Effect of freezing-thawing protocols on the performance of semen from three rabbit lines after artificial insemination

The effect of different freezing and thawing protocols on the results observed after artificial insemination with semen from three different rabbit lines (two maternal lines selected for litter size at weaning, lines A and V, and one line selected for growth rate from weaning to slaughter, line R) was studied. The sperm were frozen with a Tris-citric acid-glucose extender which included 1.75 M DMSO and 0.05 M sucrose as cryoprotectants. The straws were cooled to 5 degreesC for 45 min and then some of them were frozen in a freezer at -30 degreesC for 30 min, whereas the other group of straws were frozen in liquid nitrogen vapor (LNV, 5 cm above the liquid nitrogen level) for 10 min. Straws were thawed at two different temperatures: 50 or 70 degreesC for 10-12s. Significant differences were observed between freezing-thawing protocols, obtaining better results in fertility rate (percentage of pregnant females) when sperm had been frozen in LNV (fertility rate increased between 30 and 50 points in all the lines); the best prolificacy was observed when sperm had been frozen in LNV and thawed at 50 degreesC (70% versus 32% fertility rate, P < 0.01 and 7.4 versus 5.9 total number of young born, P < 0.01 when sperm had been frozen in LNV or at -30 T and thawed at 50 T, respectively). As for the rabbit line, significant differences were observed between lines in fertility rate (62 and 68% versus 45% fertility rate for lines A, V and R, P < 0.01), and total number of young born (5.8 versus 6.9 versus 4.6 total number of young born for lines A, V and R, P = 0.02). The best results for all lines in both fertility and total number of young born were observed when sperm had been frozen in LNV and thawed at 50 degreesC (85% versus 84% versus 50% fertility rate and 6.7 versus 8.3 versus 7.3 total number of young born for lines A, V and R, respectively), when compared to the results of the control group, frozen at -30 degreesC and thawed at 50 degreesC (30% versus 52% versus 19% fertility rate and 6.7 versus 6.4 versus 4.5 total number of young born for lines A, V and R, respectively). In conclusion, the best results (fertility rate and prolificacy) for all the rabbit lines were obtained after freezing in liquid nitrogen vapor and thawing at 50 degreesC, being more pronounced in the line selected for high growth rate (line R). (C) 2002 Elsevier Science Inc. All rights reserved.