Effect of Different Freezing Rates on Post-Thaw Semen Quality in Yak

A study was conducted on yak semen to evaluate the effect of different freezing rates on the quality of frozen semen. The semen was frozen following three freezing rates viz., Freezing rate I (@ 5ºC/minute from +4oC to -10oC, @ 40ºC/minute from -10ºC to -100ºC and @ 20ºC/minute from -100ºC to -140ºC), II (@ 4ºC/minute from +4ºC to -12ºC, @ 40ºC/minute from -12ºC to -40ºC and @ 50ºC/minute from -40ºC to -140ºC and III (@ 5oC/minute from +4ºC to -10ºC, @50ºC/minute from -10ºC to -100ºC and @ 20ºC/minute from -100oC to -140ºC). The sperm motility, live sperm, HOST-reacted sperm, total incidence of acrosomal changes and extracellular release of Alanine aminotransferase and Aspartate aminotransferase of yak semen extended in Tris extender differed significantly (P <0.01) between stages of processing and freezing but did not differ significantly between freezing rates and due to stage × freezing rate interaction. Analysis of variance revealed that in frozen semen, a freezing rate significantly (P<0.05) influenced the percentage of live sperm with Mitochondrial Membrane Potential (MMP) but did not significantly influence the percentage of sperm with DNA damage. The percentage of live sperm with MMP in frozen semen was significantly higher for freezing rate III than for freezing rate II but the difference between freezing rates I and III was not significant.