Red LED Treatment Confers Superior Early Post-Thaw Sperm Kinetic Resilience and Identifies Progressive Motility as the Most Informative Functional Marker in Dogs.

LED-based photobiomodulation has attracted increasing interest as a potential strategy to improve post-thaw sperm performance, but its biological relevance in canine semen remains poorly defined. This study investigated whether blue and red LED exposure influences baseline sperm quality and the dynamic pattern of post-thaw sperm deterioration in dogs. Semen from six healthy adult dogs was collected, diluted, allocated to control (no LED), blue LED, or red LED groups, and cryopreserved under standardized conditions. After thawing, sperm quality was evaluated by computer-assisted sperm analysis (CASA), hypo-osmotic swelling test (HOST), eosin-nigrosin staining, and a rapid thermoresistance test. Relative motility retention at 10 min post-thaw was calculated as a synthetic indicator of short-term kinetic resilience. No significant baseline differences were detected among LED conditions for any seminal or CASA-derived parameter. In contrast, post-thaw sperm kinetics were clearly time-dependent and treatment-sensitive. Total motility showed a strong temporal decline and a significant treatment-dependent decay pattern, whereas progressive motility exhibited significant effects of treatment, time, and treatment × time interaction, indicating greater sensitivity for detecting functional differences among LED conditions. The red LED condition preserved a substantially greater fraction of both total and progressive motility during the first 10 min after thawing, demonstrating superior short-term post-thaw kinetic resilience. In conclusion, red LED was the most effective condition for preserving early post-thaw sperm kinetic resilience in canine semen, and progressive motility proved to be the most sensitive functional marker for detecting treatment-related differences.