Assessment of adaptive and physiological trait expression in one-year-old male calves from Braford, Brangus, Tulianguy, Hereford, and Angus breeds in northern Uruguay

The Uruguayan livestock system, especially in the north of the country, is characterized by being pastoral, open-air, and highly dependent on climatic conditions. The increasing occurrence of droughts, high temperatures, and decreased rainfall has increased the need to select animals with greater adaptive and physiological capacity in the face of climatic stress. This study evaluated the expression of physiological and adaptive responses to heat stress in one-year-old castrated male calves belonging to five beef breeds: Angus, Hereford, Brangus, Braford, and Tulianguy. The experiment was conducted during the summer of 2023–2024 in Paysandú, Uruguay, under field conditions. Rectal temperature (RT), respiratory rate (RR), and surface temperatures on the forehead and flank were recorded under three levels of wind- and radiation-adjusted temperature and humidity index (THI _hadj = 75, 85, and 101). A total of 46 animals were evaluated. Environmental data were obtained using an automatic weather station, and physiological variables were analyzed using repeated measures models, considering breed, THIhadj value, and their interaction as fixed effects. The results showed a significant increase in all physiological variables as THIhadj increased. The Angus and Hereford breeds ( Bos taurus ) had the highest RT values (and RR, especially under THI _hadj = 101 conditions), indicating greater susceptibility to heat stress. In contrast, Brangus, Braford, and Tulianguy (composite or adapted breeds with Bos indicus influence) showed more stable responses, confirming greater thermal resilience. These findings reinforce the importance of selecting climate-adapted animal biotypes to achieve sustainable livestock systems in hot and variable environments such as northern Uruguay.