Case Report: Unexplained field infertility in an AI purebred Piedmontese bull with normal conventional semen parameters

This case report describes a purebred Piedmontese bull selected for artificial insemination (AI). Despite meeting all conventional semen quality thresholds, the bull showed complete reproductive failure during progeny-testing field inseminations: across all inseminations performed, no pregnancies were confirmed, resulting in a field fertility of 0% and indicating complete infertility under field conditions. To investigate this discrepancy, the bull’s semen was compared with a reference population of 19 Piedmontese bulls managed under identical conditions. Routine ejaculate characteristics, including volume, concentration, and both fresh and post-thaw motility, fell entirely within the expected range, as well as all CASA-derived kinematic parameters. In contrast, flow cytometric evaluation revealed severe functional abnormalities. DNA fragmentation was markedly elevated, and reactive oxygen species (ROS)-related fluorescence intensity was markedly increased, whereas superoxide production was lower than in the control group. Additionally, free thiol content was significantly reduced. Taken together, these alterations indicate not only extensive DNA damage but also a markedly impaired ability of spermatozoa to counteract oxidative stress. These findings demonstrate that profound sperm functional defects may remain undetected through conventional AI-Centre screening, yet critically impair fertility in vivo. Incorporating advanced functional assays into routine monitoring may improve early identification of subfertile sires and enhance reproductive efficiency in bovine breeding programs.