Semen stored in a laboratory in Sydney has been defrosted and successfully used to impregnate 34 Merino ewes, with the resulting live birth rate as high sperm frozen for just 12 months.

The authors believe the sperm used is the oldest viable stored semen of any species in the world and definitely the oldest sperm used to produce offspring.  The original semen samples were donated from sires owned by the Walker family. Those samples, frozen in 1968 by Dr Steven Salamon, came from four rams, including 'Sir Freddie' born in 1963, owned by the Walkers on their then property at Ledgworth. The Walkers 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 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," said  Professor Simon de Graaf of the University of Sydney of the work, which has not been published yet. "That style of Merino has since largely fallen from favor as the folds led to difficulties in shearing and increased risk of fly strike."

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

Out of 56 ewes inseminated, 34 were successfully impregnated. This compares 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 percent for the 50-year-old semen against 59 percent for recently frozen sperm, a statistically equivalent rate.