Seasonal changes in testosterone and thyroxine concentrations in Mediterranean rams and bucks and their relationship with sperm cryoresistance.

• In the conditions of the study, bucks were more seasonal than rams. • Plasma testosterone and thyroxine were negatively related to buck sperm freezability. • Plasma testosterone and thyroxine were unrelated to ram sperm cryoresistance. • For cryopreservation, it is recommended to collect buck sperm in spring and winter. This study examines the relationship between seasonal changes in plasma testosterone and thyroxine concentrations and the relationship between these changes and the sperm resistance to the freezing-thawing process in Merino x Merino Precoz crossbreed rams and Florida breed bucks. Plasma testosterone and thyroxine concentration, sperm head area, sperm variables and their cryoresistance ratios were analyzed throughout a year. Both species showed a rise in thyroxine preceding testosterone fall. Plasma thyroxine concentrations in rams were higher during autumn than in spring and summer (P <0.05), whereas testosterone concentrations were not influenced by season. Conversely, bucks showed seasonal differences in plasma testosterone levels, with higher concentrations during summer and autumn than winter and spring (P <0.05), whereas thyroxine concentrations were not influenced by season. In bucks, a negative correlation was detected between plasma testosterone concentrations and the cryoresistance ratio for membrane integrity (R=-0.88; P <0.05) from April to September and between plasma thyroxine concentrations and the cryoresistance ratios for total motility (R=-0.66; P <0.05), curvilinear velocity (R=-0.61; P <0.05), the amplitude of lateral head displacement (R=-0.77; P <0.05) and the beat-cross frequency (R=-0.74; P <0.05), throughout the year. The results suggest that the annual variation in sperm cryoresistance may be related to seasonal endocrine changes, at least in goat. Winter and spring are the most appropriate seasons to collect and freeze sperm from Florida goat. [ABSTRACT FROM AUTHOR]