By Dean Kreager
If you are using artificial insemination to maximize the genetic potential of your herd, proper semen handling is critical to fertility. Frozen semen will never be more fertile than the day it was frozen but there are many things you can do that decrease the fertility.
To help understand the importance of semen handling I will give you a brief background in the science. Semen is frozen and stored in liquid nitrogen because nitrogen is stable and relatively safe to work with compared to alternatives. Liquid nitrogen has a temperature of -320° F. Originally dry ice, -109° F, was used but its ability to keep the samples cold enough was limited and liquid nitrogen provided superior results with extended storage.
It would seem like frozen is frozen so why would temperature fluctuations at these low temperatures harm the frozen semen? When water freezes it expands. Think of the exploding pop can in the freezer or garage that almost everyone has experienced. Sperm are cells and are 70 to 80% water. If you try to freeze them the water will form ice crystals and destroy the cells. To overcome this problem a cryoprotectant such as glycerol is added to the sperm. This lowers the freezing point and draws water out of the cells allowing them to freeze with only very small ice crystals that do not kill the cells. During this process the freezing point of the solution surrounding the cells is lowered to a point that also allows it to begin to thaw at temperatures well below freezing. Every time this temperature is reached and cooled again more ice crystal damage can occur. Even though dry ice will keep the semen for some time at -112° F there is evidence that warming the samples above -200° F can cause damage.
A liquid nitrogen tank is like giant thermos. It consists of an outer and inner container with a vacuum space between the two. They are attached to each other at the neck. When you look into the neck of the tank you will notice a frost line. All handling of the straws should take place below this frostline using the specially designed tweezers to prevent thermal damage to the straws. The temperature at the frost line will be about -120° F so care must still be taken to prevent warming damage.
Most semen thawing instructions recommend thawing at 95 to 98° F. This temperature is important! Just as the freezing rate controls ice crystal damage the thawing does also. Usually the thaw time will be about 45 seconds. Sitting longer in the thawing bath will not hurt the sample as long at the temperature is maintained but avoid going longer than 10 to 15 minutes. Alternative thaws, such as a shirt pocket thaw, may work for some samples but don’t assume it is good for all samples. Artificial insemination companies use a variety of semen extenders and freezing methods and each may have unique characteristics. Temperature is also very important. When you are ready to load your insemination rod, a cold rod or a hot rod can rapidly change the temperature of the sperm in the straw and reduce fertility. See the video clip above for a comparison of a shirt pocket thaw (lower left on the screen) vs a standard 95° F water bath thaw (upper right) on motility.
Everyone has probably pulled up straws of semen and wiped the frost off the side to read the identification. This 5 second procedure may seem harmless but compare the results in the video below of doing this 3 times to a straw and returning it (lower left) vs a standard 95° F water bath thaw (upper right) on motility . The straw represented in the lower left was never completely removed from the goblet and the procedure was completed inside the neck of the tank.
Source : osu.edu