Cryopreservation of boar semen: Where we are

The preservation of boar semen can be divided into two methods as follows: fresh boar semen and frozen boar semen. The first method is only to preserve the sperm for a short period, ranging from 3 to 10 days and depends on the semen extender used, however, its benefit is only in use for artificial insemination on a particular farm or for a short distant transportation. The latter method has been developed in order to keep the semen of superior genetic boars for long distant transportation or preservation of their genetics for future production or to set up a new herd in case of unforeseen outbreaks of a particular disease such as African Swine Fever and Highly Pathogenic Porcine Reproductive and Respiratory Syndrome. Nevertheless, the results of boar semen cryopreservation do not produce that high a yield, because during the cryopreservation process semen faced with several steps of temperature fluctuation, so called cold stress which leads to oxidation that can produce oxidative molecules (reactive oxygen species, ROS) such as superoxide anion (O2^-), hydrogen peroxide (H2O2), peroxyl radical (ROO•) and the very reactive hydroxyl radicals (OH^•). These ROS have a detrimental effect on the sperm structure and result in low motility and fertilizing ability. This might be the reason that many studies have paid attention to diminishing the negative effect of ROS on sperm quality by supplementation of selective antioxidants (antioxidant enzymes, amino acids, vitamins, natural antioxidants etc.) into the freezing extender with the hope that those antioxidants will cope with the oxidative molecules by improving the in-vitro post-thawing semen quality and also the fertility test on farm. This review aims to provide and discuss the results of some studies on boar semen cryopreservation both with success or non-success and provide ongoing research for further development of boar semen cryopreservation.