50-year-old semen still works, scientists find

Sheep semen stored in a lab for over 50 years has been successfully used to impregnate dozens of ewes in a breakthrough project.

The team behind the work believe this is the oldest sample of frozen semen successfully used to produce offspring.

Sperm samples belonging to four Merino rams alive in the 1960s, including one known as “Sir Freddie”, were unfrozen so the animals could father lambs despite being dead for years.

Out of 56 ewes inseminated with the decades-old semen, 34 were successfully impregnated, making the frozen sperm as effective as samples taken just a year previously.

“This demonstrates the clear viability of long-term frozen storage of semen,” said Dr Simon de Graaf, one of the scientists behind the project at the University of Sydney.

“The results show that fertility is maintained despite 50 years of frozen storage in liquid nitrogen.”

The scientists found that the lambs born through this process had the distinctive body wrinkle that characterised Merino sheep in the middle of last century.

This feature was specifically bred for in an effort to maximise skin surface area and therefore wool yields, but it fell out of favour due to difficulties with shearing and increased risk of disease.

Besides demonstrating the health of long-frozen sperm, the scientists said their experiments gave them a window into the history of animal breeding.

“We can now look at the genetic progress made by the wool industry over past 50 years of selective breeding. In that time, we’ve been trying to make better, more productive sheep,” said Dr de Graaf.

Dr Jessica Rickard undertook initial tests to assess whether the semen, stored as pellets in liquid nitrogen, still showed the activity and healthy DNA required for insemination.

“We believe this is the oldest viable stored semen of any species in the world and definitely the oldest sperm used to produce offspring,” she said.