Effect of freezing rate of ram spermatozoa on subsequent fertility in vivo and in vitro

Ram spermatozoa are most susceptible to damage during freezing between the temperatures of −10°C and −25°C. The objectives of the present study were to examine how freezing rate through this critical temperature zone affected the fertility of spermatozoa as assessed in vivo and in vitro. Semen from six adult rams was frozen at two different rates (“fast”: 5°C/min from +5 to −25°C; “slow”: 0.5°C/min from +5 to −25°C). In Experiment 1, semen from the fast and slow treatments was used to fertilize ovine oocytes that had been matured in vitro. Semen from the fast treatment yielded a higher cleavage rate (57% vs. 26%; P