Genetic evaluation for heat tolerance in growing Angus cattle

Abstract Text: The purpose was to investigate the existence of heat stress on preweaning growth in Angus cattle and to develop a genetic evaluation to improve heat tolerance. The American Angus Association provided weight data, and records from the Southern United States (n = 82,669) were used because of the hot, humid summer months. Heat stress was measured using heat load, defined as the average degrees of temperature-humidity index greater than 24 ^oC for 30 days prior to the weigh date. Forty-five percent of cattle experienced heat loads greater than 0. Heat load was used in a reaction norm to assess phenotypic plasticity, and the results were compared with a univariate analysis. For both models, random effects included direct genetic, maternal genetic, maternal permanent environment, and residual; and fixed effects included a linear age covariate, age of dam class, sex, herd, and year. Moderate differences in heat load resulted in strong direct genetic correlations (r > 0.80), but large heat load differences had weaker direct genetic correlations. The same pattern occurred for Spearman rank correlations for proven bulls (n = 1,048) with r = 0.30 between no heat load and extreme heat load. Selection decisions should differ depending on heat load, and producers could benefit from environment-specific selection tools. As heat load increased, the maternal genetic effect remained consistent even though heat stress decreased milk production in dairy cows. To compensate for the expected reduction in dam milk production during heat stress, calves may have consumed creep feed or other forages to maintain growth. In addition to comparing results, accuracy was assessed by the ability to predict phenotypes for young animals. Predictivity did not improve when using the reaction norm (r = 0.31) instead of the univariate model (r = 0.30). Thus, the univariate analysis performed as well as the reaction norm and sufficiently evaluated genetic differences in growth despite heat stress.  Additional research is needed on methods for assessing heat tolerance in national cattle evaluations. Researchers implemented heat load successfully in species raised in confinement including dairy and swine, but heat load was confounded with contemporary group, calving season, seasonal fluctuations in forage quality and quantity, and fescue toxicity for beef cattle. A more robust measure of heat load would aid in understanding the effect of heat stress on preweaning growth and creating selection tools for improving beef cattle heat tolerance.

Keywords: beef cattle, heat stress, weaning weight