Author: Paul F. Johnston DVM, Dipl ABVP (Equine Specialty), Dipl ACT

What is frozen semen? Frozen semen is fresh semen that has been frozen and stored in liquid nitrogen at -196 degrees C.

What is chilled semen? Chilled semen is fresh semen that has been collected, processed with a semen extender, and packaged in a special container which will cool/chill it to 5 degrees C at a rate of 0.5 degrees C /minute.  At our clinic we call these containers "boyfriends in a box".  Chilled semen is viable for only 1 to 2 days and is usually shipped via courier or air freight to mares in heat and about to ovulate. Generally chilled semen is as fertile as breeding with natural cover or more so in some mares. 

How is the semen frozen?  The freezing process is quite complex requiring special semen extenders to protect the sperm during freezing and a very specific and controlled freezing rate (change in temperature vs. time). 

What effect does freezing have on sperm? The most important consequence on sperm that has been frozen and thawed is diminished longevity (life span) and alteration of sperm cell membranes leading to less effective binding by special cells in the oviducts (mare fallopian tubes). Binding to cells in the oviduct enhances the sperms ability to fertilize an ovum (egg). Binding also preserves sperm for up to several days by lowering the sperms metabolic rate. These factors demand that frozen semen be thawed and inseminated into the mare somewhere in the range of 6 hours prior to and 6 hours following ovulation. 

How long does frozen semen last? Once frozen, the semen is good indefinitely and will last decades provided it is stored properly. 

What are the advantages of frozen semen?

 1) Frozen semen allows us breeding access to stallions located overseas or in the U.S., where the distance and technicalities of shipping semen make it difficult to get chilled semen to a mare before it dies in the container.

2) Using this technology, semen can be collected and stored months to years in advance of insemination.  All too often a stallion may be unavailable for breeding or collection on a certain day due to show or race schedules, or convenient shipping arrangements can not be made with the airline or courier service. 

3) Several doses can always be kept on hand which reduces repeat collection and shipment cost.
4) Frozen semen is a form of insurance for stallion owners.  Hundreds of doses can be stored in case of unexpected death of the stallion, and for use long after the end of his natural life. 

5) Low cost to the breeder if semen is purchased by the dose and conception is achieved with the first dose.  Generally when you purchase a breeding to a particular stallion you pay the stud fee by purchasing a minimum of 3 doses of semen.  For example, if stallion X's stud fee is $2400.00, the stallion owner will send 3 doses of semen.  This is because, on average 3 breedings are often required to achieve a conception.  If your mare conceives on the first dose your stud fee was in effect $800.00.  However, it is important to remember that the law of averages usually catches up with you.

What are the disadvantages of frozen semen:

1) Lower conception rates compared to fresh or chilled semen.  With chilled
semen from a fertile stallion, we usually expect a 70% pregnancy rate when inseminating a fertile mare. The average pregnancy rate when using frozen semen is a 40 to 50% per breeding. This varies depending on the stallion.  We often have 70% pregnancy rates with some stallions and 30% or less with others. 

2) Breeding frozen semen requires more intensive (and subsequently, more expensive) monitoring of your mare's cycle by your veterinarian.  Multiple daily ultrasound
exams may be required.  However, an alternative to this is timed, controlled ovulation using reproductive hormone administration (see
website listed below for more details).  On average a breeding (per heat cycle) veterinarian versus $200.00 - $300.00 with fresh or chilled semen. 
3) Diminished success rates with older problem mares.  Older less fertile mares will often conceive when inseminated 1 to 2 days prior
to ovulation and subsequently treated with uterine lavage to remove residual fluid which accumulates in the uterus following breeding.  This
accumulation is a normal occurrence in most mares.  However, in young mares it is quickly expelled and the uterus returns to normal
to prepare itself for an embryo following fertilization.  In older mares it will persist and the embryo once formed, has no suitable
environment to develop in.  As previously discussed, frozen semen should be inseminated very close to ovulation.  This leaves less time for the
uterus to respond to lavage and prepare itself for an embryo.  Conversely fresh or chilled sperm quickly traverse the uterus and
take up residence in the oviduct.  They can remain viable here for 1 to 2 days (some stallion variation) and patiently wait for an ovum
(egg) which is released from the follicle at ovulation.  In the meantime, the uterus can be treated to prepare a favorable environment for the developing embryo. 

In summary, there are many variables to consider before using frozen semen in order to achieve a successful outcome. In my opinion, their order of importance is as follows:

1) Fertility of the mare. 

2) Fertility and response to freezing of the stallion selected.

3) Experience, expertise and access to diagnostic ultrasound by your veterinarian.

4) Luck.
Artificial insemination using frozen semen is now routinely performed for many Maritime horse breeders. Most breeders will agree that it has strongly enhanced the quality the locally bred horses.

 East Coast Equine Reproduction Centre