State-of-the-art of boar sperm preservation in liquid and frozen state

Pig breeding is mainly conducted through Artificial Insemination (AI) in Western and developing countries. Apart from requiring specific catheters and trained staff, preserving boar semen in proper conditions is needed to ensure high reproductive performances. Although, at present, boar sperm may be preserved in liquid (15-17oC) or frozen states, more than 95% of AIs are conducted using liquid semen. The present work reviews the state-the-art of these two preservation technologies. Thus, the composition and types of extenders for liquid-stored semen are discussed, together with the specific requirements for boar sperm, which are stored at 15-17oC. Commercial extenders for liquid semen are compared and the effects of storage on sperm quality are also summarised. In the second part of the manuscript, the main features of boar sperm cryopreservation are described and reference to cryodamage is also made. These cryoinjuries mainly affect sperm motility, membrane permeability and chromatin integrity. Furthermore, the individual variability in the sperm resilience to withstand cryopreservation procedures is reviewed and a brief summary about freezability markers is also included. Final sections briefly discuss the improvement of freezing extenders with additives, such as seminal plasma and antioxidants, and highlight the relevance of using a proper AI technique to avoid dramatic drops in reproductive performance.