Adaptabilidade ao calor e índices ambientais para vacas da raça Holandesa no semiárido

Abstract

This experiment was conducted in the municipality of São Bento do Una in Pernambuco’s semi-arid region during spring/summer and autumn/winter. Its aim was to assess the heat adaptability of Holstein cows originating from the state of Paraná and to estimate thermal comfort indices. The experiment took place from November 2010 to September 2011. Measurements of the following physiological parameters were taken from 18 Holstein dairy cows in the morning (6 h) and afternoon (16 h): rectal temperature (RT, °C), respiratory rate (RR, mov./min-¹), skin temperature of white areas (STW, °C) and black areas (STB, °C), and surface temperature of white pelage (STWP, °C) and black pelage (STBP, °C). Milk production was measured daily. The environment was continously monitored by an automatic metereological station and the recorded weather data was used to calculate the following indices for thermal comfort: temperature humidity index (THI), Black-Globe Temperature Humidity Index (BGTHI), equivalent temperature index (ETI) and the radiant thermal load index (RTL, W/m²). The experimental delineation that was used was completely casual, a 2x2 factorial scheme being the period of day (morning and afternoon) and season of the year (spring/summer and autumn/winter). The result of the correlation analysis showed that all of the meteorological variables showed significant correlations with all of the physiological parameters. Predominantly black cows showed a significant difference (P<0.01) for RT, STBP, STB and STW, which were higher than those recorded for predominantly white cows. All of the physiological variables were higher (P<0.01) in spring/summer than in autumn/winter and in the afternoon rather than in the morning. Significant correlations were observed between environmental variables and thermal comfort indices with physiological variables. The results obtained prove that RT is the best indicator of heat stress in lactating Holstein cows. Regression analyses with adjustments to the model were carried out to estimate critical levels of thermal comfort. Critical THI, based on rectal temperature and respiratory rate, was estimated, being equal to 78 and 75, respectively.