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Reproduction in domestic animals = Zuchthygiene2026; 61(6); e70244; doi: 10.1111/rda.70244

Annual Variation in Stallion Testicular Measurements and Its Association With Hormonal Profiles and Semen Quality.

Abstract: In a stallion breeding-soundness examination, testicular volume may be estimated either from total scrotal width or by measuring the height, width and length of both testes. This study aimed to determine whether testicular morphometric parameters obtained by ultrasonography fluctuate throughout the year, and whether these variations affect semen quality and hormone production in stallions located at low latitudes in the Southern Hemisphere. Eight healthy light-breed stallions were evaluated every 15 days for 1 year, encompassing the breeding (October-April) and non-breeding(May-September) seasons. Assessments included testicular ultrasonographic measurements, semen quality, hormonal profiling and thermographic evaluation. Testicular morphometric parameters, particularly total testicular volume and testicular width, exhibited seasonal variation, with greater values observed during the breeding season. A moderate correlation was identified between total testicular width and total testicular volume, indicating that width contributes to testicular volume variation. Despite the increase in testicular volume during the breeding season, total sperm number per ejaculate remained stable throughout the year, and only subtle seasonal changes were observed in seminal and hormonal parameters. Minor and total sperm defects increased during the breeding season, coinciding with higher ambient temperature, the temperature-humidity index (THI) and scrotal surface temperature. Cortisol concentrations were higher during the non-breeding season, suggesting physiological adaptation to environmental conditions. These findings indicate that stallions maintained at low latitudes exhibit limited reproductive seasonality and suggest that seasonal changes in testicular morphometry are more closely associated with thermoregulatory adaptations than with marked alterations in spermatogenic activity.
© 2026 The Author(s). Reproduction in Domestic Animals published by Wiley‐VCH GmbH.
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Publication Date: 2026-06-03 PubMed ID: 42233443DOI: 10.1111/rda.70244Google Scholar: Lookup
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  • Journal Article

Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

Overview

  • This study investigated how stallion testicular size changes throughout the year and whether these changes relate to semen quality and hormone levels in stallions living in low latitude regions of the Southern Hemisphere.

Study Purpose and Background

  • Testicular volume can be estimated during stallion breeding-soundness exams by either total scrotal width or by measuring height, width, and length of each testis using ultrasound.
  • The research aimed to find out if these testicular measurements vary seasonally and whether this variation affects semen quality and hormone production.
  • The focus was on stallions located at low latitudes, where less reproductive seasonality is expected compared to stallions in higher latitudes.

Methodology

  • Eight healthy light-breed stallions were studied.
  • Measurements were taken every 15 days over one full year to cover both breeding (October-April) and non-breeding (May-September) seasons.
  • Evaluations included:
    • Ultrasonographic measurements of testes (height, width, length, total volume)
    • Semen quality analysis
    • Hormone profiling (including cortisol levels)
    • Thermographic evaluation of scrotal surface temperature

Key Findings – Testicular Morphometry

  • Testicular measurements showed seasonal variation with larger total testicular volume and testicular width during the breeding season.
  • A moderate correlation was found between testicular width and total testicular volume, indicating width is an important factor in volume changes.

Key Findings – Semen Quality and Hormonal Profiles

  • Total sperm count per ejaculate remained largely stable throughout the year despite the increase in testicular volume during breeding season.
  • Subtle seasonal changes were seen in seminal parameters and hormone levels.
  • Minor and total sperm defects increased during the breeding season, coinciding with:
    • Higher ambient temperature
    • Higher temperature-humidity index (THI)
    • Higher scrotal surface temperature
  • Cortisol hormone concentrations were higher during the non-breeding season, suggesting a physiological response or adaptation to environmental stressors during that time.

Interpretation and Implications

  • Stallions living at low latitudes exhibit limited reproductive seasonality, meaning their reproductive parameters and semen quality do not fluctuate drastically between seasons.
  • Seasonal increases in testicular size appear more connected to thermoregulatory adjustments rather than to large changes in spermatogenesis or sperm production.
  • The rise in sperm defects during hotter periods suggests environmental heat stress affects sperm integrity even if overall sperm numbers remain stable.
  • Higher cortisol levels outside the breeding season could reflect stress or metabolic adaptations to changing conditions.
  • These findings may influence how breeding soundness is assessed in stallions from low latitude regions and underscore the importance of considering environmental factors such as temperature and humidity in reproductive evaluations.

Cite This Article

APA
Braga TRC, Freitas ML, Martins DMV, Viu MAO, Oliveira RA. (2026). Annual Variation in Stallion Testicular Measurements and Its Association With Hormonal Profiles and Semen Quality. Reprod Domest Anim, 61(6), e70244. https://doi.org/10.1111/rda.70244

Publication

Reproduction in domestic animals = Zuchthygiene
ISSN: 1439-0531
NlmUniqueID: 9015668
Country: Germany
Language: English
Volume: 61
Issue: 6
Pages: e70244

Researcher Affiliations

Braga, T R C
  • University of Brasília, Faculty of Veterinary Medicine, ReproequiUnB, (FAV/UnB), Brasília, Distrito Federal, Brazil.
Freitas, M L
  • University of Brasília, Faculty of Veterinary Medicine, ReproequiUnB, (FAV/UnB), Brasília, Distrito Federal, Brazil.
Martins, D M V
  • University of Brasília, Faculty of Veterinary Medicine, ReproequiUnB, (FAV/UnB), Brasília, Distrito Federal, Brazil.
Viu, M A O
  • University of Jataí, Faculty of Veterinary Medicine, Câmpus Jatobá - Cidade Universitária, Goiás, Brazil.
Oliveira, R A
  • University of Brasília, Faculty of Veterinary Medicine, ReproequiUnB, (FAV/UnB), Brasília, Distrito Federal, Brazil.

MeSH Terms

  • Animals
  • Male
  • Testis / anatomy & histology
  • Testis / diagnostic imaging
  • Horses / physiology
  • Horses / anatomy & histology
  • Seasons
  • Semen Analysis / veterinary
  • Ultrasonography / veterinary
  • Organ Size
  • Sperm Count / veterinary
  • Hydrocortisone / blood
  • Breeding
  • Temperature

Grant Funding

  • Fundação de Apoio à Pesquisa do Distrito Federal
  • 001 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior

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