The Next Step

Once the stallion is collected, the next step is evaluation of the semen. At this point, there are three main places that the semen can go. If there is a mare that is ready to be bred on site, she will be inseminated with all but a very small drop of the sample collected. This is usually about 50mL, depending on the stallion. The small drop left over is looked at under a microscope and the percent of motile sperm is estimated. A "good" stallion's number of progressively motile sperm (these are the ones that have a movement in a forward direction and do not only turn in circles) will probably never be above 80%. 50% is pretty good and the cut off as far as how comfortable my facility was using it was about 28%. A few things affect a sample that have horribly low numbers (like 5 and 6%). One of these could be the age of the stallion, we saw some two year olds that did not have many mature sperm yet. Also, for a stallion that is not collected often or ever, sometimes we did a depletion collection to get rid of any mis-formed or bad sperm. The idea behind this was that the second collection would be sperm that were more recently made and the numbers for the stallion's motility would be higher and a more true indication of how the stallion will perform in the future.

So, once the collection is finished, if the semen is not being put into a mare on site, that is the mare that is ready to br bred and waiting for that semen, it is either shipped as cooled semen or frozen. Semen that is being shipped fresh, that is by air, to be used ASAP, gets centerfuged once and put into an extender. In a collection the majority of the volume is a liquid called seminal plasma. The study of the benefit and purpose of this liquid is a topic of much research today. The gist of it is that seminal plasma not only lends some nutrient to the sperm, but also helps it to deal with the environment of the mare's uterus. It is theorized that seminal plasma also helps to reduce infection in the mare, though this has yet to be solidly proven. As far as storage or shipment, the plasma does not keep the sperm alive as long as other nutritious extenders developed by man. So once the sample is centerfuged, the seminal plasma is poured off. In some facilities they do it very precisely so that all they get is the pellet of sperm. However, with the uncertainty that seminal plasma is all together useless, the technician at my facility only poured off most of the plasma. For fresh semen, it was centerfuged once and then a extender was added and it was put on ice and sent out.

Horses that were being frozen, that is their semen was being frozen, were centerfuged twice and a greater amount of the seminal plasma was removed. The point of the extender is to offer nutrients to the sperm and keep them 'happy' when they are thawed and put into the mare. In order to determine which extender is the best for any given stallion, we would do a test freeze. For this the stallion was collected, the sample centerfuged twice and the seminal plasma poured off. The semen is then separated into four equal parts and each part received an extender (we used four though there are many more than that available). The semen samples are them put into half mL straws and these are then frozen with liquid nitrogen by a machine with a computer. The computer regulates how fast the semen is frozen and this is dependant on the extender used. There were two curves on the machine I am familiar with, a short and long. Two of the extenders we tested were short and two long, so that when testing extenders there were really only two variables being tested. Once these test straws were frozen, they were thawed (50 degrees celcius for 7 seconds) and looked at again under the microscope. The extender that yielded the most viable, progressively motile sperm compared to the original count of the fresh semen was the one that any subsequent samples were frozen with.

More on the benefits of frozen compared to fresh semen later.