The semen from each stallion varies in its response to different extending (preserving) fluids used in the freezing process. When we first collect semen from a stallion, we need to find out which extender formula works best for that particular stallion (i.e. which will be the most fertile when thawed). To do this, we carry out a more complicated freezing and testing procedure when we first collect, and then compare the different batches when thawed.

Once the straws are frozen, we will thaw and compare one from each batch, aiming to obtain straws containing at least 350 million progressively motile sperm with a progressive motility (forward motion) of more than 30% after the thawing process. We expect to freeze approximately 10 mare (insemination) doses from the average fertile stallion collection, but this can vary markedly, ranging anywhere from 2-30 mare doses per ejaculate.

In comparison with chilled semen or a natural service, frozen semen often results in a somewhat reduced fertility. A smaller number of sperm are inseminated than with a natural service, and a percentage of sperm in a straw will succumb to the cryopreservation process, either dying or being irreparably damaged. Also, in practice, some stallions´ semen fares better than others. The generally accepted fertilisation rate when all things are equal on the mare side is about 1.7 inseminations per pregnancy, an acceptable standard when using frozen semen. .

If you are buying frozen semen to use and store at the clinic, we suggest you enquire as to the stallion´s most recent (frozen semen) fertility rate. The only accurate statistic of a stallion´s fertility with frozen semen is his rate of inseminations per pregnancy, ideally measured over a large number of mares. Many semen centres only report a stallion´s overall pregnancy rate, without mentioning how many inseminations were required for each pregnancy. If we are freezing your stallion´s semen here at the clinic for the first time we suggest a test insemination with this semen on one of your own mares. The reason for this is that, as soon as we can positively confirm a pregnancy (at around 14 days after insemination), you can start advertising your stallion´s frozen semen as fertile!