Decades after his 1960s heyday, Sir Freddie the ram has sired offspring from beyond the grave in what researchers believe is a project that shows the world’s oldest viable stored semen.

A team at the University of Sydney has defrosted Sir Freddie’s 50-year-old semen and that of three other rams and successfully impregnanted it in 34 merino ewes.

“This demonstrates the clear viability of the long-term frozen storage of semen,” associate professor Simon de Graaf said.

“The lambs appear to display the body wrinkle that was common in merinos in the middle of last century, a feature originally selected to maximise skin surface area and wool yields.”

‘Sir Freddie’ in 1969, one of the four rams whose semen was frozen in 1968. Photograph: Walker family

De Graaf said that style of merino has since largely fallen from favour as the folds led to difficulties in shearing and increased risk of fly strike.

His colleague Jessica Rickard said it was believed to be the oldest viable stored semen of any species in the world and the oldest stored sperm used to produce offspring.

“What is amazing about this result is 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.

Rickard said this compared to recently frozen semen from 19 sires used to inseminate 1,048 ewes, of which 618 were successfully impregnated.

The pregnancy rate for the 50-year-old semen was 61% against 59% for recently frozen sperm, which is a statistically equivalent rate, she said.

Merino lambs born in 2018 from 50-year-old semen at the Balmoral Field Day sheep trial near Coleraine in Victoria. Photograph: Morgan Hancock

Sir Freddie was born in 1963 and died in the 1970s.

The Walker family, who run an 8,000-sheep merino stud called Woolaroo at Yass Plains in New South Wales, donated the original ram semen samples in the 1960s from their then property Ledgworth.

Dr Steven Salamon, who escaped a gulag during the cold war and migrated to Australia, froze the semen samples in vats of liquid nitrogen in 1968, as part of a time-capsule project.