EFFECT OF COLD SHOCK AFTER THAWING ON ACROSOMAL MAINTENANCE AND MOTILITY OF BOVINE SPERMATOZOA FROZEN IN PLASTIC STRAWS

Two experiments were conducted to determine the effects of thawing rate and post-thaw cold shock on survival of bull spermatozoa diluted in skim milk and packaged in .3-ml Continental US straws. Average acrosomal maintenance and progressive spermatozoal motility during post-thaw incubation for 0, 3 and 6 h at 37.degree. C were considerably higher for straws thawed rapidly in warm water at 35.degree. C, whether or not exposed to cold shock at 5.degree. C after thawing, than for straws thawed slowly in 5.degree. C iced water. There was no evidence of cold shock damage when thawing time at 35.degree. C was restricted (10 s) to prevent the temperature of the thawed semen from rising above 5.degree. C. Cold shock injury occurred when seminal temperature during thawing was permitted to rise above 5.degree. C (23.degree. and 35.degree. C) before the straws were placed in iced water. Cold shock injury was more pronounced for acrosomal retention than for motility of sperm. Greatest damage to acrosomal maintenance occurred when the postthaw temperature of the semen was lowered quickly from 35 to 5.degree. C; retained acrosomes were reduced from 67.7% for control straws thawed at 35.degree. C/80 s to 57.9% for straws plunged into a 5.degree. C iced bath for 30 s after being thawed in the same manner. Regardless of thawing method, caution should be exercised to minimize the exposure of thawed straws and inseminating devices to cold.