Melhor índice de estresse térmico para novilhas leiteiras mestiças / Most appropriate heat stress index for half-blood dairy heifers

ABSTRACT

Background: Heat stress indexes integrate several variables of the thermal environment in a single figure and predict their impact on animal welfare and performance. The correct interpretation of these indexes is of help in the choice of more adequate measures to attenuate the stress caused by the heat. Therefore, the aim of this research is to examine some of the heat stress indexes mentioned in the literature and to decide which ones best reflect the potential impact of meteorological conditions on the thermal homeostasis of half-blood dairy heifers bred in Uberlândia, Triângulo Mineiro area, Minas Gerais, Brazil.Materials, Methods & Results: Eight half-blood dairy heifers were exposed to direct sunlight from 9 h to 13 h in the months of November and December of 2016, and also January and February of 2017, 5 days per month on average, in Uberlândia, MG, Brazil. After this challenge, the respiratory rate and the rectal temperature were measured. Simultaneously to the collection of physiological variables, the temperatures of dry bulb and wet bulb, as well as that of black globe, were quantified, as was the wind speed. Afterwards, the solar radiation, the relative humidity, the mean radiant temperature and some heat stress indexes were calculated, the latter being Temperature-Humidity Index, Black Globe-Humidity Index, Equivalent Temperature Index, Environmental Stress Index, Respiratory Rate Index, Thermal Load Index, Environmental Specification Index, Thermal Comfort Index for Dairy Cattle, Comprehensive Climate Index and Cattle Heat Stress Index. These indexes were, then, correlated with the physiological variables. The averages of room temperature, black globe temperature, solar radiation, wind speed, radiant temperature and relative humidity were, respectively, 29.96°C, 41.73°C, 831.02 W/m2 , 0.11 m/s-1, 318.14 K and 50.51%. Rectal temperature averaged 38.8ºC while respiratory rate averaged 41.97 breaths per minute-1.[...](AU)