Natural Service in Cattle: Physiological Insights and Bull Management for Field Veterinarians 

Introduction 

Reproductive efficiency remains one of the strongest drivers of economic performance in cattle herds, particularly in regions where natural service continues to dominate due to infrastructure, resource, and labour constraints. In many smallholder, low-input, or pasture-based systems, natural service is not only the most practical and cost-effective breeding method but also one that can offer distinct physiological advantages when properly managed.  

Physiological Benefits of Natural Mating 

A key advantage of natural service is the reproductive tract’s vascular response during and after mating. Doppler ultrasonography demonstrates that natural mating induces significantly higher vaginal and uterine blood flow compared with AI. Elmetwally et al. (2024) reported a striking 199% increase in vaginal blood flow six hours post-mating and a 79% rise in uterine blood flow by nine hours. This enhanced perfusion likely supports sperm transport, uterine receptivity, and early embryo development. These physiological insights provide the foundation for understanding why natural mating may outperform AI under certain field conditions¹ . 

Uterine Blood Flow and Early Pregnancy Stability 

The importance of uterine perfusion is further supported by research on early pregnancy detection. A 2024 BMC study showed that corpus luteum (CL) blood flow measured by Doppler ultrasonography strongly correlates with early pregnancy maintenance, with pregnant cows demonstrating significantly higher CL perfusion and accompanying gene-expression patterns supportive of luteal function². These findings reinforce that increased blood flow during and after mating is not incidental; it directly contributes to early embryonic survival. 

So, vascular health of the reproductive tract is central to conception success, creating a robust physiological rationale for optimizing natural service protocols. 

 Fertility Variation Among Cows and Bulls 

Even with optimal mating conditions, fertility outcomes vary widely. A 2023 review emphasizes that cow ovarian physiology, follicular growth patterns, metabolic status, and breed differences all influence ovulatory timing and conception probability³ 

Similarly, bull factors including libido, semen morphology, scrotal thermoregulation, and musculoskeletal fitness must be assessed before the breeding season. Research consistently shows that suboptimal bull fertility is a primary cause of herd reproductive failure. This underscores the need for routine Breeding Soundness Exams (BSE) and continuous in-season monitoring of mating behaviour^4,5. 

Breeding Soundness Evaluation (BSE): A Research-Informed Requirement 

Modern BSE guidelines, supported by multiple recent studies (2021–2025), emphasize three critical domains.  

First, the physical examination assesses lameness, vision, and overall musculoskeletal integrity, all of which directly influence a bull’s ability to serve^6,7.  

Second, reproductive tract assessment includes measurement of scrotal circumference, a parameter strongly correlated with testicular mass and daily sperm output^6,8.  

Third, semen quality evaluation, specifically sperm motility, morphology, and viability remains central to determining breeding soundness, with updated guidelines outlining minimum acceptable thresholds for each parameter^6,7. 

Evidence shows that bulls with reduced semen quality, particularly abnormal sperm morphology or low motility, are more likely to have lower conception or non-return rates, indicating reduced fertility performance^9,10,11. Conventional BSE parameters alone may miss subtle functional defects, which explains why some bulls that pass standard thresholds still exhibit subfertile outcomes. Integrating BSE findings with behavioural observations during mating has been recommended to improve prediction of real-world breeding performance¹². 

Bull-to-Cow Ratio and Seasonal Mating Management 

Effective natural service depends on selecting bulls whose mating workload matches herd size. Evidence from extensively managed herds shows that when cow-to-bull ratios become too high, herd fertility declines noticeably. Herds with more than approximately 40 cows per bull demonstrate significantly lower pregnancy outcomes, indicating that overuse of bulls contributes to reduced reproductive performance¹³. To maintain breeding success, practical measures such as rotating bulls, monitoring body condition, and observing mating behaviour remain essential throughout the breeding period. 

Pre-Mating Assessment of Cows 

Pre-mating assessment is essential for optimizing cow fertility. Ensuring cows are in moderate body condition supports timely resumption of ovarian cyclicity, with under-conditioned cows showing delayed ovulation and lower conception rates^14,15. Adequate trace-mineral nutrition, including selenium, zinc, copper, and manganese, supports postpartum recovery, immune function, and reproductive potential¹⁶. Monitoring the postpartum interval and managing uterine health reduces reproductive delays and associated fertility losses^16,17. Veterinary guidance in nutrition and reproductive management helps prepare cows physiologically for breeding, improving the likelihood of successful natural service and overall herd fertility. 

Early Pregnancy Diagnosis and Monitoring 

Using colour-Doppler ultrasonography to assess corpus luteum vascularity offers a promising early pregnancy diagnostic approach. A recent study demonstrated that evaluation of CL blood flow on day 20 after insemination can reliably identify non-pregnant cows, enabling early rebreeding and potentially reducing days open². Early identification of open cows followed by re-synchronization may thus improve herd reproductive efficiency. 

 From Mating Plan to Calf Outcome: The Veterinary Role 

Integrating physiological insights, bull evaluation, cow readiness, and early pregnancy monitoring provides a comprehensive approach to ensuring high pregnancy and calving rates. Veterinarians serve as the central link connecting science-driven breeding decisions with practical on-farm implementation. When natural mating is optimized using current evidence, herd reproductive outcomes improve predictably. 

 References (All Open Access) 

1. Elmetwally MA, Meinecke-Tillmann S, Herzog K, Bollwein H. Effects of natural mating, artificial insemination and intravaginal deposition of raw semen or seminal plasma on vaginal and uterine blood flow in German Holstein cows. BMC veterinary research. 2024 Jun 26;20(1):277.  

2. De Silva M, Wickramasingha D, Abayalath N, Alexander B, Wijayagunawardane M, Lee KF, Kodithuwakku S. Early pregnancy diagnosis in cows using corpus luteum blood flow analysis based on colour Doppler ultrasonography and mRNA analysis. BMC Veterinary Research. 2024 Dec 20;20(1):572. 

3. Triwutanon S, Rukkwamsuk T. Factors affecting first ovulation in postpartum dairy cows under tropical conditions: A review. Open Veterinary Journal. 2023 Dec 31;13(12):1536. 

4. Butler ML, Bormann JM, Weaber RL, Grieger DM, Rolf MM. Selection for bull fertility: a review. Translational Animal Science. 2020 Jan;4(1):423-41. 

5. Barth AD. The use of bull breeding soundness evaluation to identify subfertile and infertile bulls. Animal. 2018 Jun;12(s1):s158-64. 

6. Bedford-Guaus S. Breeding soundness examination of bulls. In: MSD Veterinary Manual [Internet]. 2024 [cited 2025 Dec 2]. Available from: https://www.msdvetmanual.com/management-and-nutrition/breeding-soundness-examination-of-the-male/breeding-soundness-examination-of-bulls 

7. Quail L, Galbreath J. Bull Breeding Soundness Examinations. NDSU Extension Publication AS1755 [Internet]. Revised 2025 Sep [cited 2025 Dec 2]. Available from: https://www.ndsu.edu/agriculture/extension/publications/bull-breeding-soundness-examinations 

8. Flores JA, Moreno FB, Monteiro BM, et al. Relationship of testicular biometry with semen variables in the breeding soundness evaluation of Nellore bulls. Theriogenology. 2018;113:131-6. 

9. Karoui S, Díaz C, Serrano M, Cue R, Celorrio I. Non-return rate and its relationship with semen quality and sperm morphology in bull semen. Acta Vet Scand [Internet]. 2018 [cited 2025 Dec 2];60:55. Available from: https://actavetscand.biomedcentral.com/articles/10.1186/s13028-018-0396-2 

10. Reichenbach HD, Wimmers K, Alm K, Torner H, Kanitz W. Associations of sperm morphology and head defects with reduced fertility in AI bulls. J Reprod Fertil [Internet]. 1991 [cited 2025 Dec 2];93(2):389–95. Available from: https://pubmed.ncbi.nlm.nih.gov/1950245/ 

11. Amann RP, DeJarnette JM. Impact of sperm motility and morphology limitations on bull fertility: a review. Biol Reprod [Internet]. [cited 2025 Dec 2]. Available from: https://academic.oup.com/biolreprod/article/106/5/854/6523729 

12. DeJarnette JM, Marshall CE. Using semen characteristics in combination with field behaviour to predict bull breeding performance. Biol Reprod [Internet]. [cited 2025 Dec 2]. Available from: https://academic.oup.com/biolreprod/article/106/5/854/6523729 

13. Montoya-Monsalve G, Sánchez-Calabuig MJ, Blanco-Murcia J, Elvira L, Gutiérrez-Adán A, Ramos-Ibeas P, et al. Impact of overuse and sexually transmitted infections on seminal parameters of extensively managed bulls. Animals [Internet]. 2021;11(3):827. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC7999385/ 

14. Nazhat SA, et al. Importance of body condition scoring in reproductive performance of dairy cows: a review [Internet]. Open J Vet Med. 2021 [cited 2025 Dec 2]. Available from: https://www.scirp.org/pdf/ojvm_2021072714525019.pdf  

15. Roche JR, et al. Body condition score and fertility in dairy cows: mechanisms and management. Anim Reprod Sci [Internet]. 2013 [cited 2025 Dec 2]; Available from: https://www.sciencedirect.com/science/article/pii/S0378432013001821  

16. Van Emon ML, Sordillo LM, Lavergne SN, et al. Impacts of bovine trace mineral supplementation on production, health, and reproduction: a review. Front Vet Sci [Internet]. 2020 [cited 2025 Dec 2]; Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7765511/  

17. Sheldon IM. Preventing postpartum uterine disease in dairy cattle. Reprod Domest Anim [Internet]. 2020 [cited 2025 Dec 2]. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7234917/