Genetic Parameter Estimation for Breeding Soundness Examination Traits in Angus Bulls
Objective: The objectives of this study were to estimate heritabilities, repeatabilities, and genetic relationships between scrotal circumference and semen quality traits to better understand their potential application in bull fertility genetic evaluation.
Study Description: Eleven purebred Angus breeders provided 5,877 breeding soundness examination records on 4,996 bulls. The data included measurements of scrotal circumference, percentage of motility, gross motility score, percentage of primary abnormalities, percentage of secondary abnormalities, percentage of normal spermatozoa, and percentage of abnormal spermatozoa. Genetic parameters were estimated with statistical software also used by the American Angus Association for genetic evaluations.
Results: Heritability describes the portion of trait variation explained by genetics, indicating the strength of the relationship between breeding values and observed traits. Scrotal circumference was moderately heritable, while semen quality traits such as motility and morphology were lowly heritable. Repeatability, or the consistency of repeated measurements, was moderate for most semen quality traits, but high for scrotal circumference and the percentage of primary abnormalities. Genetic correlations measure how strongly the breeding values of two traits are related. A favorable relationship was found between scrotal circumference and normal spermatozoa, suggesting that selecting for larger scrotal circumference could increase the proportion of normal sperm. A favorable phenotypic relationship was also observed between normal spermatozoa and motility, meaning sperm that moved better tended to have more normal morphology. However, an unfavorable genetic relationship appeared between scrotal circumference and motility. This was unexpected, because normally developed sperm should also move better. These findings suggest that further research is needed to determine whether an optimal scrotal circumference range exists to maximize semen quality.
The Bottom Line: These results indicate that it is feasible to improve bull fertility with genetic selection, but further research is needed to determine the best methods for developing a commercially available genetic evaluation for bull reproduction.
Read the full report. Ellinghuysen, Danielle M.; Tarochione, Audrey S.; Hartman, Ashley R.; Semler, Michael W.; Grieger, David M.; Weaber, Robert L.; Bormann, Jennifer M.; and Rolf, Megan M. (2026) “Genetic Parameter Estimation for Breeding Soundness Examination Traits in Angus Bulls,” Kansas Agricultural Experiment Station Research Reports: Vol. 12: Iss. 1.