EN 
PT_BR 
Heat stress is one of the main challenges for dairy farming in the summer, especially in hot and humid climates. High-producing cows, such as Holstein cows, produce a greater amount of metabolic heat and therefore have more difficulty dissipating this excess when temperatures rise.
Maíza Scheleski da Rosa, the acting technical superintendent of the Association of Holstein Cattle Breeders of Rio Grande do Sul (Gadolando), explains that the thermal comfort of these animals occurs within ranges similar to those of an autumn day, between 8°C and 18°C. “The more milk the cow produces, the greater the heat generated by its own metabolism. This makes the animal more sensitive to high temperatures, especially when there is high humidity,” she states.
According to Maíza, the most appropriate parameter for assessing risk is the Temperature-Humidity Index (THI), which combines temperature and humidity into a single value. “The ideal THI for the breed is below 68. When the temperature exceeds 20°C, especially with high humidity, the cow begins to suffer from the heat,” she explains.
According to the technique, animals under heat stress reduce their dry matter intake, consequently leading to a drop in production. Fertility can be impaired, metabolic problems increase, and animals become more susceptible to disease.
Physical signs can be observed during daily handling. Faster breathing, excessive salivation, and spending more time standing, with less time lying down ruminating, indicate that well-being is compromised. "These behaviors show that the animal is trying to dissipate heat and is not in an ideal condition of comfort," he notes.
To improve thermal comfort, Maíza points out that some structural and management measures make a difference in both confined and pasture-based systems. “When we talk about barns or freestalls, it’s essential to invest in good ventilation, whether natural or with fans and exhaust systems. Whenever possible, it’s also advisable to use water spray cooling systems in conjunction with ventilation,” she states.
The technique also draws attention to the structure of the facilities. Roofs with thermal insulation and adequate shaded areas help reduce the effects of high temperatures. Providing a constant supply of clean, fresh water is another essential measure, according to the expert.
Adjusting the feed to coincide with the cooler times of the day is also recommended. "Cows tend to eat better when they are not suffering from the heat," he explains.
In pasture-based systems, the availability of shade, whether natural or artificial, is essential, as is constant access to quality water. Organizing activities for the early morning and late afternoon helps to avoid long journeys during the hottest periods. Forage planning also contributes to reducing excessive walking and, indirectly, decreasing heat stress.
The expert notes that signs such as rapid breathing, salivation, and increased standing time indicate that the animal is already suffering from the heat. "Thermal comfort is not a luxury, but a necessity, because when the cow is comfortable, she is well, she eats better, produces more milk, reproduces more efficiently, and remains in the herd longer," she states.
