Bull fertility is not a simple pass-or-fail trait, according to experts with the Kansas State University Beef Cattle Institute (BCI).

During a recent Cattle Chat podcast, specialists emphasized that fertility should be evaluated over time and under real breeding conditions, rather than relying solely on a single test result.

BCI experts explained that subfertility refers to a bull’s reduced ability to achieve acceptable pregnancy rates within a defined breeding period. Subfertile bulls may still settle cows, but often at lower rates or over longer timeframes. This can result in fewer total pregnancies, a more spread-out calving season and reduced overall herd efficiency.

Guest and veterinarian Dr. Stephen Hughes of South Africa expressed interest in how bull fertility is evaluated in the United States. In response, BCI experts noted that fertility is often described as a bull’s ability to impregnate approximately 25 cows within a 65- to 70-day breeding season, although no single definition applies to every operation or production system.

“Biologically, about two-thirds is about as good as we can do,” said Todd Gunderson, a veterinarian who has conducted extensive research on breeding soundness exams (BSEs).

Gunderson emphasized that while BSEs are an important management tool, they cannot measure everything that contributes to fertility. He says observing bulls during the breeding season — watching mobility, libido and ability to service cows — is a critical part of evaluating reproductive performance.

BCI experts highlighted both the value and the limitations of breeding soundness exams.

“The breeding soundness exam tells us the bull can get it done today,” said veterinarian Brad White, noting that fertility can change with age, health status and environmental conditions. Regular evaluation is especially important as bulls move beyond their first breeding season.

K-State veterinarian Bob Larson added that overall bull health and semen quality play a major role in reproductive success. A bull that passes a fertility exam as a yearling is not guaranteed to remain fertile for life. Gunderson agreed, explaining that BSE results tend to be more reliable once bulls reach maturity and that yearling and mature bulls often produce different fertility outcomes.

They encourage producers to keep in mind that successful breeding programs rely on a combination of soundness exams, ongoing observation and good recordkeeping to identify potential fertility issues early and maintain a tight, efficient calving season.

For more information on bull fertility, check out the BCI Cattle Chat podcast. For questions, reach out on social media or send an email to [email protected].