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Journal of animal breeding and genetics = Zeitschrift fur Tierzuchtung und Zuchtungsbiologie2026; doi: 10.1111/jbg.70056

Thermal Thresholds and Genetic Sensitivity in Barrel Racing Performance of Quarter Horses Across Temperature-Humidity Indices.

Abstract: Heat stress is an increasingly important challenge for the performance and welfare of equine athletes, particularly in competitions conducted across diverse climatic conditions. Understanding the genetic basis of performance responses to increasing thermal load is therefore essential to support robust genetic evaluation and sustainable selection strategies. The objective of this study was to evaluate barrel racing performance of Brazilian Quarter Horses across thermal environments defined by two widely used indicators, the temperature-humidity index (THI) and the wet bulb globe temperature (WBGT), and to estimate genetic parameters associated with baseline performance and environmental sensitivity. A total of 351,993 barrel racing time (BRT) records from 13,960 Quarter Horses were analyzed. Segmented regression models were applied to least squares means to identify heat stress thresholds, while random regression models with polynomial functions were used to estimate covariance components, genetic parameters, and correlations along the thermal gradients. Distinct thermal thresholds were identified for both indices, at THI = 74.7 and WBGT = 23.6, beyond which performance deteriorated more rapidly. Heat stress conditions were observed in 31.15% and 20.71% of barrel racing events according to the THI and WBGT thresholds, respectively, emphasizing the practical relevance of the evaluated thermal gradients. Random regression models assuming continuous thermal variation provided the best overall fit to the data. Additive genetic variance remained relatively stable across thermal environments, whereas permanent environmental and residual variances increased under higher thermal load. Mean heritability under thermoneutral conditions was approximately 0.21 for THI and WBGT, declining modestly to 0.17-0.18 under extreme heat stress. Genetic correlations between thermoneutral and extreme environments remained high for both indices (at approximately 0.97), indicating strong genetic continuity of performance and no substantial reranking of estimated breeding values across the thermal gradient. Overall, THI and WBGT yielded highly consistent results in terms of thresholds, genetic parameters, and selection outcomes. Although WBGT showed slightly greater sensitivity under extreme conditions, THI offered smoother response patterns and clear operational advantages due to its simplicity and ease of calculation. No evidence of substantial genotype-by-environment interaction was detected for BRT. However, the results suggest that incorporating genetic tolerance to heat stress as a complementary selection criterion may help sustain barrel racing performance under increasingly challenging climatic conditions.
© 2026 The Author(s). Journal of Animal Breeding and Genetics published by John Wiley & Sons Ltd.
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Publication Date: 2026-06-05 PubMed ID: 42246621DOI: 10.1111/jbg.70056Google Scholar: Lookup
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Summary

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Thermal stress significantly affects barrel racing performance in Brazilian Quarter Horses, with specific temperature-humidity thresholds identified that mark rapid performance decline; genetic analysis shows stable heritability across temperature gradients and suggests that selection for heat tolerance could improve sustained performance.

Study Objective and Context

  • Investigate the impact of heat stress on barrel racing performance in Quarter Horses.
  • Focus on two environmental indices: Temperature-Humidity Index (THI) and Wet Bulb Globe Temperature (WBGT) to represent thermal environments.
  • Estimate genetic parameters related to baseline performance and sensitivity to thermal stress.
  • Support genetic evaluation and sustainable breeding strategies adapted to changing climatic conditions.

Data and Methodology

  • Analyzed 351,993 barrel racing time (BRT) records collected from 13,960 Brazilian Quarter Horses.
  • Used segmented regression models on least squares means to identify critical heat stress thresholds where performance deteriorates more rapidly.
  • Applied random regression models with polynomial functions to estimate genetic variance components, heritability, and correlations across thermal gradients.
  • Compared the effectiveness and outcomes of two thermal indices: THI and WBGT.

Key Findings: Thermal Thresholds

  • Identified critical heat stress thresholds at THI = 74.7 and WBGT = 23.6.
  • Above these thresholds, barrel racing performance declined more sharply, indicating onset of heat stress effects.
  • Heat stress conditions were common: 31.15% of races exceeded the THI threshold, and 20.71% exceeded the WBGT threshold.
  • This highlights the real-world relevance of thermal stress to competitive events.

Genetic Parameter Estimates and Heritability

  • Genetic additive variance remained relatively stable across the full range of thermal environments.
  • Permanent environmental and residual variances increased as heat stress worsened, indicating more non-genetic variability.
  • Heritability of barrel racing performance under thermoneutral conditions was approximately 0.21 for both THI and WBGT.
  • Under extreme heat stress, heritability slightly decreased to the range of 0.17 to 0.18.
  • High genetic correlations (~0.97) between thermoneutral and extreme heat environments suggest strong genetic consistency and negligible reranking of breeding values across thermal gradients.

Comparison of Thermal Indices (THI vs. WBGT)

  • Both indices produced highly consistent results for thresholds, genetic parameters, and selection implications.
  • WBGT showed slightly higher sensitivity to extreme heat conditions.
  • THI yielded smoother performance response patterns and was simpler and easier to calculate, offering operational advantages in routine genetic evaluations and management decisions.

Genotype-by-Environment Interaction and Selection Implications

  • No substantial genotype-by-environment interaction was detected, meaning horse rankings for breeding value did not change significantly with thermal environment.
  • This strong genetic continuity simplifies selection decisions as the same horses tend to perform consistently across environments.
  • However, the study suggests incorporating genetic tolerance to heat stress as a complementary criterion in breeding programs.
  • Selection focusing on heat tolerance could sustain barrel racing performance as climate conditions become more challenging.

Practical and Scientific Contributions

  • Provides valuable insights into the impact of thermal stress on equine performance, specifically in barrel racing.
  • Offers a large-scale genomic and phenotypic analysis using advanced statistical models to elucidate environmental sensitivity.
  • Supports development of climate-resilient breeding programs for Quarter Horses by integrating thermal tolerance into genetic evaluations.
  • Highlights the practical relevance of using simple yet effective environmental indices like THI to monitor and manage heat stress in equine sports.

Cite This Article

APA
Santana ML, Bignardi AB. (2026). Thermal Thresholds and Genetic Sensitivity in Barrel Racing Performance of Quarter Horses Across Temperature-Humidity Indices. J Anim Breed Genet. https://doi.org/10.1111/jbg.70056

Publication

Journal of animal breeding and genetics = Zeitschrift fur Tierzuchtung und Zuchtungsbiologie
ISSN: 1439-0388
NlmUniqueID: 100955807
Country: Germany
Language: English

Researcher Affiliations

Santana, Mário Luiz
  • Grupo de Melhoramento Animal de Mato Grosso (GMAT), Instituto de Ciências Agrárias e Tecnológicas, Universidade Federal de Rondonópolis (UFR), Rondonópolis, MT, Brazil.
Bignardi, Annaiza Braga
  • Grupo de Melhoramento Animal de Mato Grosso (GMAT), Instituto de Ciências Agrárias e Tecnológicas, Universidade Federal de Rondonópolis (UFR), Rondonópolis, MT, Brazil.

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