The effect of polyunsaturated fatty acids on bovine sperm, in vitro

Artificial Insemination (AI) is the single most important tool in facilitating the genetic improvement of dairy cows. Both frozen and liquid semen are used in AI, however the majority (~95%) of insemination doses worldwide (and in Ireland) employ use of frozen semen (Vishwanath & Shannon, 2000). Only 5% of insemination doses used in Ireland are liquid semen, however, during peak breeding season of April and May, this number could rise to 25%. Use of frozen semen in AI has the benefit of long term storage as it causes the biochemical and developmental changes in sperm to stop, however, after the freeze thaw process, more than 50% of sperm cells are damaged (Vishwanath & Shannon, 2000), meaning a higher concentration of sperm cells per dose is required and therefore, less insemination doses per ejaculate are achieved. The use of stored fresh semen would remove this problem of damaged sperm cells after a freeze thaw process. Liquid semen would also allow a lower concentration of sperm cells per insemination dose, thereby, maximising the use of semen from high genetic merit bulls. It has been reported that although liquid bull semen maintains motility for up to 4 weeks post collection, in vivo fertility declines significantly after 4-5 days post collection (Vishwanath & Shannon, 1997). The reason for this sudden drop in fertility remains to be elucidated; however, it could be attributed to a number of factors, such as changes in the lipid profile and a subsequent loss in cell membrane fluidity, perhaps due to attack from reactive oxygen species (ROS). The addition of long chain fatty acids to bull sperm in vitro may have a stabilising effect on the sperm membrane, through fatty acids constituents being incorporated into the sperm cell membrane, as reported in a study by Neill & Masters (1971) or by inhibiting or reducing the lipid peroxidation (LPO) of the sperm membrane, which is high in polyunsaturated fatty acids.