While significant attention has been directed towards improving female fertility, male reproductive performance often receives less focus. However, loss in bull fertility can cause substantial loss to a potential calf crop. 

Bull scrotal circumference (SC) refers to the measurement around the widest part of a bull’s scrotum. It serves as a proxy for testicular size and, by extension, reproductive potential. Through the years, Angus members have collected and recorded more than 1.28 million scrotal circumference measurements on yearling bulls, which are then used to predict scrotal circumference expected progeny differences (EPDs).  

Link to female fertility

For years, SC has been linked to puberty and fertility in the bull’s daughters that you keep in your herd. Studies dating back to the mid-1980s show favorable outcomes between cow pregnancy rates and bull semen quality. In fact, Wiltbank and Parish (1986) saw an increased pregnancy rate of 5% in cows mated to bulls having 80% or more normal sperm compared to cows bred to bulls not selected for greater than 80 normal sperm. 

Breeding soundness exams (sometimes referred to as BSE) continue to include scrotal size as well as motility and morphology semen quality metrics to help determine a satisfactory breeder prior to turnout. 

To successfully complete a breeding soundness exam, a bull must have at least 30% sperm motility, 70% normal sperm morphology and a minimum scrotal circumference based on age. 

Relationships among traits

Scrotal circumference is heritable at 0.48, so genetic selection can rapidly take place at that level. 

A negative genetic correlation between a sire’s scrotal circumference and his daughters’ age at puberty means that as scrotal size increases, the age at which daughters reach puberty decreases. Daughters reaching puberty at younger ages are more likely to be cycling at the beginning of breeding season and, as a result, are more likely to become pregnant early on. 

However, in recent years, as tools including heifer pregnancy (HP) have been developed, producers note not all high SC EPD bulls are high HP EPD bulls, and vice versa. 

The genetic correlation in the current Angus population between SC and HP, which is a sire’s daughter’s ability to get pregnant in her first breeding season, is predicted near zero. This means the genes that control SC are not the same genes that control HP. It can also be extrapolated that if heifers have reached pubertal age at breeding, a genetic predisposition to secure a pregnancy in the first breeding season still exists. 

Another interesting note about SC is it appears that its genetic trend has slowed in recent years, showing a gain in SC EPD of just 0.03 from 2020-2023 compared to an increase of 0.11 from 2017-2020. Fig. 1 depicts the genetic trend by birth year for SC EPD from 1990-2023. 

Evidence supports that selecting for an increased scrotal size maintains its benefits of reflecting daily sperm production, which is why it is still used as a benchmark in breeding soundness exams to determine if a bull is satisfactory as a potential breeder.  

Ongoing research and collaboration

Currently Angus Genetics Inc. (AGI) and the Association are collaborating with the animal breeding group at Kansas State University to further examine male fertility traits in Angus bulls by analyzing data points collected during breeding soundness exams. 

In total, data collected on 5,292 Angus bulls is being examined. The work and its prospective results should help Angus breeders better understand how scrotal size relates to sperm progressive motility, primary and secondary abnormalities, and percent normal morphology as well as estimate the heritability estimates and genetic correlations of these male fertility traits. 

Preliminary estimates show interesting results, including favorable genetic correlations between SC and percent normal morphology of sperm. However, unfavorable genetic correlations between SC and progressive motility appeared. More analysis is needed to further understand the biological implications of these preliminary results.

Conclusion

Even so, SC continues to be a trait that helps seedstock and commercial producers alike gain perspective during the breeding season. Its importance cannot be overstated, as it serves as a reliable indicator of reproductive potential and a key determinant of breeding success. As the industry continues to evolve, embracing advancements in genetics and technology, the significance of