Liquid storage of stallion spermatozoa - Past, present and future

The ability to store stallion spermatozoa between the events of semen collection and insemination has facilitated improved rates of gain in selective breeding programs by enabling the transport of spermatozoa. While cryopreservation is the only viable means of storing spermatozoa for long intervals, the higher costs and reduced fertility of cryopreserved spermatozoa have led to most breeders opting to use liquid stored spermatozoa. Stallion spermatozoa is commonly cooled during liquid storage (approximately 4-5 °C), and there has been an enormous body of research dedicated to development of protocols and media to facilitate sperm survival including identification of energy sources, antioxidants, pH buffers and toxic metabolite scavengers, along with membrane-stabilising components to reduce deleterious effects of cold shock. Despite these efforts, the upper time limit for cooled sperm storage is ∼ 72 h and there are many stallions whose spermatozoa cannot tolerate the process of cooling. As such, media have been developed to allow spermatozoa to be liquid stored at higher temperatures (15 - 22 °C), and these efforts have led to development of a medium that can effectively store stallion spermatozoa for at least 7 d with no appreciable loss of fertilising capacity. Furthermore, there is an increasing body of research aimed at providing substrates that allow spermatozoa to repair and regenerate during storage, thereby challenging the paradigm that post-ejaculatory sperm damage is irreversible. This review aims to summarize stallion sperm liquid storage strategies and the developments that led to the technologies available today.