Fertility of fresh and frozen rabbit semen inseminated at different times is indicative of male differences in capacitation time

Some reports indicate that sperm from different males differ in capacitation time, and other reports suggest that freezing sperm may affect their capacitation time. These two variables were specifically studied in rabbits in a fertility trial with 96 does inseminated with approximately 1.6 million motile fresh or frozen sperm from three different bucks at 15, 10, 5, and 0 h before expected ovulation. Fresh semen averaged 84% live (unstained) sperm and 88% had normal acrosomes; corresponding values for frozen sperm were 48% and 54%. On the basis of does that became pregnant, average litter size with fresh semen was 5.5 and with frozen semen was 4.8 (p > 0.05), but overall, does bred with frozen semen produced fewer young (p < 0.05). On the basis of total does and total semen, average litter size from insemination at 15, 10, 5, and 0 h was 2.8, 4.2, 3.8, and 1.7, and average litter size for the three bucks was 4.0, 1.8, and 3.6. There was no interaction of type of semen (fresh or frozen) with the other variables in the model (p > 0.05). Bucks and time of insemination affected both the proportion of does that were pregnant and litter size (p < 0.01). A major interaction between buck and time of insemination (p < 0.01) was due apparently to both differential sperm survival and probable capacitation time among bucks. This major interaction should be considered in designing in vitro and in vivo fertility studies, and for selecting males for use in artificial insemination.