Genetic evaluations for growth heat tolerance in Angus cattle

Area of Science:

• Animal Genetics
• Environmental Physiology
• Beef Cattle Production

Background:

• Heat stress poses a significant challenge to livestock production, affecting animal growth and performance.
• Genetic evaluation models need to account for environmental factors like heat stress to accurately assess breeding values.
• Angus cattle are economically important, making the study of their heat tolerance crucial for sustainable beef production.

Purpose of the Study:

• To evaluate the impact of heat stress on Angus cattle growth traits, specifically weaning weight (WW) and yearling weight (YW).
• To develop and assess a model for the genetic evaluation of heat tolerance in Angus cattle.
• To identify which growth stage (weaning or yearling) is more suitable for developing selection tools for heat tolerance.

Main Methods:

• Utilized a large dataset of WW and YW records from the American Angus Association, focusing on records from the Upper South region.
• Characterized heat stress using a heat load function based on the temperature-humidity index (THI) prior to weigh dates.
• Employed univariate models and reaction norms with B-splines to analyze genetic parameters for heat tolerance under varying heat loads.

Main Results:

• Direct genetic correlations for WW decreased significantly with increasing heat load, indicating reranking of sires under extreme conditions.
• Heat stress had minimal impact on the genetic merit of YW, suggesting less influence on this trait.
• Maternal heritabilities for growth were consistent across heat loads, with high maternal genetic correlations, and no genotype × environment interaction was observed for the maternal component.

Conclusions:

• Weaning heat tolerance is a more promising trait for developing selection tools compared to yearling weight.
• Phenotypic plasticity exists for the direct genetic effects of WW, but traditional evaluations are adequate for non-extreme conditions.
• Maternal genetic effects on growth appear robust and not significantly influenced by heat stress or genotype-environment interactions.