Antibiotic-Free Hypothermic Storage of Boar Semen at 5 °C with Next-Day On-Farm Cooling: Fertility and Biosafety Under Field Conditions.

Background: Antibiotic-free approaches to boar semen preservation are gaining importance to counter emerging antimicrobial resistance. Hypothermic storage at 5 °C, instead of the conventional 17 °C, is a promising strategy to eliminate antibiotics still commonly used in extenders. For practical adoption, the method must be simple and compatible with on-farm routines. Objective: To assess fertility when cooling was initiated on farm one day after delivery, and to evaluate the robustness of cold-stored semen to temporary warming and subsequent re-cooling, mimicking typical handling on insemination days. Methods: Individual ejaculates (n = 34) from six boars were extended in Androstar® Premium either without antibiotics (5 °C) or with gentamicin (17 °C control). One day after collection, antibiotic-free doses were cooled on farm to 5 °C and used alongside controls in routine insemination of 270 sows. Sperm quality was evaluated by computer-assisted semen analysis and flow cytometry, and bacterial counts were monitored. In a separate test, cold-stored doses were exposed to 20 °C for 60 min and re-cooled to 5 °C. Results: Farrowing rates and litter sizes did not differ between groups (p > 0.05). In antibiotic-free samples after 120 h, bacterial counts were mostly not detectable or low (<102 CFU/mL). Sperm motility and plasma membrane integrity in cold-stored doses remained >80%, comparable to controls (p > 0.05). Temporary warming did not affect sperm quality or bacterial counts. Conclusions: Antibiotic-free semen storage at 5 °C is easy to implement in practice and maintains fertility under field conditions. Broader validation under routine conditions is encouraged in support of the One Health concept.