The semen of some stallions tolerates freezing well, while the semen of others does not. Researchers are trying to discover the reasons for this in order to develop freezing protocols that decrease the variability of sperm, thereby increasing the number of sperm able to withstand freezing.

“In our lab we are trying to understand why some stallions are good freezers (their sperm survives in a percentage above the average after freezing and thawing) and others are not (few sperm survives after freezing),” said Fernando Juan Peña, DVM, PhD, Diplomat in the European College of Animal Reproduction, of the University of Extremadura in Spain.

Many factors impact the viability of semen, including oxidation during cryopreservation causing sperm cells to age rapidly, the composition of the lipids of the sperm membranes, and bacterial contamination, said Peña.

The researchers discovered that any bacterial contamination, even bacteria that do not cause infection, might compete for the nutrients in the semen extender, change the pH, and release toxins to the media, which would all compromise sperm viability