Endangered species may have found a new lifeline after sheep were impregnated using 50-year-old frozen sperm.Samples taken from four rams including one called Sir Freddie in 1968 are described as the "world's oldest known viable semen".Scientists from the University of Sydney inseminated 56 Merino ewes with the half-century-old semen, and 34 became pregnant.The 61% success rate was higher than that of sperm frozen for only a year - which impregnated 59% of inseminated ewes.Associate Professor Simon de Graaf said the trial demonstrated the "clear viability of long-term frozen storage of semen", raising new hope for the propagation of endangered species.The semen was stored as small pellets in large vats of liquid nitrogen at -196 degrees, the university said.Image:Sir Freddie - one of four rams whose sperm was frozen in 1968, seen in 1969.

Pic: Walker family/University of SydneyAfter it had been defrosted, post-doctoral research fellow Dr Jessica Rickard tested its motility, velocity, viability and DNA integrity."What is amazing about this result is we found no difference between sperm frozen for 50 years and sperm frozen for a year," she 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."Associate Professor de Graaf said the lambs appeared to have a "body wrinkle that was common in Merinos in the middle of the last century - a feature originally selected to maximise skin surface area and wool yields".He added: "That style of Merino has since largely fallen from favour as the folds led to difficulties in shearing and increased risk of fly strike."Explaining the significance of the trial using decades-old semen, he said: "We can now look at the genetic progress made by the wool industry over the past 50 years of selective breeding."In that time, we've been trying to make better, more productive sheep."This gives us a resource to benchmark and compare."