In the recent past, breeders have routinely removed seminal plasma from stallion semen before cooling—a practice thought to be beneficial to the sperm. Now, a group of French researchers has confirmed that, at least for their group of stallions, seminal plasma removal does help stabilize the sperm. However, that stability didn’t appear to make much difference in pregnancy rates.

“For anyone still not convinced, our study goes to show that the functioning of the spermatozoa is very complex—and highly individual,” said Isabelle Barrier-Battut, PhD, researcher at the French National Stud of Le Pin and at the French Horse and Equitation Institute in Normandy. “Biology has never pretended to be an exact science, has it?”

Indeed not. By testing 66 ejaculates from 14 breeding stallions at the country’s National Stud—some of which were considered “poor coolers,” meaning their fertility rates were low with cooled semen—Barrier-Battut and colleagues determined that, in fact, there are no definites when it comes to semen. Different semen's reactions to seminal plasma removal, centrifugation, extenders, cooling, and thawing can be as varied as the stallions themselves.

In her study, Barrier-Battut and her fellow researchers removed seminal plasma from half the semen samples before cooling them in a refrigerator for 48 hours. Once they had been warmed up again to near body temperature, the researchers put the spermatozoa in the samples through a series of tests. First, they evaluated motility by observing speed, direction, straightness, and progression. Second, the team used the hypo-osmostic swelling test, in which they expose the sperm to a special mix of salts and antibiotics. If this makes the flagella (the “tail”) swell, the sperm are considered to be more stable. Finally, the team carried out acrosome (a specialized structure on the head of the sperm that helps penetrate the egg during fertilization) analysis, in which they exposed the sperm to a chemical that’s supposed to mimic some of the events occurring at fertilization and then stained it with a particular dye. The researchers investigated how stained the acrosomes were by using a fluorescent microscope. Their results showed that removing the seminal plasma yielded much better sperm membrane stability, on average. However, individual results varied significantly from one stallion to another