Genetic and phenotypic insights into thermal load and racing performance of Quarter Horses.

Overview

• This study examined how heat affects the racing performance of Brazilian Quarter Horses and explored the genetic factors behind their ability to tolerate heat stress during races.
• It found that while Quarter Horses generally perform better within a certain temperature range, there is genetic variation in heat tolerance that also impacts their overall performance, suggesting that breeding programs should consider heat tolerance to improve race outcomes in warmer climates.

Background and Purpose

• Thermal load problem: Physical exertion during horse racing increases internal heat, which can harm horse welfare and reduce performance.
• Knowledge gap: Although there are ways to manage heat effects phenotypically (like cooling strategies), the genetic basis of heat tolerance in horses is not well understood.
• Research goals:
  • Assess how different thermal conditions affect racing times of Brazilian Quarter Horses.
  • Investigate genetic factors linked to heat tolerance by analyzing performance across varying temperature conditions.

Methods

• Data collection: Analyzed 21,857 race times from 5,542 Quarter Horses racing between 1984 and 2016 in Sorocaba, São Paulo, Brazil, which has a mild subtropical climate.
• Environmental variables: Considered multiple measures of temperature and thermal indices; identified the average air temperature over the four days prior to races (T4) as most influential on performance variability.
• Statistical approach:
  • Used a reaction norm model, which allows estimation of how genetic effects on a trait change across an environmental gradient (here, T4 temperature).
  • Examined additive genetic variance, heritability, and genetic correlations relating to racing times at different temperature levels.

Key Findings

• Performance and temperature: Quarter Horses performed better at higher temperatures within their thermal comfort zone, indicating that mild heat may not be detrimental and can even improve race times.
• Genetic variation in heat tolerance: Identified horses with genetically thermotolerant and heat-sensitive profiles, meaning some are genetically better adapted to perform well under heat stress.
• Heritability and genetic variance: Both changed across temperature gradients, showing that genetic control of performance depends on thermal environment.
• Genetic trade-off: Negative genetic correlation between baseline performance (intercept) and heat tolerance (slope) was found, indicating selecting purely for fast racing times may reduce heat tolerance.
• Selection trends: Past breeding focused on faster times but inadvertently reduced thermotolerance in the population.

Implications and Recommendations

• Thermotolerance in breeding: Future breeding should balance selection for speed with selection for heat tolerance to sustain performance under hotter climates.
• Climate change relevance: As global temperatures rise, integrating thermotolerance becomes crucial for animal welfare and maintaining racing success.
• Potential for genetic improvement: Understanding the genetic basis of thermal tolerance allows informed decisions to enhance heat resilience without sacrificing performance.

Conclusion

• The study provides new insights that both performance and heat tolerance have a genetic basis and vary with temperature, highlighting the need for combined genetic strategies to improve Quarter Horse racing performance under thermal stress conditions.