Effect of freezing temperature, at which straws were plunged into liquid nitrogen, on the post-thaw motility and acrosomal status of ram spermatozoa

The present study was conducted to observe the effect of initial freezing temperature on subsequent survival and acrosomal integrity of Malpura and Bharat Merino ram spermatozoa during post-thawing incubation. Semen samples were diluted in TEST–yolk-glycerol extender, loaded in 0.25 ml straws and cooled down to −25, −75 or −125 °C freezing temperature using a programmable cell freezer. Computer assisted sperm analysis and acrosomal integrity of thawed samples were assessed after thawing and at hourly intervals during incubation at 37 °C for 4 h. The percentage of motile cells in samples frozen at −125 °C were 80.3 and 63.7 after post-thawing and -thawing incubation, compared to 75.9 and 39.7 at −25 °C or 73.9 and 51.8 at −75 °C temperatures, respectively. The spermatozoa with normal acrosome were also significantly, respectively, higher in samples frozen at −125 °C, compared to −25 and −75 °C temperatures. There were no significant breed variations on percentage of motile, percentage of rapidly motile cells, percentage of normal acrosomes, curvilinear velocity and lateral head displacement except straight line velocity and average path velocity of spermatozoa. The results indicated that −125 °C initial freezing temperature conferred the best cryopreserving ability to ram spermatozoa for post-thawing thermoresistance test compared to −25 or −75 °C freezing temperature.