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A 50-year-old frozen semen from a ram, believed to be the oldest in the world, is still achieving high fertility rates with the successful impregnation of 34 living sheeps, according to researchers from the University of Sydney.

The semen, frozen in 1968 by Steven Salamon of the varsity, included four rams of Ledgworth, Merryville and Boonoke genetics, owned at the time by the Walker family of Ledgworth at Yass. To prove sperm could survive several years, the semen was stored as small pellets in large vats of liquid nitrogen at minus 196 degree Celsius.

"We believe this is the oldest viable stored semen of any species in the world and definitely the oldest sperm used to produce offspring," Jessica Rickard, a post-doctoral student in the Sydney Institute of Agriculture, said in a statement.

The findings can pave the way for young cancer patients who may want to save semen samples before starting radiation treatment. This would allow them to have children later in life, the researchers said. In the study, the team thawed the semen to determine if the stored semen was viable for artificial insemination.

The team also undertook in vitro tests on the sperm quality to determine the motility, velocity, viability and DNA integrity of the 50-year-old sperm. Out of the 56 ewes inseminated with the 50-year-old semen, 34 (61 per cent) were scanned as pregnant.

This was compared with 59 per cent in the recently frozen semen (in 2018) from 19 sires used to inseminate 1,048 ewes, of which 618 were successfully impregnated. "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 Simon de Graaf, Associate Professor from the varsity. "What is amazing about this result is we found no difference between sperm frozen for 50 years and sperm frozen for a year," Rickard added.