Artificial insemination (AI) with liquid-stored semen is a globally accepted method (an effective method) of breeding pigs (Johnson et al., 2000). In Western Europe, ≥90% of sows have been bred by AI for ≥20 years (Rodríguez-Gil & Estrada, 2013), whereas in Japan, it is finally becoming widespread at 60% range, according to the Japan Pork Production Association (https://jppa.biz/jppa-docum ent/). The outcome of AI is largely dependent on the semen quality and insemination procedure. In practice, liquid semen stored at 15-20°C for 1-5 days has been used for intra-cervical insemination in pigs (Johnson et al., 2000;Knox, 2016). Especially, semen quality is guaranteed using optimal extenders, and high-quality semen with fertilising capacity, which is inherently linked to the quality of the spermatozoa themselves (Tsakmakidis et al., 2010).However, spermatozoa are vulnerable to oxidative damage due to a low amount of cytoplasm, high concentrations of polyunsaturated fatty acids, and inadequate antioxidant protection (Aitken