufficient milk is a requisite for raising a healthy well-grown foal. All mares should pass a standard with regard to temperament and disposition. At the least, mares should be halter broke and gentle enough to catch in a small pen or stall. Fall and winter are also good times to manage vaccination, deworming, and Coggins tests, before mares are admitted into the recipient herd. Testing and vaccination for equine arteritis virus should be addressed where geographically indicated.

If determination of the day of ovulation becomes the primary focus, then a difficult aspect of recipient mare management, which is endometritis, is easily overlooked. Therefore, a second, equally important goal of management of open recipient mares is to ensure that they are reproductively sound and ready to receive an embryo. Begin by considering only mares that are 3 to 12 years of age for admission into the recipient herd. This should eliminate most mares with substantial chronic endometritis that would interfere with the ability to maintain a pregnancy to term. Next, evaluate the anatomy of the perineum and vulva. Very thin mares often have poor perineal anatomy, which is improved when they gain weight. If indicated, a Caslick’s operation should be performed. The importance of this cannot be overstated because bacterial contamination at the time of embryo transfer is a frequent cause of embryonic loss. It is apparent, then, that transrectal palpation and ultrasound examinations during estrus serve not only to establish the day of ovulation but also to evaluate the condition of the uterus. If indicated, uterine swabs should be procured for microbial culture. In our view, cultures should be taken only when the mare is in advanced estrus. The mare should have at least a 35-mm follicle that is beginning to mature, and the uterus should demonstrate substantial edema at the time of uterine culture. If these conditions are not met, false-negative culture results may be obtained. In addition, care should be taken not to contaminate the uterus during the process of obtaining the culture. Concurrently, control of delayed or inadequate uterine clearance is important. Florida workers recommend repeated oxytocin (20 IU given intravenously) injections to eliminate intrauterine fluid, but not more often than every 4 hours. These mares should be monitored for at least 1 day after ovulation to be certain no fluid is present in the uterus as they enter diestrus.

Proper management of donor mares centers on the ultimate goal of recovering as clean an embryo as possible that is of a size that transfers well. With this in mind, practitioners should realize that contrary to conventional thought, donor mares with endometritis become pregnant on a regular basis and an embryo is commonly found in a very dirty flush. Regardless of the number of times the embryo is washed, getting these embryos to thrive after embryo transfer is difficult. Therefore, a prudent practice is to make sure the donor mare’s uterus is free of microorganisms at the time of insemination. Once-daily transrectal palpation and ultrasound examinations allow the practioner to follow follicular development and ovulation and, just as important, to monitor the conditions of the uterus. The time and effort spent keeping the donor mare’s uterus clean are rewarded with increased rates of successful embryo transfer. Keep in mind that bacterial contamination of the donor’s uterus can come from many sources. Among these are inadvertent contaminations during the breeding process, contaminated semen, and, increasingly commonplace, residual infection and endometritis from previous reproductive procedures.

During the week between ovulation and the uterine flush, donor mares should be kept as stress-free as possible. Evidence shows that performance mares in training may have reduced embryo recovery rates (Mortenson et al., 2009). These reduced rates may result, in part, from the fact that some mares have a significant increase in body temperature during strenuous physical activity that can have an adverse affect on embryonic development or survival.

Older mares and mares that ovulate small follicles may benefit from progesterone supplementation during the first week after ovulation. The most practical way to do this is with administration of altrenogest (0.044 mg/kg orally, once daily) until the embryo recovery attempt is performed. Other candidates for progesterone supplementation are the mare that has multiple follicles with asynchronous ovulations (separated by 3 to 4 days) and the mare that does not develop a prominent CL after ovulation.

Methods used for embryo recovery have been described previously (McCue et al., 2001; Vanderwall, 2000). In general, recovery is accomplished with three to four transcervical uterine lavages, with the effluent fluid passed through a filter that captures the retrieved embryo.