Renewed Genetic Evaluation of Heat Tolerance in Italian Holsteins

Abstract

Heat stress is a significant and growing challenge for the Italian dairy industry, adversely affecting milk production, fertility, and animal welfare. This study presents a renewed genetic evaluation for heat tolerance in Italian Holstein cattle, expanding upon a previous 2021 index that focused solely on milk yield. The primary objective was to develop a more comprehensive selection tool by incorporating five production traits: milk yield (kg), fat yield (kg), protein yield (kg), fat percentage, and protein percentage. Test-day production records were augmented with meteorological data from 137 weather stations across Italy to calculate a 7-day average Temperature-Humidity Index (THI) for each record. A repeatability mixed model was first employed to identify the specific THI thresholds at which each of the five traits begins to decline. Subsequently, a random regression mixed model was implemented to estimate genetic parameters and calculate Estimated Breeding Values (EBVs) for both general production merit and specific heat tolerance. THI thresholds were identified for all traits, with milk yield declining above a THI of 70. Heritabilities for heat tolerance traits were found to be low to moderate, ranging from 0.12 for fat yield to 0.37 for protein percentage, indicating sufficient genetic variation for selection. An aggregate index, the Heat Tolerance Index (IHT), was developed by assigning economic weights to each trait Heat Tolerance EBV. Validation results demonstrated the index's efficacy: daughters of high-IHT bulls (+1 SD) lost 0.91 kg/d less milk during summer compared to daughters of low-IHT bulls (-1 SD). This renewed evaluation provides a robust tool to select for more resilient animals, offering a tangible strategy to mitigate economic losses and improve animal welfare in a warming climate.