Factors affecting preservation and fertility of bull sperm: a brief review

This paper is a brief review of the factors that determine the number of sperm required for insemination to obtain high fertility and ways that sperm viability might be prolonged. Damage to sperm during freezing results in a requirement, after thawing, of about 6 x 10(6) motile sperm (> 10 x 10(6) total) per insemination to achieve near-maximal fertility, whereas 2.5 x 10(6) motile fresh sperm result in high nonreturn rates. Multiple inseminations to bracket the time of ovulation are usually not economical except in superovulated cows. Earlier unpublished work on sperm packaging for slow release in the cow and methods for stabilizing membranes to increase sperm survival time in the cow are discussed. Current studies are directed towards reducing catabolic metabolism of sperm and studying membrane changes during freezing and thawing and during incubation with bovine oviduct epithelial cells. Studies with bull sperm indicate that the choline and ethanolamine phosphoglyceride components of their membranes represent an unstable configuration. Exposure of sperm to liposomes with the sterol cholesterol can alter the phospholipid bilayer and increase capacitation time. Similar approaches may produce sperm with a longer fertilizing life following insemination. New procedures in vitro permit low cost modelling of fertilization, which will facilitate research by reducing the cost of studies in vivo.