(CNN)Scientists in Australia have set a new record by using the world's oldest known viable semen to impregnate dozens of merino sheep.
Lambs born from world's oldest stored semen
The sperm had been frozen since 1968, but the live birth rate was as high as when using semen frozen for 12 months.
"The birth of these lambs clearly demonstrates that artificial insemination with frozen semen is a safe and reliable reproductive technology now and into the future," Simon de Graaf, an associate professor at the University of Sydney's Institute of Agriculture, which conducted the research, told CNN via telephone.
A total of 56 ewes were inseminated and 34 produced lambs, representing a pregnancy rate of 61%. This compares favorably with a 59% pregnancy rate for recently frozen sperm, according to the researchers.
Fellow researcher Jessica Rickard said: "We believe this is the oldest viable stored semen of any species in the world and definitely the oldest sperm used to produce offspring."
The semen had been stored in liquid nitrogen at -196 degrees Celsius (-320.8 Fahrenheit). Rickard, who thawed the sperm before testing it for motility, velocity, viability and DNA integrity, was surprised by how well it had been preserved.