Environmental and male variation factors of freezability in rabbit semen

The aim of this study was to analyze the environmental and male effects that could have an influence on sperm freezability using a recursive model. A total of 853 ejaculates from 217 males belonged to a paternal rabbit line were collected and frozen. Six different traits were evaluated: the sperm concentration (10(6) spermatozoa per mL), the acrosome integrity on fresh (%) and frozen-thawed semen (%), the sperm motility on fresh (%) and frozen-thawed semen (%), and the percentage of viable sperm on frozen-thawed semen (%). In addition, two synthetic traits were computed, the relative reduction of acrosome integrity (%) and relative reduction of motility (%) after the freezing-thawing process. A multiple-trait recursive model was used to analyze the relationships between the semen traits considered. For the fixed effects studied, the season had the highest effect on postthaw semen characteristics. Results of the analysis of recursive coefficients showed that fresh semen concentration and motility influence the future freezability of the semen. All traits studied presented moderate repeatabilities ranging from 0.11 to 0.38. These results provide conclusive evidence that sperm freezability in rabbits could be heritable. Regarding male correlations, there were large positive male correlations between fresh traits (r(m) = 0.77 to 0.57), and between direct frozen-thawed traits (r(m) = 0.72 to 1). Male effects on fresh and direct frozen-thawed traits were generally positively correlated. This correlation was moderate to high for fresh semen motility with all frozen-thawed traits (r(m) = 0.41 to 0.74) and for frozen-thawed semen and all fresh traits (r(m) = 0.5 to 0.74), these results suggest that these traits could be genetically related. Further studies involving more males and ejaculates should be conducted in the future to estimate the heritabilities and genetic correlations of postthaw semen traits in rabbits.