Thermal equilibrium of Nellore cattle in tropical conditions: an investigation of circadian pattern

<p>The aim of this study was to evaluate the circadian pattern of the thermal equilibrium of Nellore cattle in a tropical environment. The study was conduct at the Animal Biometeorology Laboratory of the São Paulo State University, Jaboticabal, Brazil (21.25° S, 601 m altitude) in December of 2016. Six Nellore cattle with similar weight, age and body condition were assigned in two Latin Square experimental design (24 classes of hours (1-2h; 3-4h; …; 23 - 24h; 24 -1h) during twelve days). Evaluations were performed with subjects protected from direct solar radiation and rain overnight. Respiratory parameters as percentage of oxygen (O<sub>2</sub>; %), carbon dioxide (CO<sub>2</sub>; %), water vapor pressure (e{T<sub>E</sub>}; kPa) respiratory rate (R<sub>R</sub>; breaths min<sup>-1</sup>) and ventilation (V<sub>E</sub>; L s<sup>-1</sup>) were quantified using an indirect calorimetry system; furthermore, hair coat surface (T<sub>S</sub>, °C), skin (T<sub>EP</sub>, °C) and rectal temperature (T<sub>R</sub>, °C) were recorded with thermocouple. Metabolic heat production (q<sub>met</sub>), sensible heat flow (q<sub>sens</sub>), heat loss from respiratory tract (q<sub>er</sub>) and cutaneous evaporation (q<sub>es</sub>) were calculated. Air temperature (T<sub>A</sub>,°C), relative humidity (R<sub>H</sub>, %), partial vapor pressure (e{T<sub>A</sub>}, kPa), solar irradiance (G<sub>S</sub>, W m<sup>−2</sup>), black globe temperature in shade (T<sub>G</sub>,°C), and under direct sunlight (T<sub>GSUN</sub> ,°C) were measured at regular one-minute intervals during each day with a portable weather station. Data were analyzed by the least square method. T<sub>AR</sub> showed, on average, 9°C of amplitude, with higher value at 13:00 h (30.9 ± 0.2 °C). Average metabolism of 158.43 ± 13.97 W m<sup>-2 </sup>was observed, presenting highest value of 174.62 ± 2.63 W m<sup>-2</sup> at 11:00 h, during the morning; thenceforth, it was gradually decreasing, and start to increase one hour after feeding time. The sensible heat flow through the body surface was represented, in highest percentage, by long wave radiation. Interestingly, even with the fall of the T<sub>AR</sub> at night, we observed lower values for q<sub>rad</sub>. q<sub>es</sub> was more expressive during the warmer temperatures. The q<sub>er</sub> seems to be a mechanism of minor significance for the thermal of Nellore cattle. Under tropical condition, from 21 to 30 °C thermoregulatory responses of Nellore cattle remain stable over the 24 hours when protected from direct solar radiation and rain overnight, which seems to be a good indicative of lower energetics costs for homoeothermic maintenance.</p>