Extension specialist explains bull and female contributions to fertility

Bull fertility has long been a cornerstone of profitable beef operations, yet predicting which sires will consistently deliver pregnancies remains a challenge. 

While advances in genetics, nutrition and reproductive management have improved pregnancy outcomes, consistent sire performance remains difficult to predict. Even among bulls which meet accepted industry standards, reproduction outcomes can vary widely.

For seedstock producers, this variation carries implications beyond a single breeding season, influencing customer success, genetic progress and long-term sustainability. If a producer’s bottom line depends on calves hitting the ground, this isn’t something to gamble with.

“Efficiency is one of the key components of beef cattle sustainability,” says Saulo Zoca, assistant professor and Extension beef cattle specialist at the University of Tennessee. 

This encompasses more than just environmental sustainability, Zoca explains. It includes maintaining a profitable, generational business where herd fertility plays a key role.

The function of fertility 

Fertility is often discussed as a pass-or-fail trait, but research continues to show it exists along a spectrum. 

Bulls which are technically fertile can still differ substantially in how efficiently they establish pregnancies and how consistently those pregnancies are maintained. 

Subfertile bulls indicate cattle which may successfully breed some females but consistently achieve fewer pregnancies overall and more late conceptions. 

Beyond conception, fertility also influences pregnancy retention. Research led by Ky Pohler, associate professor at Texas A&M University, shows a bull’s contribution extends past fertilization and into early development.

“The question is how this contribution plays forward in pregnancy,” Pohler says. “When a cow is open or loses a calf, who pays the price most of the time? The cow does. She ends up being sold. What is the contribution of the sire in these situations?”

This distinction helps explain why bulls meeting standard semen quality thresholds can still differ widely in fertility, reinforcing the need to evaluate it as a biological process rather than a single checkpoint.

The role and limits of a BSE

When a bull’s fertility is subpar, the economic effects are immediate. However, fertility itself is influenced by a wide range of factors, including disease, injury, structural soundness, nutrition and environmental stressors. These variables make it a complex trait to evaluate.

The breeding soundness exam (BSE) remains the primary tool for managing sire fertility risk. Long established and widely accepted, it evaluates a bull’s physical soundness, scrotal circumference and semen quality, including motility and morphology. 

“The BSE is not only our current method, but it is the standard and, at this point, essentially the only method we have to evaluate bull fertility,” Zoca says.

Semen evaluation focuses on motility and morphology, with thresholds of at least 30 percent motility and 70 percent normal morphology for natural service and 60 percent motility for artificial insemination (AI) semen before freezing. Conducted properly, BSEs serve as a risk management tool rather than a performance predictor. 

“When I talk to producers about BSEs, I encourage them to think of it as an insurance policy,” Zoca says. “When you do a BSE, you hope all bulls pass, but if a bull is not going to pass, you want to know before he is turned out with cows.”

Despite these measures, around 20 percent of bulls fail BSEs for physical or semen-related issues, underscoring the need for proactive evaluation. 

Zoca advises to test bulls before each breeding season. 

“Even if you use the same bull for multiple breeding seasons in a year – such as spring and fall calving – you still need to retest the bull before each season,” he states.

These failures highlight the importance of testing, but they also point to a deeper issue. Among bulls that pass, significant fertility differences still exist.

Data reveals sire-to-sire variation

Commercial data highlights just how large fertility variation can be. 

Pohler describes one Angus fixed-time AI cohort in which pregnancy rates ranged from nine to 64 percent across sires. Equally important to consider is how fertility can fluctuate over time. 

“If a bull can get cows pregnant by day 24 or 30, does it mean those pregnancies are likely to continue?” Pohler asks. “We studied eight Angus bulls and looked at pregnancy loss in the first 24-plus days. Four bulls accounted for most of the early pregnancy loss. When we looked later in gestation, only one bull stayed consistent.”

Data from commercial Holstein AI programs reinforces this point. 

One bull passed his BSE, but when put to the test, the results showed another story. This bull only accomplished a 10 percent pregnancy rate, and 40 percent of those conceptions didn’t make it to term. 

“There is no relationship between a bull’s ability to get cows pregnant and the ability of those cows to maintain pregnancy,” Pohler says. “This bull passed all quality control measures. From a sperm standpoint, he looked fine, but there was a clear issue in the field.”

For seedstock producers, these examples pose a critical reminder relying solely on standard BSEs or a bull’s early conception success may not predict long-term reproductive performance in the herd. Tracking pregnancy outcomes across multiple breeding seasons and environments is essential for identifying sires which perform consistently.

New tools for evaluating bull fertility

Advances in reproductive science are expanding how fertility can be evaluated, but they also reinforce an important reality – no single measurement captures a bull’s true reproductive impact.

“We used to think if sperm were alive, they were good,” Zoca explains. “Later, we realized not all live sperm are equally fertile.”

Emerging technologies allow researchers to uncover new insights. Innovation including flow cytometry and multi-omics approaches allow researchers to evaluate thousands of sperm cells with greater resolution.

“The key is not just identifying markers, but developing tools which can be used practically by AI centers, veterinarians and producers in the field,” Zoca says.

Pohler emphasizes fertility evaluation must extend beyond fertilization. 

“The same bull that gets cows pregnant by day 24 or 30 does not necessarily maintain those pregnancies. Bulls rerank, and we’ve seen this in both beef and dairy data,” Pohler says.

These insights emphasize a bull’s reproductive capacity establishes the upper limit for herd performance, but it cannot overcome limitations in female fertility, embryo viability or herd management. This is where the attention shifts from the bull alone to the entire reproductive system, including female contributions.

Advancements in ET and IVF

While the bull’s reproductive potential defines what is possible, the success of pregnancies ultimately depends on female reproductive health and embryo quality. 

For seedstock operations, advanced reproductive technologies offer additional opportunities to manage fertility and reduce risk through methods such as embryo transfer (ET) and in vitro fertilization (IVF). 

“Donor management has a lot to do with embryo quality,” says Dr. Lee Jones, senior professional services veterinarian at Boehringer Ingelheim. “We know embryo grades have an impact on performance, and we know fresh versus frozen embryos can sometimes affect performance.”

On the flip side, recipient management remains equally important. 

“Body condition score targets of five or six are ideal. We typically like recipients to be at least 45 days postpartum before synchronization,” Jones says. 

But it doesn’t stop at conception. The management of these cattle influences factors well beyond for both the recipient female and the calf she’s carrying.  

“Recipient management also affects fetal development, body condition at calving, colostrum quality, calf health and postpartum fertility,” Jones says. “Pregnancy diagnosis is one checkpoint where we can assess body condition and gestation stage and make adjustments if needed.”

Tools such as EmVision created by EmGenisys now provide producers with a way to assess embryo viability more precisely. Using video analysis and machine learning, the technology system scores embryos to help select those with the highest potential for successful pregnancies and reduce early embryonic loss.

“This score helps rank embryos from most healthy to least healthy, and producers can decide how they fit into their practice,” says Cara Wells, chief executive officer and founder of EmGenisys. “It empowers embryologists with objective data to make better decisions in the field.”

The system works in real time using simple equipment already available in most embryo transfer operations, such as a microscope and smartphone. Videos of embryos are uploaded to the platform, which immediately evaluates each embryo’s activity and assigns a confidence score for pregnancy potential. 

Over time, the EmVision platform has been trained on thousands of embryos with known outcomes, allowing the machine learning system to recognize patterns beyond what the human eye can detect. 

According to Wells, “Embryos with moderate, balanced activity tend to make the best pregnancies. Our system helps identify those embryos and avoids ones unlikely to succeed, moving the needle on efficiency and live birth rates.”

Advances in ET and IVF are giving producers tools to manage fertility with greater precision than ever before.

From careful donor and recipient management to scoring embryos with EmVision, every decision along the reproductive chain can influence pregnancy success, calf health and long-term herd performance.

Jones explains fertility is a year-round cycle, requiring commitment from before conception to well after birth, regardless of the industry sector. 

“It can be virtuous or vicious. Everything builds on what came before,” Jones states.

By combining traditional management practices with modern technologies, producers are moving beyond guesswork. While these tools don’t replace experience, they allow operations to make more informed, repeatable decisions and set the stage for a more data-focused approach to fertility.

Data-driven fertility selection

Fertility is both a science and an art. Success depends on bull selection, embryo quality, recipient management and attention to detail at every stage. 

From selecting sires with consistent performance to managing donors and recipients and now scoring embryos with new technologies, producers have more tools than ever to reduce risk and improve outcomes.

“Ultimately, we want reproduction to become almost a passive process, monitored carefully and adjusted when necessary, in a way that is sustainable, effective and efficient,” Jones says.

Integrating traditional practices with advanced technologies allows producers to make informed, repeatable decisions, ensuring fertility delivers both predictable results and lasting profitability. 

Briley Richard is a freelance writer from El Dorado, Kan. This article was originally published by Angus Journal on Feb. 11.