Estimation of Genetic Parameters of Heat Tolerance for Production Traits in Canadian Holsteins Cattle

SIMPLE SUMMARY: Heat stress is a major problem in dairy cattle and has several negative consequences for both health and production. The possibility to genetically select for more heat tolerant animals can provide additional means for the dairy industry to address this environmental issue. When production records and environmental information, such as temperature and relative humidity are available, the genetic merit of the animals for milk production can be estimated under different environmental conditions. Animals can then be ranked according to their genetic merit under heat stress conditions for selection purpose. Therefore, the objective of this study was to estimate the genetic parameters for heat tolerance of milk, fat, and protein yields in Canadian Holstein cows. The correlation between the estimated genetic merit under thermal comfort and heat stress conditions indicated an antagonistic relationship between the level of production and heat tolerance. However, this correlation was moderate, which suggests that there exist animals with a high genetic merit for production that are also heat tolerant, which allows for genetic selection for heat tolerance to be carried out while still improving production. The results of this study support genetic selection for heat tolerance contributing towards the goal of improving the overall heat tolerance of Canadian Holstein cattle. ABSTRACT: Understanding how cows respond to heat stress has helped to provide effective herd management practices to tackle this environmental challenge. The possibility of selecting animals that are genetically more heat tolerant may provide additional means to maintain or even improve the productivity of the Canadian dairy industry, which is facing a shifting environment due to climate changes. The objective of this study was to estimate the genetic parameters for heat tolerance of milk, fat, and protein yields in Canadian Holstein cows. A total of 1.3 million test-day records from 195,448 first-parity cows were available. A repeatability test-day model fitting a reaction norm on the temperature-humidity index (THI) was used to estimate the genetic parameters. The estimated genetic correlations between additive genetic effect for production and for heat tolerance ranged from −0.13 to −0.21, indicating an antagonistic relationship between the level of production and heat tolerance. Heritability increased marginally as THI increased above its threshold for milk yield (0.20 to 0.23) and protein yield (0.14 to 0.16) and remained constant for fat yield (0.17). A Spearman rank correlation between the estimated breeding values under thermal comfort and under heat stress showed a potential genotype by environmental interaction. The existence of a genetic variability for heat tolerance allows for the selection of more heat tolerant cows.