The key mare, stallion, and management factors shaping Thoroughbred pregnancy success
An Applied Animal Science analysis of 8,800 matings over nine years reveals the strongest predictors of conception—and underscores the value of breeding‑shed records
Champaign, IL, March 23, 2026—High rates of pregnancy success are crucial to the success of commercial Thoroughbred breeding farms, which usher in the next generation of horses for elite racing and equestrian sports. Although many factors are known to influence pregnancy rates, outcomes can still vary widely across different operations. A new study published in Applied Animal Science sought to narrow down and better understand which of these factors are most important for conception. Their results highlight how horse behavior and physicality, semen characteristics, and even the month of mating can meaningfully influence per-cycle pregnancy rates—providing actionable information for stallion and farm managers to use in their operations.
In a retrospective analysis, a team led by Karen Bennett-Wimbush, PhD, from Ohio State’s College of Food, Agricultural, and Environmental Sciences, Wooster, OH, USA, analyzed detailed breeding shed records from 2009 to 2017 taken from a commercial Thoroughbred breeding farm. Bennett-Wimbush explained that the scale and consistency of the dataset made the findings especially valuable. “Because the same stallion manager collected the same types of information over nine consecutive breeding seasons, we were able to evaluate how individual mare characteristics, stallion differences, and breeding‑shed observations affected pregnancy outcomes. This kind of long-term, detailed recordkeeping is rare, and it allowed us to identify patterns that would be invisible otherwise.”
The data revealed that pregnancy rates were different between the 20 stallions included, with an overall average of 63%. Mare classification, reproductive history, seasonal timing of mating, and the physical structure of the vulva also played significant roles.
Behavioral cues proved to be powerful indicators of reproductive readiness. “Behavioral estrus intensity is a valid indicator of elevated estrogen levels,” Bennet-Wimbush noted. “Mares scoring zero or one on the tease scale were substantially less likely to conceive, which supports what we see clinically—poor estrous expression often reflects suboptimal timing.”
Caption: A new study published in Applied Animal Science examined breeding shed records over nine seasons from a single Thoroughbred breeding farm to narrow down which factors are most critical to pregnancy success (Credit: Sandy Hatfield).
Semen characteristics recorded immediately after mating provided additional helpful insights. Urine contamination in dismount samples was associated with a decline in pregnancy rate. In contrast, factors such as the number of mounts, the number of semen pulses, time of day, and the presence of red or white blood cells did not significantly affect outcomes.
Interestingly, one of the strongest predictors of success was the stallion manager’s subjective cover grade, which incorporated stallion stability, mare movement, and dismount sample size. “Even though cover quality is a personal measure, it consistently aligned with pregnancy outcomes,” Bennett-Wimbush said. “Experienced managers can recognize subtle differences in how a mating proceeds, and those observations clearly matter.”
Taken together, these data provide actionable, science-based tips for management on both the mare and stallion side of the equation. Bill Weiss, PhD, PAS, Dipl. ACAS, editor in chief of Applied Animal Science, emphasized the impact of the overall findings. “High pregnancy rates are essential to horse breeding farms; however, the rate varies widely across breeding farms. Using thousands of records, this study identified important management practices, semen characteristics, and horse behaviors that affect pregnancy rate. This information can be used by breeding farm managers to improve pregnancy rates.”
The research team hopes the study encourages farms to maintain detailed breeding‑shed records and to use those data proactively. “Looking back at well-kept records can reveal which practices are working and where improvements can be made,” said Bennett-Wimbush. “In an industry where every pregnancy matters, that knowledge is incredibly valuable.”
The article appears in the April issue of Applied Animal Science.
Notes for editors
“Factors affecting pregnancy rates on a commercial Kentucky Thoroughbred breeding farm: A retrospective analysis of breeding shed records,” by K. Bennett-Wimbush, J. Suagee-Bedore, and S. Hatfield
(https://doi.org/10.15232/aas.2025-02738), Applied Animal Science, volume 42, issue 2 (April 2026), published by FASS and Elsevier.
This article is openly available at https://doi.org/10.15232/aas.2025-02738.
To schedule an interview with the author(s), please contact Karen Bennett-Wimbush at wimbush.4@osu.edu or aussies4wranglin@gmail.com.
About Applied Animal Science
Applied Animal Science (AAS) is a gold open access, peer-reviewed scientific journal and the official publication of the American Registry of Professional Animal Scientists (ARPAS). In continuous publication since 1985, AAS is a leading outlet for animal science research and is indexed by Scopus and ESCI (Clarivate’s Emerging Sources Citation Index). The journal welcomes novel manuscripts on applied technology, reviews on the use or application of research-based information on animal agriculture, commentaries on contemporary issues, short communications, and technical notes. Topics that will be considered for publication include (but are not limited to) feed science, farm animal management and production, dairy science, meat science, animal nutrition, reproduction, animal physiology and behavior, disease control and prevention, microbiology, agricultural economics, and environmental issues related to agriculture. Themed special issues also will be considered for publication. www.appliedanimalscience.org
About the American Registry of Professional Animal Scientists (ARPAS)
The American Registry of Professional Animal Scientists (ARPAS) is the organization that provides certification of animal scientists through examination, continuing education, and commitment to a code of ethics. Continual improvement of individual members is catalyzed through publications (including the AAS journal) and by providing information on educational opportunities. ARPAS is affiliated with five professional societies: American Dairy Science Association, American Meat Science Association, American Society of Animal Science, Equine Science Society, and Poultry Science Association. www.arpas.org
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