comportamiento en pastoreo durante el verano en vacas Charolais en el litoral norte del Uruguay

In northern Uruguay, cattle production is mainly focused on meat production, carried out in extensive pastoral systems characterized by high temperatures and marked seasonal variations in forage availability. In this context, the breeds selected must combine productive performance with adaptability to hot climates and resistance to diseases and parasites. The breeds most used in production are British breeds such as Hereford and Aberdeen Angus; however, in the environmental conditions of the north of the country, their welfare is compromised by high summer heat load. The Charolais breed is characterized by its hardiness, high weight gain, and adaptability to diverse environments, which positions it as a genetic resource of interest. In this context, the objective of this study was to determine the grazing behavior of Charolais breeding cows, according to adjusted THI, managed on natural pasture, under conditions of heat stress during the summer in Paysandú, Uruguay. This involved determining the daily time spent grazing, in the shade, resting in the sun, ruminating, and drinking water. The study consisted of visual monitoring for three consecutive days from 7:00 a.m. to 6:00 p.m., recording the animals' activity (grazing, resting in the sun, resting in the shade, ruminating, and drinking water) every 10 minutes. At the same time, the Temperature and Humidity Index adjusted for wind speed and solar radiation (THIadj) was quantified to measure the risk of heat stress in the animals evaluated, being 78 (alert), 81 (danger), and 80 (danger), respectively, for each day of measurement. Statistical analysis showed differences in behavior according to variations in THIadj, with a decrease in grazing as the index increased (r = -0.19), an increase in the use of shade (r = 0.35), and a substitution of resting (r = -0.37) and water-seeking behaviors (r = -0.24) for staying in the shade. Likewise, there was no correlation between THIadj, and the time spent drinking water. On average, within the total hours of observation, cows spent 4.6 hours grazing, 2.6 hours resting in the sun, 0.6 hours ruminating, 3.6 hours in the shade, and 0.4 hours at the watering hole, with a frequency of visits to the water of 2, 4, and 3 times per day, respectively. In conclusion, the results confirm the importance of access to shade as a determining factor for the welfare and productive efficiency of cattle exposed to hot conditions. For this reason, it is relevant to consider the necessity to include the environmental component to daily planning.