SURVIVAL OF RABBIT SPERMATOZOA FROZEN AND THAWED AT DIFFERENT RATES WITH AND WITHOUT SEEDING

The ability to freeze rabbit spermatozoa with resulting high survival and fertility, would be very useful experimentally and have practical application. The experiments reported here were designed to explore different methods of freezing and thawing rabbit spermatozoa to improve their survival. Dutch male rabbits on a regular semen collection schedule provided sufficient spermatozoa per ejaculate to allow factorial arrangements which including an egg yolk-acetamide extender processed differently. This extender provides substantial protection during freezing, but previous reports indicated that there was considerable damage to spermatozoa during freezing and thawing. The results of the experiments reported here demonstrated consistently that mechanical seeding of extended semen at -6-degrees-C, so that it did not supercool to -14-degrees-C, substantially improved post-thaw progressive motility of spermatozoa (P < 0.01). Holding the semen at -6-degrees-C to equilibrate before seeding, or slow freezing after seeding had only minor effects. Freezing at -15-degrees-C min-1 from - 6-degrees-C after seeding to -100-degrees-C and then transfer to liquid nitrogen was equal or superior to slower rates of freezing (-5 and -10-degrees-C min-1). Thawing semen at 25-degrees-C was superior to thawing at 45 or 65-degrees-C (P < 0.01). With optimal freezing procedures, the post-thaw percentage of progressively motile spermatozoa usually exceeded 60%.