Successful pregnancies in cows following double freezing of a large volume of semen.

The objective of the following paper is to describe a new technology for large volume and double freezing of semen in 12 mL test tubes. Semen from two different bulls was frozen with a new technique using 12 mL test tubes and was refrozen after thawing in mini straws. All freezing was done in a "Multi thermal gradient" (MTG) freezing apparatus, which moves the container at a constant velocity (V) through a thermal gradient (G) producing a controlled cooling rate B = (G) x (V). Each of the two bulls ejaculated were evaluated for post thaw motility in the lab and then in a field trial which was carried out in a split sample mode. We inseminated 105 cows after a double freezing/thawing cycle, and another 123 cows were inseminated with semen frozen in mini-straws and a conventional method. The results showed a 75 +/- 5% post thaw motility after freezing a 12 mL test tube and 50 +/- 5% after a second freezing/thawing in mini-straws, respectively. Controlled vapour freezing showed a 60 +/- 10% post thaw motility. The results of the field trial showed a pregnancy rate of 44% (47/105) for the double freezing group in comparison to 45.5% (56/123) for the controlled group. These results can be beneficial for large volume freezing, and therefore for bull semen cryobanking in a large volume which will be followed by second freezing in a regular insemination volume.