Estudio de variables fisiológicas en condiciones climáticas de verano en la raza Charolais en el litoral norte de Uruguay

Heat stress is one of the main challenges for livestock production in subtropical regions, where high temperatures, high relative humidity and intense solar radiation affect productive efficiency, animal welfare and the sustainability of pastoral systems. In Uruguay, the frequency and intensity of extreme heat events have increased in recent decades, especially on the northern coast, where climatic conditions increasingly exceed critical thresholds for livestock. In this context, understanding the physiological responses of breeds used in beef production is essential to develop management and genetic selection strategies aimed at climate resilience. The Charolais breed, of French origin and recognized for its high meat yield, had an important presence in Uruguay, although its population decreased due to its large size and higher nutritional requirements. However, its light coat and recent evidence of intra-racial variability in heat tolerance suggest a higher adaptability than expected, justifying its re-evaluation in subtropical environments. The objective of this study was to evaluate physiological responses to heat stress in Charolais cattle during the summer in Paysandú, Uruguay. We worked with 21 animals of different categories (bulls, bulllets, cows, heifers, calves and heifers) kept in natural range. Seven measurements were taken between December 2024 and January 2025, at critical times (07:00 and 14:00 h). Climatic variables were recorded and the Temperature and Humidity Index adjusted for wind speed and solar radiation (ITHaj) was calculated. During November-February, the average ITHaj was 79.3±5.8, corresponding to thermal emergency conditions, with a maximum of 91.5 in mid-afternoon. The herd remained 49 days in emergency, 27 in danger and only 4 days in thermal comfort. A positive correlation was observed between ITHaj and rectal temperature (TR; r = 0.43) and between ITHaj and respiratory frequency (FR; r = 0.61). During the morning (ITHaj 47.7-73), animals maintained TR between 38.3 and 39.4 °C, FR between 40 and 56 rpm, forehead temperature (TFr) between 31.3 and 36.6 °C and flank temperature (TFl) between 34 and 36.6 °C, indicating thermal stability. In the afternoon (ITHaj 78.5-90), all variables increased significantly: TR up to 40.2 °C, FR > 90 rpm, TFr around 46 °C and TFl up to 44.7 °C, reflecting increased dissipation effort through hyperventilation and peripheral vasodilation. Adult categories showed the largest increases, while young animals presented more moderate responses, suggesting better thermal recovery. The results confirm a positive association between ITHaj and physiological variables, evidencing that the thermal environment is the main determinant of the physiological response of Charolais cattle in summer. TFl stood out as a sensitive surface indicator of the degree of stress. In conclusion, there is individual variability that can be exploited for the selection of more resilient animals, and it is important to integrate ITHaj with physiological measurements for welfare monitoring and adaptive management in subtropical pastoral systems.