Heterospermic semen: implications for managing bull-to-bull variation in fertility

Heterospermic (HS) bull semen involves mixing the semen from two or more sires in the same semen straw. It has the potential to protect against sub-fertile bulls which pass semen quality control checks, due to the presence of sperm from other males in the same straw. Previous studies suggest that its use could increase conception rates due to a more heterogeneous population of sperm in the female reproductive tract. Despite HS semen use increasing in recent years, mainly driven by the introduction of sex-sorted semen and increased usage of HS beef semen on dairy cows, few studies have focused on the physiological mechanisms underpinning its fertility. This review aims to summarise the existing studies on HS bull semen, present hypotheses for its potential role in sperm interaction with the female reproductive tract, introduce the topic of HS sex-sorted semen and, finally, discuss the challenges for the large scale production and implementation of this product in breeding programmes.