Thermal comfort and seminal traits in stallion reproduction: New methods to understand these relationships.

Thermal conditions significantly influence stallion semen quality, with poorer seminal traits observed in summer due to heat stress; this study demonstrated how non-invasive methods like Doppler ultrasonography and infrared thermography can effectively assess these seasonal effects on stallion reproduction.

Objective and Background

• Stallion breeding commonly focuses on visible physical traits rather than semen quality.
• The study aimed to explore relationships between semen parameters and thermal comfort using non-invasive, integrative methods across differing seasons (summer vs. winter).

Methods

• Twenty-four stallions were evaluated twice, in winter (cooler temperatures) and summer (warmer temperatures).
• Semen collection was performed to analyze seminal quality, focusing on parameters like motility, vigor, and sperm defects.
• Testicular thermography (infrared thermal imaging) was used to assess testicular surface temperature as an indirect measure of thermal stress.
• Doppler ultrasonography measured blood flow velocities in the testicular artery (systolic and diastolic velocities) to evaluate testicular perfusion, which is important for spermatogenesis.
• Physiological and environmental heat stress parameters were quantified, including:
  • Temperature-Humidity Index (THI) calculated by the formula: THI = [0.8 × (Environmental Temperature + Relative Humidity/100)] × [(Environmental Temperature – 14.4) + 46.4]
  • ThStress (Thermal Stress) index
• Data analysis included both univariate and multivariate statistical methods using SPSS® software.

Key Results

• Heat stress indicators (THI and ThStress):
  • Were significantly higher in summer (mean environmental temperature ~28.6 °C) compared to winter (~21.1 °C).
• Semen quality:
  • Sperm defects increased in summer (22.4% in winter vs. higher in summer).
  • Motility and vigor of sperm were higher in winter (81.2% motility, vigor 3.2) compared to reduced values in summer (64.2% motility, vigor 2.7).
• Testicular blood flow:
  • Systolic and diastolic velocities were significantly greater in winter (33.3 cm/s and 5.9 cm/s, respectively) than in summer (24.4 cm/s and 4.0 cm/s).
  • Better blood flow in winter likely facilitates improved testicular function and sperm quality.
• Correlations:
  • Strong correlations were found between thermal comfort indices and seminal traits during summer, indicating heat stress negatively affects semen quality.
  • In winter, Doppler and thermographic measurements correlated well with semen quality, supporting these as reliable assessment tools.

Conclusions and Implications

• This study successfully characterized the relationships among thermal comfort, blood flow, and semen quality in stallions, highlighting clear seasonal differences.
• Heat stress during summer detrimentally impacts sperm quality and testicular blood flow.
• The use of Doppler ultrasonography and infrared thermography provides valuable, non-invasive tools for fertility assessment and can help:
  • Determine optimal breeding seasons based on thermal and circulatory parameters.
  • Identify stallions under heat stress that may have compromised semen quality.
• Overall, integrating environmental and physiological monitoring may improve stallion breeding programs and reproductive outcomes.