Copper nanoparticles as a precision nutrition strategy to enhance reproductive resilience and in vitro sperm fertilizing capacity in heat stressed breeding bulls.

Heat stress (HS) is a major environmental challenge that compromises fertility in breeding bulls by disrupting physiological homeostasis and reproductive function. This study evaluated the potential of dietary copper nanoparticles (CuNPs) to improve reproductive resilience and thermotolerance in Sahiwal breeding bulls exposed to summer HS. Eighteen clinically healthy bulls were randomly assigned to three dietary treatments (n = 6/group): control (no CuNPs), S1 (10 mg CuNPs/kg dry matter (DM), and S2 (20 mg CuNPs/kg DM), and maintained under peak HS conditions for 16 weeks. The supplementation of CuNPs significantly improved thermoregulation by reducing rectal temperature, pulse rate, respiration rate, and circulating cortisol while preserving rumen motility. Supplemented bulls exhibited greater semen quality and sperm functional competence, including improved plasma membrane integrity, acrosome integrity, and DNA stability. These modifications were accompanied by enhanced testicular blood perfusion and preservation of ultrasonographic echotexture. Oxidative stress was also attenuated by CuNPs, as indicated by reduced malondialdehyde concentrations and enhanced antioxidant defenses, including superoxide dismutase, catalase, glutathione peroxidase, and total antioxidant capacity. Improved redox status was associated with higher reproductive hormone levels and enhanced immune function. During in vitro fertilization, semen from supplemented bulls achieved higher cleavage and blastocyst rates and produced a greater proportion of transferable grade embryos than the control group. The beneficial effects were more pronounced in the S2 group. These findings demonstrate that dietary copper supplementation in nanoparticle form enhance thermotolerance, redox homeostasis, testicular function, and in vitro sperm fertilizing capacity in heat-stressed Sahiwal breeding bulls.