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In a liquid environment, at high dilutions, fertility of bull sperm is maintained for 3-5 days when stored at ambient temperatures (10-21 degrees C), after which time it steadily declines at a rate of 3-6% per day. This decline in fertility occurs irrespective of whether the sperm are stored at 5 degrees C or at 15 degrees C, but the rate is greater once storage temperatures exceed 25 degrees C. Sperm motility can be maintained for extended periods in an environment where the extracellular oxidative stress is minimized by reducing the oxygen tension, by addition of antioxidants and chelating agents; however, this will not prevent a significant drop in fertility after five days of storage at ambient temperature. The requirement of energy by the sperm-motility apparatus demands a high level of respiratory activity. This system is very active and the free radicals produced in vivo during this process could lead to chromatin damage. As no internal repair mechanism exists in sperm, an extraneous supply of protectants, or an environment where damage is minimized, is essential to maintain its fertilizing potential. The lack of extended storage potential of sperm, even in the presence of antioxidants, seems to suggest that although oocyte-penetrating ability of the sperm could still be intact, the high rate of intracellular metabolic activity could lead to mitochondrial DNA damage and chromosomal abnormalities that would compromise the viability of the resulting conceptus.

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It is approaching three decades since the first public evidence of sex-sorting of semen. The technology has progressed considerably since then with a number of institutions and researchers collaborating to eventually bring this to application. The technical challenges have been quite substantial and in the early years the application was limited to only heifer inseminations. Comparable fertility of sex-sorted semen with conventional semen has been an aspirational benchmark for the industry for many years. Significant investment in research in the primary biology of sex-sorted sperm and associated sorting equipment ensured steady progress over the years and current methods particularly the new SexedULTRA-4M™ seems to have now mostly bridged this fertility gap. The dairy and beef industry have adopted this technology quite rapidly. Other animal industries are progressively testing it for application in their specific niches and environments. The current state of the art in the fundamentals of sex-sorting, the biology of the process as well as new developments in machinery are described in this review.