Factors Affecting the Survival of Ram Spermatozoa during Liquid Storage and Options for Improvement

SIMPLE SUMMARY: The success of semen preservation is vital for the use of artificial reproductive technologies in sheep. However, reduced temperatures can cause significant damage to the sperm cell. Recent investigations in other species have identified room-temperature liquid storage as a viable alternative if spermatozoa are protected from the increased risk of lipid peroxidation, a side effect of unaltered metabolism. The following review aims to summarise the factors which contribute to the survival of ram spermatozoa during liquid storage and the role of pro-survival factors and antioxidants in helping to ameliorate the damaging effects caused by lipid peroxidation on fertility. This would contribute towards establishing a new method of semen preservation for the sheep industry which maximises fertility following storage and artificial insemination. ABSTRACT: Semen preservation is an essential component of reproductive technologies, as it promotes genetic gain and long-distance semen transport and multiplies the number of ewes able to be inseminated per single ejaculate. However, the reduced temperature during cold storage at 5 or 15 °C inflicts sub-lethal damage to spermatozoa, compromising sperm quality and the success of artificial breeding. New and emerging research in various species has reported the advantages of storing spermatozoa at higher temperatures, such as 23 °C; however, this topic has not been thoroughly investigated for ram spermatozoa. Despite the success of storing spermatozoa at 23 °C, sperm quality can be compromised by the damaging effects of lipid peroxidation, more commonly when metabolism is left unaltered during 23 °C storage. Additionally, given the biosafety concern surrounding the international transport of egg-yolk-containing extenders, further investigation is critical to assess the preservation ability of synthetic extenders and whether pro-survival factors could be supplemented to maximise sperm survival during storage at 23 °C.