Age-Related Testosterone and Oestradiol Profiles in Dairy Bulls: Implication for Age Groups Reproductive Efficiency and Breeding Soundness Evaluation

Abstract

Aims: The aim of this study was to characterize testosterone and oestradiol levels in dairy bulls in nine age groups (6 months to ≥8 years) to provide contextual reference values for reproductive efficiency and breeding soundness assessment in tropical dairy production systems of Ghana. Study Design: Longitudinal observational study. Place and Duration of Study: Various dairy farms in Ashanti, Eastern, and Greater Accra Regions in Ghana, from January 2022 to December 2023. Methodology: Blood samples from 90 dairy bulls from nine age groups (n=10 per group) were subjected to analyses using validated commercial enzyme immunoassays. Thorough breeding soundness assessments involved physical parameters, breeding behaviour observation, and monitoring of reproduction performance. Bulls were divided into age groups including 6 months, 1, 2, 3, 4, 5, 6, 7, and ≥8 years. Testosterone and oestradiol levels were evaluated using validated commercial enzyme immunoassay kits with intra-assay CV <7% and inter-assay CV <10%. Results: Testosterone levels had extremely significant age-depending variation (F₈,₈₁ = 42.3, P < .001), with the lowest levels in bulls aged 6 months (1.2 ± 0.21 ng/mL), increasing from 1-4 years with the highest levels at 4 years (8.9 ± 0.38 ng/mL). There was a gradual decrease (P <.05) from 5 (7.8 ± 0.32 ng/mL), tapering at ≥8 (5.1 ± 0.25 ng/mL, P <.001) compared with 4-years-old. Oestradiol levels were highest in 1-years-old bulls (45.6 ± 2.6 pg/mL, P <.01), with comparable lower levels in other age groups (18.2-24.7 pg/mL). Bulls of 4-year-old had peak testosterone levels, exhibited higher breeding behaviour scores (8.9 ± 0.22, P < .001), and 27% higher conception rates for breeding soundness than young or aged bulls. Body condition score was well correlated with testosterone levels (r = 0.68, P < .001). Strong positive associations were observed between testosterone levels and traditional breeding soundness criteria. Negative associations between physical parameters such as scrotal circumference (r = -0.31, P < .01) and breeding behaviour scores (r = -0.42, P < .01) and oestradiol were recorded. There was seasonal fluctuation in testosterone levels with the peak in the minor rainy season and minimum in the dry season (15-20% variation). Environmental conditions such as temperature and humidity had low correlations with hormone levels (r = -0.18 for temperature and r = -0.12 for humidity). Breed influenced (P<.05) testosterone levels such that Sanga bull had the highest followed by Friesian-Sanga crosses and Jersey bulls in descending order. Age-related patterns were consistent in the different age groups. Conclusion: The results offer critical reference values for hormone testing for breeding soundness in tropical dairy production systems. Bulls achieve peak breeding potential at an age of 4, with progressive regression until 5–7 years, with implications for evidence-based bull decision making as well as extended utilization strategies.