Thermography and physiology of stress in dairy calves in outdoor holding pens covered with geosynthetics/Termografia e fisiologia do estresse em bezerras mantidas em bezerreiros tropicais cobertos com geossinteticos

HIGHLIGHTS: The use of shading for dairy calves reduces thermal discomfort in tropical regions. Geosynthetics can be used as cover material in outdoor holding pens for dairy calves. The physiological responses of calves are affected by the roofing materials used in the rural installations. This study aimed to assess the environmental variables, thermal comfort indices and physiological responses of calves in outdoor holding pens shaded with geosynthetics. Twenty crossbred females (Giroland, Jersey and Holstein) in the suckling phase (from birth to 90 days old) with an average initial live weight of 40.6 kg were used. A completely randomized block design was used, in a 4 * 3 factorial scheme with five replicates. The roofing materials (polyethylene mesh, geocomposite drainage layer, nonwoven geotextile and woven geotextile) were the first factor and time periods (8 to 10 a.m., 12 to 2 p.m. and 4 to 6 p.m.) the second factor. The following environmental variables were measured to calculate thermal comfort indices: temperature-humidity index, black globe-humidity index and enthalpy. The physiological variables analyzed were respiratory rate, rectal temperature and skin temperature. Environmental variables and thermal comfort indices did not differ between the different roof types, however, a significant difference (p < 0.01) was observed between the time periods, with 12 to 2 p.m. being the most critical period. The lowest average respiratory rate (60.3 breaths min1) and rectal temperature (38.9 [degrees]C) were recorded for the animals kept under the geocomposite drainage layer roof. There was a significant difference (p < 0.05) for interaction between treatment and time periods for the cannon area. The geosynthetics studied can be used as roofing material for outdoor holding pens, with the geocomposite drainage layer being the most indicated for tropical regions. Key words: dairy cows, thermal images, shading, physiological variables Este estudo teve como objetivo avaliar as variaveis ambientais, indices de conforto termico e respostas fisiologicas de bezerras criadas em bezerreiro do sistema tropical sombreados com geossinteticos. Foram avaliadas 20 femeas das racas (Girolando, Jersey e Holandesa) com peso inicial medio de 40.6 kg durante a fase de aleitamento (nascimento ate 90 dias de idade). O delineamento experimental foi de blocos casualizados completos, aplicado em um arranjo fatorial 4*3, com 5 repeticoes. Os materiais de cobertura (malha de polieti