(CNN) Scientists in Australia have set a new record by using the world's oldest known viable semen to impregnate dozens of merino sheep.
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.
"What is amazing about this result is we found no difference between sperm frozen for 50 years and sperm frozen for a year," she said in a press release.
The samples came from four rams owned by the Walkers, a family of sheep farmers who now keep 8,000 sheep in Yass Plains, New South Wales.
One of the rams, known as Sir Freddie, was born in 1959 and bought for 345 guineas in 1961.
The lambs produced during the research are now being kept at a farm in the town of Coleraine, Victoria.
They will be monitored over the next two years to see how they compare with lambs born outside the program, so scientists can work out how decades of selective breeding have changed Merino sheep.
The fact that fertility is maintained during long-term storage will be of some benefit to humans, de Graaf told CNN, particularly for groups such as men who want to freeze their semen before undergoing chemotherapy.
De Graaf added that it will also be a huge help in efforts to save threatened species from extinction.
"The work demonstrates that we can preserve the genetics of rare, endangered animals today so their sperm cells can be used for artificial insemination, IVF or even more advanced reproductive technologies into the future," he said in an email.
In captive breeding of endangered species, scientists try to slow genetic change as much as possible, said De Graaf.
"One of the ways to do this is by extending the generation interval of captively bred endangered species so the use of semen frozen for a very long time is a great way of achieving this," he added.
Other research in the field includes a "frozen zoo" at the University of Georgia's Regenerative Bioscience Center.
In 2015, scientists announced plans for a genetic storage center that uses skin cell extracted from a sedated animal in a noninvasive procedure.
Researchers convert those cells into stem cells by introducing a series of specialized reprogramming genes. And from there, those stem cells can be made into sperm or eggs for future use.
And in 2018, scientists led by Professor Thomas Hildebrandt at the Leibniz Institute for Zoo and Wildlife Research in Berlin successfully used in-vitro fertilization techniques to develop hybrid rhino embryos -- "test-tube rhinos" -- which could help save the endangered northern white rhinoceros species, according to a study.
The team was also able to extract stem cell lines from southern white rhino embryos, a subspecies closely related to northern white rhinos, which could be used to make reproductive cells such as eggs and sperm to create embryos.
These techniques could become a valuable tool in the conservation of rhino populations.