Dr Paula Bell-Cross is well renowned for her commitment and skill in the field of Equine Reproduction. Every breeding season Paula is busy in her dedicated reproduction lab here at the Valley Equine Veterinary Centre, performing stallion collections, semen freezing, embryo transfer and artificial inseminations.

Valley Equine Veterinary Centre adopts an advanced approach to all areas of equine reproduction by continually implementing and adapting the latest information and techniques in the fields of broodmare management and assisted reproduction.

Our veterinarians are involved in all aspects of equine reproduction from, preparing mares for service, stallion dummy training, artificial insemination, embryo transfer, monitoring high- risk pregnancies and foaling down.


The various techniques of assisted reproduction are widely used in the breeding of Pleasure and Performance horses. Dr Bell-Cross has experience in all aspects of equine reproduction and we regularly collect stallions, inseminate mares and transfer embryos for our clients.

Artificial Insemination

Artificial insemination (A.I.) is the placement of semen into the uterus of a mare at the time of ovulation. It is performed by means of a catheter inserted through the mare’s cervix by a veterinarian.

A.I. is used in horses for a number of reasons including the availability of the stallion, as an aid in managing mares with reproductive difficulties, to decrease the workload and increase the efficiency of the stallion, in the event of injury to the mare or stallion which may prevent normal service and in controlling venereal disease.

Successful A.I. involves regular ultrasound examination and monitoring of the mare’s cycle and necessitates close communication between the mare owner, the veterinarian and the semen provider.

Three methods of A.I. are used in horses:

Fresh semen is collected and used almost immediately on the farm at which it is collected, this method enables more efficient use of the stallion and may help pregnancy rates for stallions of marginal fertility.

Chilled semen involves collection, dilution with extender, cooling and transport of semen to the mare for use within 36 hours. This method can be used from any fertile stallion from any location that can ensure reliable transportation of the semen to the mare.

Frozen semen is collected and extended then frozen and stored in straws in liquid nitrogen. This technique allows access to semen from horses which are located overseas, currently competing, injured or even deceased. Using frozen semen is convenient because the semen is available to be used when needed, but requires more intensive management of the mare as the technique demands timing of insemination close to ovulation of the mare’s follicle.

Frozen Semen

Frozen semen has several distinct advantages over fresh and cooled semen.

The genetics of a valuable stallion can be preserved indefinitely by collecting and storing doses of frozen semen during the non-breeding season.

Frozen semen can be sent to the mare farm as soon as the contract or breeding agreement is signed, eliminating the need for same-day or overnight courier service, as is required with cooled semen

Frozen semen can be exported overseas

The primary disadvantages of frozen semen in an equine breeding program are stallion variability in response to freezing, cost to collect, freeze and store semen, intensified mare management, and slightly lower pregnancy rates per cycle.

Sperm from some stallions do not tolerate freezing and thawing. Consequently, post – thaw motility and pregnancy rates can vary greatly between stallions. Performing a ‘test freeze’ is valuable in determining if semen from a stallion can be frozen successfully and which freezing extender is most suitable for each stallion. A minimum of 30% progressive motility should be present post-thaw for a stallion to be considered a ‘good freezer’. Proper storage and handling of frozen semen is critical for future success.

The number of straws required for a breeding dose is dependent on the total number of sperm in the straw and the insemination technique. The mare is examined 3 to 4 times per day as ovulation approaches and the mare is inseminated immediately after ovulation is detected.

Prediction of ovulation and timing of insemination with frozen semen is facilitated by administration of ovulation-inducing agents, such as hCG or deslorelin. Typically, a mare in oestrus would be given one of these agents once a follicle larger than 35mm in diameter is detected. In most instances, ovulation will occur in 36 to 40 hours after hCG or deslorelin administration.

Average pregnancy rates per cycle for mares bred with frozen semen are 30 to 40%. Some stallions may have per cycle pregnancy rates of 50 to 70% using frozen semen, while frozen semen from other stallions will be unsuccessful in getting mares in foal. In general, pregnancy rates per cycle using frozen semen are 5 to 10% lower than with cooled-transported semen. Management of mares for breeding with frozen semen may seem intimidating. However, by following a few simple guidelines, frozen semen can be used very successfully in a breeding program.

Embryo Transfer

Embryo transfer simply involves the removal of an embryo from the uterus of one mare and transferring into the uterus of another mare.

This procedure is suitable for all sport and performance horses. In particular it is a beneficial assisted reproductive exercise for:

Frozen semen can be exported overseas

Valuable mares may have more than one foal per year

Sub fertile mares can have their embryos transferred to young, reproductively healthy mares

Older mares can donate embryos to young recipients

Embryos can be transferred from 2 and 3 year olds

Mares with a damaged cervix as a result of a difficult foaling can have embryos transferred

Ideally the best donor mares are mature, reproductively healthy mares. In order to maximise success rates, detailed attention needs to be paid to the donor mare and the recipient in relation to timing of ovulation.

The donor mare is examined daily while in season by palpation and ultrasound in order to determine the exact time of ovulation. Recovery of embryos is usually attempted on day 7 (ovulation day is day 0). Day 6 and day 8 embryos can be transferred, but the success rate diminishes dramatically after this time.

A sterile catheter is inserted through the cervix and into the uterine body and the uterus is then filled with a special flush media. This media is then allowed to flow back through the catheter and passed through a filter which traps the embryo. The contents of the filter are then poured into a dish and examined for the presence of an embryo under a microscope.

Embryos that are day 7 are generally less than 1mm in diameter. The embryo is then graded from 1 to 5, grade 1 and two being ideal and grade 5 as of no value. The recovered embryo is then washed and maintained in holding media until transferred into a recipient mare.

Embryo recovery rates may be as high as 75% or as low as 25%. Factors that influence this recovery rate are:

Age and fertility of donor mare

Quality of semen used

Day of recovery

Number of ovulations

It is critical that the oestrous cycle and the day of ovulation of the recipient mare is closely synchronised with that of the donor mare. Ideally the recipient mare should ovulate from one day before and no more than three days after the donor mare. Ovulation dates outside of this synchronisation will dramatically reduce transfer success rates.

As the act of synchronisation cannot be an exact science, it is therefore best to try to synchronise at least 2 to 3 recipients in order to obtain one that would fit into that critical window. Ideally it is best to have the recipient ovulate 1 to 2 days after the donor.

The embryo is transferred non-surgically using a specialised embryo transfer instrument which is passed through the cervix and the embryo is deposited into the uterus. At this stage it is not unusual to administer certain medications to enhance the success rate of the transfer by helping the mare maintain the transferred embryo.

As the embryo is already 7 to 8 days old when transferred it only needs another 4 to 5 days before the embryo can be observed with ultrasound to determine if the transfer has been successful.

The success rate of transfer is usually about 70%. It is important to follow up with subsequent pregnancy examinations as some recipient mares do not stay pregnant to term. This is more likely to occur when using older donor mares and when the embryo transferred is of a lower grade.


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