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In conclusion, enrichment of semen extender with Se-NPs at a concentration of 1.0 μg/ml improved post-thaw sperm quality of Holstein bulls, and consequently in vivo fertility rate by reducing apoptosis, lipid peroxidation and sperm damage occurring by cryopreservation.įreezing semen is a key procedure that facilitates assisted reproductive technology (ART) applications for both humans and animals. In vivo fertility rate was higher in T3 (90%) than in T1 (59%). Total antioxidants capacity (TAC) in seminal plasma increased (P < 0.05) and malondialdhyde (MDA) concentration decreased ( P < 0.01) in T3 as compared to T1, but did not differ in T4 from that in T1. Percentage of viable sperm increased (P < 0.05), while percentages of early apoptotic, apoptotic and necrotic sperm cells decreased (P < 0.05) in T3 as compared to T1. Results showed that T2 and T3 had a positive effect (P < 0.05) on post-thawing sperm progressive motility, livability and membrane integrity as compared with the control. Finally, the effect of Se-NPs on fertilization capacity was checked in vivo using n = 81cows. Total antioxidant capacity and lipid peroxidation markers were determined in seminal plasma of semen in each treatment. Apoptosis and sperm ultrastructure were also examined. After thawing, semen of each treatment was evaluated for sperm quality parameters, including sperm progressive motility, livability, morphological abnormalities, plasma membrane integrity and chromatin integrity. Diluted semen was packed in straws (0.25 ml) and stored in liquid nitrogen (−196 ☌) for one month. Semen of all bulls were pooled and diluted in a tris-yolk fructose (TYF) extender supplemented with Se-NPs at concentrations of 0 (T1, control), 0.5 (T2), 1.0 (T3) and 1.5 (T4) μg/ml for a final sperm concentration of 80 × 10 6 sperm cells/ml. Five healthy, fertile Friesian bulls were used, and the ejaculates were obtained using an artificial vagina. The aim of this study was to evaluate the effect of adding different concentrations of selenium nano-particles (Se-NPs) in semen extender on bull sperm cryopreservation.