Effect of cooling rate on the survival of cryopreserved rooster sperm : comparison of different distances in the vapor above the surface of the liquid nitrogen

The aim of the present trial was to study the effect of different freezing rates on the survival of cryopreserved rooster semen packaged in straws. Slow and fast freezing rates were obtained keeping straws at different distances in the vapor above the surface of the nitrogen during freezing. Adult Lohmann roosters (n = 27) were used. Two experiments were conducted. In Experiment 1, semen was packaged in straws and frozen comparing the distances of 1, 3 and 5 cm in nitrogen vapor above the surface of the liquid nitrogen. In Experiment 2, the distances of 3, 7 and 10 cm above the surfaces of the liquid nitrogen were compared. Sperm viability, motility and progressive motility and the kinetic variables were assessed in fresh and cryopreserved semen samples. The recovery rates after freezing/thawing were also calculated. In Experiment 1, there were no significant differences among treatments for all semen quality variables. In Experiment 2, the percentage of viable (46%) and motile (22%) sperm in cryopreserved semen was greater when semen was placed 3 cm compared with 7 and 10 cm in the vapor above the surface of the liquid nitrogen. The recovery rate of progressive motile sperm after thawing was also greater when semen was stored 3 cm in the vapor above the surface of the liquid nitrogen. More rapid freezing rates are required to improve the survival of rooster sperm after cryopreservation and a range of distances from 1 to 5 cm in nitrogen vapor above the surface of the liquid nitrogen is recommended for optimal sperm viability.