By W.L. Flowers, North Carolina State University Department of Animal Science
After collection, semen used for artificial insemination is exposed to some significant changes in its thermal environment. It leaves the boar somewhere around 38 degrees C and usually ends up being stored at between 16 to 18 degrees C. The general assumption is that it is best for semen to be cooled steadily from the time it is collected and then maintained at a relatively constant temperature during storage before it is inseminated. The sow’s reproductive tract is the perfect incubator for semen, so once it is inseminated there really isn’t much to worry about in terms of its thermal environment unless the sow, herself, is heat-stressed.
As semen cools and warms up, there are several metabolic and structural changes that occur. These are illustrated in Figure 1A. As semen cools down between 38 and somewhere around 25 degrees C, the metabolic activity of spermatozoa decreases. The main result of this is that their motility is reduced to the point that they often appear to be “quivering” in one place without any net forward movement. This is actually a good thing in terms of their survival during storage. Decreasing their metabolism prolongs their survival and reduces the amount of waste products that the extender has to neutralize. Somewhere around 20 degrees C the plasma membranes lose their fluidity. They change from being fairly soft and pliable (similar to gelatin) and become more rigid (similar to butter). This change occurs because much of the plasma membrane is composed of proteins and phospholipids. Both proteins and lipids become more rigid and less pliable as the temperature decreases. Finally, sperm cells from most boars would have increased susceptibility to cold shock at temperatures less than 15 degrees C depending on the type of extender that they were stored in. Cold shock produces irreversible damage to the spermatozoa that often significantly reduces their viability and fertility.
As mentioned previously, the ideal situation for spermatozoa after collection is for them to be cooled steadily and then maintained at a stable temperature between 16 and 18 degrees C until they are inseminated. The worst situation is for them to be exposed to fluctuations in temperatures as illustrated in Figure 1B. Fluctuations that occur between 38 and 25 degrees C create situations in which the metabolism of the sperm cells is continually speeding up and slowing down. This typically reduces the viability of sperm cells in that their cellular machinery really isn’t designed to speed up, then slow down, then speed up again. A similar situation occurs with temperature variations around 20 degrees C, but they affect the physical state of the plasma membrane components. Rapid phase changes between a fluid and a semi-solid eventually will change the tertiary structure of both the proteins and phospholipids which, in turn, compromise the structural and functional integrity of spermatozoa and also reduce their viability and integrity.
These temperature changes have relevance to commercial operations based on the results of a recent study that was conducted in the southeastern United States. We monitored temperatures in 30 semen storage units on sow swine farms. Ten of the units had temperatures that decreased below 15 degrees C and eight of the units had temperatures that increased above 20 degrees C. Based on the information in Figure 1, the ones that decreased below 15 degrees C would be at risk for cold shock, while the ones with variability around 20 degrees C would be at risk for having damaged plasma membranes. Nearly 60% of the semen storage units studied exhibited thermal environments at some point that were not conducive for optimal maintenance of sperm viability.
There are many different ways to address undesirable temperature fluctuations of extended semen on sow farms and many of these are most likely situation or farm-dependent.
Nevertheless, it is important for every production system to develop a plan to monitor the temperature of semen arriving at the farm and while it is stored prior to insemination. It is especially important to track fluctuations that cause the temperature of the semen storage unit to consistently drop below 15 degrees C and increase above 20 degrees C. This probably is best done by a third party, i.e. — someone not actually working on the sow farm, but rather an individual who could periodically collect temperature and, perhaps, semen quality data on a regular basis throughout their system.