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

Thermal load is a major welfare and performance concern for athletic horses, as physical exertion increases internal heat production. Although several phenotypic strategies have been developed to mitigate its effects, the genetic basis of thermal tolerance in horses remains largely unexplored. This study aimed to (1) evaluate the influence of thermal indices on the racing performance of Brazilian Quarter Horses, and (2) investigate the genetic basis of thermal tolerance through the estimation of genetic parameters across temperature gradients. We analyzed 21,857 racing-time (RT) records from 5542 horses competing between 1984 and 2016 in Sorocaba, São Paulo, Brazil, a region with a mild subtropical climate. The performance of Quarter Horses improved at higher air temperatures and thermal indices, as races were held within their thermal comfort zone. Among all environmental variables, the average air temperature over the four days preceding each race (T4) explained the greatest proportion of variation in RT. A reaction norm model was applied to RT using T4 as the environmental descriptor. Both thermotolerant and heat-sensitive genotypes were identified. Heritability and additive genetic variance varied along the temperature scale, revealing heterogeneity in genetic control. A negative genetic correlation between intercept and slope indicated a trade-off between overall performance and heat tolerance. Genetic trends showed that selection for faster racing times was associated with reduced tolerance to higher thermal load in Quarter Horses. Therefore, future breeding strategies should explicitly integrate thermotolerance to maintain performance under increasingly challenging climatic conditions.