This study assessed kinetic parameters and oxidative stress in bull sperm after post-thaw (PT) or after sperm selection by PercollT gradient, and thermo resistance test (SS + TRT) to identify useful indicators of field fertility. For the experiment, commercial doses of frozen semen were obtained from six Aberdeen Angus bulls. Three of the bulls were classified as high fertility and three as low fertility according to the IFertT index provided by the international breeding company CRV Lagoa. Pooled semen samples were distributed between two treatment groups for analysis: post-thaw (PT) or sperm selection (SS) (PercollT) and thermal resistance test (SS + TRT). The samples were evaluated using sperm kinetics (CASA) (motility %, progressive motility %, VCL mu m/s, VSL mu m/s, VAP mu m/s, LIN %, STR % and WOB%), production of reactive oxygen species (ROS), lipid peroxidation, superoxide dismutase (SOD) enzyme activity and total antioxidant capacity. Data were analyzed using Two-Way ANOVA, considering the fertility index, the treatment used in the samples as effects, and the interaction between these factors. When a significant effect was observed, the values were compared using the Bonferroni test. A Pearson Correlation analysis was performed between the fertility indices and the sperm parameters analyzed in vitro, to evaluate the relationship between sperm quality and the fecundity rate obtained by the bulls. Sperm kinetic parameters, including total motility, progressive motile, and beat cross-frequency, were higher in low fertility compared to high fertility bulls (P < 0.05). However, curvilinear velocity was greater in high fertility bulls followed by SS + TRT. Straight-line velocity, average path velocity, linearity, and beat cross-frequency beat were higher in high fertility bulls after SS + TRT. Reactive oxygen species was correlated with fertility after SS. In addition, there was a decrease in lipid peroxidation was observed only in high fertility bulls. However, lipid peroxidation and high fertility were correlated after PT and SS + TRT. The combination of in vitro sperm kinetic parameters predicted in vivo fertility more accurately than individual kinetic parameters. The lipid peroxidation of sperm is an important indicator of fertility in bulls. High fertility bulls appeared to be more susceptible to lipid peroxidation, which was only reduced in high fertility bulls, suggesting that their sperm can repair the damage induced by oxidative stress.
