Excerpts courtesy of Jos Mottershead of Equine Reproduction 

Q:   What are the advantages and disadvantages of frozen semen?
A:   Frozen semen advantages include:
There is no requirement to schedule shipments of semen to fit the mares ovulation time.
The stallion does not have to be taken out of competition to breed, or have his competitive attitude possibly upset by having to be bred.
The market for semen is global, as there is no limitation on duration of delivery time.
There is an "insurance factor", should your stallion become ill or die, and be unable to fulfil his breeding commitments.

Disadvantages include:
Not all stallions have semen that will freeze.
Conception rates for artificial insemination with frozen equine semen are typically a little lower than those seen with fresh or cooled semen ("transported semen" or "shipped semen").
To freeze semen is not initially as cheap as to prepare cooled semen (although in the long run it can be cheaper).
The average farm will not wish to set up their own laboratory, or have the technical know how to freeze semen.

Q:   What is involved in freezing semen?
A:   Once the semen is collected from the stallion, which is the same process as collecting for cooled transported semen or on farm AI (artificial insemination), it is evaluated to establish sperm concentration and motility levels. The semen is then mixed with a centrifugation extender, and the majority of the seminal plasma is removed by centrifugation. The resulting sperm pellet is resuspended in a freezing extender which contains a nutrient medium and a cryopreservant. This mixture of sperm and extender is then loaded into straws. The straws are then lowered in temperature by being exposed to liquid nitrogen vapour for a timed period. After the completion of that timed period, they are plunged into the liquid nitrogen, from which they can subsequently removed, and stored in a liquid nitrogen storage container. Of course, this is just a simplified outline of the procedure, and there are several different techniques that vary.

Q:   If the freezing procedure is as simple as outlined above, why isn't it more common?
A:   It's not quite as simple as the outline! There are several variables, not the least of which is the stallion himself.
Not all stallions semen can be successfully frozen. Or to be more accurate, can be successfully thawed! It's estimated that between one-third and one-quarter of stallions have semen that when frozen, thawed and inseminated into a suitably receptive mare, will not result in pregnancy. Research is producing new methods all the time, and it is hoped that this figure will reduce in time.
Attention must be closely paid to all the procedures involved in the freezing process. Stallion sperm are extremely sensitive, and will die (it sometimes appears) at the drop of a hat! Any part of the process that is not carried out properly will result in a reduced percentage of sperm capable of impregnating a mare. As there are many more steps in the semen freezing process than there are for preparing cooled semen (transported semen or shipped semen), the potential for disaster is proportionately greater.

Q:   I looked at having my stallion's semen frozen, but the cost was prohibitive. Is there anything I can do to reduce the cost?
A:   Not really, but costs have dropped significantly in the last five years, so it may well be worth checking the price again. Prices are now such that it should be practical for anyone who has a stallion that is breeding a reasonable number of mares at an average stud fee to utilize freezing services, providing their stallion is a "freezer".

Q:   How do I know if my stallion is a "freezer".
A:   Unless his sperm have been frozen, there is really no reliable method to tell whether they will survive the process. Even then, because they have survived the freezing process does not mean they will be able to get a mare pregnant. Some businesses freezing semen offer a special rate for a "test freeze".

Q:   If my stallion has had his semen frozen, and we have used it to successfully to breed mares, how long will the frozen semen last that we haven't used?
A:   As long as the semen is not thawed, and is maintained in a liquid nitrogen storage tank, the working life is believed to be nearly unlimited. (10,000 years has been suggested!) Realistically, pregnancies have resulted from using semen that has been stored for ten years or so.

Q:   Is thawing the semen before breeding by artificial insemination complicated?
A:   No! But care and attention must be paid to the process. (More information can be found on thawing equine semen here).