The resulting live birth rate proved to be as high as for sperm frozen for just 12 months.

Associate Professor Simon de Graaf from the Sydney Institute of Agriculture and School of Life and Environmental Sciences at the University of Sydney said this demonstrated the viability of long-term frozen storage of semen.

"The lambs appear to display the body wrinkle that was common in Merinos in the middle of last century, which was a feature originally selected to maximise skin surface area and wool yields," Simon said.

"That style of Merino has since largely fallen from favour as the folds were found to lead to difficulties in shearing and increased risk of fly strike."

Associate Professor de Graaf's colleague on the project, Dr Jessica Rickard, said this was believed to be the oldest viable stored semen of any species in the world.

"It was definitely the oldest sperm used to produce offspring," Dr Rickard said.

"What is amazing about this result is we found no difference between sperm frozen for 50 years and sperm frozen for a year."

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Associate Professor de Graaf said that it was the reproductive biology and genetic aspects of these 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," he said.

"In that time, we've been trying to make better, more productive sheep.

"This gives us a resource to benchmark and compare."

Dr Rickard is a post-doctoral McCaughey Research Fellow in the Sydney Institute of Agriculture.

She has continued the strong animal reproduction research tradition in veterinary and biological sciences at the University of Sydney through her work in the Animal Reproduction Group.

Dr Rickard did the original work in determining whether the stored semen would be viable for artificial insemination.

This involved thawing the semen, which is stored as small pellets in large vats of liquid nitrogen at -196 degrees.

Dr Rickard 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.

Out of the 56 ewes that were inseminated as part of the project, 34 were successfully impregnated.

This compares favourably to recently frozen semen from 19 sires used to inseminate 1048 ewes, of which 618 were successfully impregnated.

This gives a pregnancy rate of 61 per cent for the 50-year-old semen, against a rate of 59 per cent for recently frozen sperm, a statistically equivalent rate.

The original semen samples were donated in the 1960s from sires owned by the Walker family.

Those samples, originally 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 Walker family now run 8000 sheep at "Woolaroo", at Yass Plains and maintain a close and proud relationship with the animal breeding program at the University of Sydney.

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