Breeding Mares Using Transported Semen – Part 2

Part Two of A Two Part Series – Jonathan Lombardo, DVM

In the last article, I explained the processes involved in collecting and processing equine semen for shipment.  The use of cooled transported semen has been of great benefit to the equine industry.  It enables the mare owner to choose from a wide variety of sires, some of whom may be located in another part of the country.  In this issue we will examine the method by which a mare is bred using artificial insemination with cooled transported semen.

Breeding the mare involves a series of steps or processes.  It is very important that these steps be followed correctly, or the likelihood of producing a pregnancy may be reduced.  Skipping a step is never a good idea, and represents false economy.  That being said, the process usually proceeds as follows:

First, a uterine culture and cytology is obtained.  A sterile swab is inserted into the mare’s uterus, twirled gently to pick up any fluid and a few cells, and carefully withdrawn.  The swab is then rolled across a glass microscope slide, which is later evaluated by a histopathologist.  This specialist will assess the sample microscopically, looking for bacteria and inflammatory cells, which would indicate an infection.  The swab is placed in a tube of bacterial transport media, which keeps any bacteria alive for the trip  to the laboratory.  At the lab, a bacteriologist attempts to grow a culture of any bacteria present.  If anything grows, it is tested to determine the types of antibiotics which may be used to treat the infection.  If nothing grows, we interpret the culture as negative.  Uterine biopsy may also be performed in a mare with a history of subfertility.  In this process, an instrument is used to obtain a small sample of the inside of the mare’s uterus (endometrium.)  This sample is placed in formalin and sent to a laboratory where it is placed onto microscope slides and evaluated by a histopathologist, who looks for evidence of endometrial scarring or inflammation.  Both of these things are capable of reducing the mare’s fertility.  If everything appears normal so far, we move to the next step.  If we identify a problem, such as a uterine infection, then that problem must be dealt with before we go any further.

The next step in the process involves the use of transrectal ultrasonography to assess the mare’s reproductive tract.  The ultrasound uses sound waves to generate a visual image on a screen, and allowing us to visualize the mare’s uterus and ovaries.  This information is used to determine if the mare is in estrus(heat) or diestrus (out of heat), and to look for any unusual structures, such as endometrial cysts.  If the mare is not in estrus, one of several drugs may be used to bring her into heat.  If the mare is in estrus, we measure the diameter of the ovarian follicle (structure which contains the egg), and look for evidence of uterine edema.  These measurements are used to determine how far the mare has progressed into estrus.  If a sufficiently large follicle is present, the mare is given a drug called HCG, which will stimulate her to ovulate 24 to 48 hours after administration.  It is important that the mare be inseminated very soon prior to ovulation.  The stallion owner has usually been notified by this time, and the semen has been collected from the stallion, processed and shipped to the mare owner.  It is removed from its packaging, drawn into a syringe, and placed within the mare’s uterus, using a long straw, called a pipette, which is passed manually through the vagina and cervix and into the uterus.  Later, the mare is examined to determine if she has ovulated in a timely manner, and assessed for retention of any fluid within her uterus.  If this fluid remains, it will impair conception.  If fluid is present, the uterus is lavaged (flushed) to remove it.  Fourteen days after ovulation the mare is ultrasounded once more to determine if she has conceived.  At this time, she is also examined to rule out a twin pregnancy, which is a serious problem if not corrected.  Typically, the mare is ultrasounded once again at 26 days of gestation to be sure she has retained the pregnancy.  It is very exciting to watch the beating of a foal’s heart on an ultrasound monitor, only 26 days after conception!

As you can see, the process of breeding a mare involves a series of steps which, although they seem complicated, are quite necessary to produce the desired end result, a pregnant mare.

Dr. Lombardo is an equine veterinarian based in Red Wing, Minnesota.  He may be contacted at or telephone (651) 755–6515.

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