Semen stored since 1968 in a laboratory in Sydney has been defrosted and successfully used to impregnate 34 Merino ewes, with the resulting live birth rate as high as with sperm frozen for just 12 months, according to an announcement from the University of Sydney in Australia.

"This demonstrates the clear viability of long-term frozen storage of semen. The results show that fertility is maintained despite 50 years of frozen storage in liquid nitrogen," said associate professor Simon de Graaf from the Sydney Institute of Agriculture and School of Life & Environmental Sciences at the University of Sydney.

"The lambs appear to display the body wrinkle that was common in Merinos in the middle of last century, a feature originally selected to maximize skin surface area and wool yields. That style of Merino has since largely fallen from favor as the folds led to difficulties in shearing and increased risk of fly strike," de Graaf said.

His colleague on this project, Dr. 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."

De Graaf said it was the reproductive biology and genetic aspects of these as-yet unpublished findings that were of most interest to him.

"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," he said. "This gives us a resource to benchmark and compare."

Rickard did the original work to determine if the stored semen was viable for artificial insemination. This involved thawing the semen, which is stored as small pellets in large vats of liquid nitrogen at -196°C. She and her colleagues then undertook in vitro tests on the sperm quality to determine the motility, velocity, viability and DNA integrity of the 50-year-old sperm, the university said.

"What is amazing about this result is (that) we found no difference between sperm frozen for 50 years and sperm frozen for a year," Rickard said.

Out of 56 ewes inseminated, 34 were successfully impregnated. This compares to recently frozen semen from 19 sires used to inseminate 1,048 ewes, 618 of which were successfully impregnated. This gives a pregnancy rate of 61% for the 50-year-old semen against 59% for recently frozen sperm, a statistically equivalent rate.

The original semen samples were collected in the 1960s from sires owned by the Walker family. Those samples, frozen in 1968 by Dr. Steven Salamon, came from four rams, including "Sir Freddie," who was born in 1963 and owned by the Walkers on their then-property at Ledgworth.

The Walkers now run 8,000 sheep at Woolaroo at Yass Plains, New South Wales, and maintain a close and proud relationship with the animal breeding program at the University of Sydney.

The research was undertaken courtesy, in part, of a grant from Australian Wool Innovation.

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