Artificial insemination with shipped semen has been gaining interest in the equine world, especially over the past few years. The increased interest has been spurred on by the acceptance of these previously prohibited procedures by various breed organizations, such as the AQHA. This article will review the use of shipped cooled semen and frozen semen. Before going into the specifics of these techniques, first we will discuss preparation of the mare for breeding season.

Mare preparation
Artificial lighting: Mares in our part of the world are seasonal breeders. During the winter months, when the days are shorter, they normally do not cycle and are not fertile. In the summer months, mares are fertile and cycle regularly, ovulating about every 21 to 22 days. Between these two seasons, mares are described as “transitional.” During these spring and fall transitions, mares have erratic, long heats but do not ovulate and, therefore, are not fertile. Repeated breeding during transitional heats will not result in pregnancy and can lead to reproductive problems.

For various reasons, the equine industry has been interested in developing techniques to move the natural breeding season ahead, so mares will have foals earlier in the year. The best method to date for accomplishing this goal is the use of artificial lighting. Between the natural light and the artificial light, the mare needs to be exposed to 16 continuous hours of light per day. This means the mare must be in her stall with adequate artificial light before dusk. A centrally located 200-Watt bulb in a 12-foot by 12-foot stall is generally adequate light. Most breeders simply set a switch timer to go “off” at 11 PM.

General health: Prior to breeding, a general examination is useful to detect any potential health problems. Vaccinations can be updated and this is good time to have her teeth floated. Special attention should be given to her body condition and nutrition. Studies have shown that increasing body condition improves fertility. On the other hand, obesity is not helpful and in fact leads to health problems. Other factors that may promote fertility are regular exercise, regular turn-out, and well-established daily routines.

In addition to a general exam, a basic reproductive exam can be performed. This includes:
• Evaluation of perineal conformation. If the vulva is set high or tilted forward, this can lead to fecal contamination, “wind-sucking”, and infections of the reproductive tract. Mares with this problem may benefit from a “Caslick’s” surgery, which closes the upper portion of the vulva.
• Vaginal speculum exam. This procedure allows inspection of the vagina, looking for problems such as uterine discharge, scarred cervix, and “urine pooling.”
• Uterine culture. Most breeders require this test. It helps determine whether any infection is present in the uterus.
• Rectal ultrasound exam. This allows the veterinarian to inspect the entire reproductive tract both by how it feels and how it looks visually on the ultrasound screen.
• Uterine biopsy. This is a useful test for mares that have breeding problems, such as older mares or mares that failed to get pregnant in previous years.

Shipped cooled semen
This the most common method of shipped semen artificial insemination in the United States. After semen is collected from the stallion, it is mixed with special semen extenders, which help to protect and increase the longevity of semen during transport. The extended semen is placed into special plastic bags such that each bag contains at least 1 billion progressively motile sperm, that is the current recommendation for a single insemination dose. The bag is then placed into a special container, which cools the semen at the proper rate, stores the cooled sample, and serves as the shipping container. Some breeders will send two insemination doses with each shipment, others send just one. Federal Express or UPS ships the semen for next day delivery. Alternatively it can be sent by commercial airline for same day delivery. Airline delivery is far more difficult to arrange since 9/11.

Cooled semen survives for a relatively short time. With shipped cooled semen artificial insemination, there are basically three events that have to be coordinated within a narrow window of time: stallion collection, mare insemination, and mare ovulation. The mare’s egg cannot be impregnated until she ovulates. If she is inseminated too early prior to ovulation, the sperm won’t survive for the egg’s arrival. Once she ovulates, the egg only survives for only about 6 hours or so. The sperm need to be there by then or there won’t be a live egg to impregnate. The general goal is to order semen, have it delivered and inseminated in the mare the next day, and have her ovulate within the next 24 hours. This is not as easy as it may sound. One problem is accurately predicting several days in advance when ovulation is going to occur. Daily ultrasound examinations and the use of certain hormones help tremendously with this determination. Weekends and holidays can pose problems in getting semen picked up and delivered.

In spite of some the challenges, shipped cooled semen has been a great too for breeders. It has allowed mare owners to keep their mares at home, save on travel and board expenses, and has broadened the accessibility to far off stallions.

Frozen semen
The process for freezing semen is much more complex. After collecting the semen it is put into extender and then has to be centrifuged in a special centrifuge that handles large volumes. This concentrates the sperm into pellets at the bottom of the centrifuge vials. Not enough force and time results in a poor yield of sperm cells. Excessive force and time result in physical damage to the sperm cells. Typically equine semen is spun at 400 g for 10 to 15 minutes. Most of the overlying fluid is removed and the sperm pellets are resuspended into a special cryoprotectant extender. Once resuspended, the semen is then loaded into straws. These straws are then placed into a floating device that floats on top of liquid nitrogen. This device holds the straws at a specific level above the liquid nitrogen, in the nitrogen vapor to pre-cool the straws. After a set amount of time, the straws are plunged into the liquid nitrogen to complete the freezing process. The straws are then transferred to a storage container.

There is a distinct advantage that frozen semen has over shipped cooled semen. Frozen semen is preserved indefinitely as long as adequate levels of liquid nitrogen are kept in the storage tank. This removes the need to coordinate stallion collection to mare’s insemination. This is especially important for stallions that are still showing or competing, for stallions that are older and have physical limitations, and for stallions from different continents, which precludes the use of shipped cooled semen. Ah! But there is a trade-off. The processing that we subject semen to in order to freeze it adversely affects sperm longevity. With frozen semen, the longevity of the thawed semen is extremely short. This shortens the optimal window of insemination far more than with shipped cooled semen. With current technology, the goal with frozen semen is to inseminate the mare within a very narrow window, between 6 hours prior to and 6 hours post ovulation. This requires even closer monitoring of the mare with ultrasound to time thawing and insemination within this narrow time frame.

With the current level of technology, the use of frozen semen is less than that of shipped cooled. But as skills and techniques improve, one could only expect the use of frozen semen to continue to expand. Frozen semen completely separates the events of stallion collection and mare insemination. The potential gains are enormous and will continue to drive that technology.