A group of sheep were impregnated using 50-year-old sperm.

Flock of sheep impregnated using 50-year-old sperm

Flock of sheep impregnated using 50-year-old sperm

Scientists from the University of Sydney inseminated 56 Merino ewes with 50-year-old semen taken from four rams in 1968 and 34 became pregnant.

The sperm used is considered the ''world's oldest known viable semen'' and its success rate was higher than that of sperm frozen for only a year.

Associate Professor Simon de Graaf said the trial demonstrated the ''clear viability of long-term frozen storage of semen'' - raising new hope for the reproduction of endangered species.

Dr. Jessica Rickard - who tested the DNA integrity of the sperm - said: ''What is amazing about this result is we found no difference between sperm frozen for 50 years and sperm frozen for a year.

''We believe this is the oldest viable stored semen of any species in the world and definitely the oldest sperm used to produce offspring.''