Avaliação do estresse térmico em vacas leiteras mestiças (Bos taurus x Bos indicus) criadas em clima semi-árido em sistema free-stall

Abstract

The study aimed to evaluate the influence of thermal stress in crossbred dairy cows (Holstein-Gir) reared in semi-arid climate in free-stall system during dry and rainy seasons in years 2010 to 2011 in the municipality of São Luis do Curu in the state of Ceará. For both, it was assessed the climatic elements such as air temperature (AT) and relative humidity (RH) inside (IS) and out of the shed (OS), from 6:00am to 6:00pm. These variables used to calculate the rate of temperature and humidity as well as the monitoring of physiological parameters such as rectal temperature (RT), respiratory frequency (RF), surface temperature (ST) and surface temperature of the udder (STU) of 14 dairy cows, collected once a week, in the morning (6am), in the afternoon (12:00am) and at evening (6pm). Data were expressed as mean and standard error and analyzed by ANOVA and the means were compared by Tukey likely to 5% error. Correlations between physiological variables (RT, RF and ST) and environmental (AT, RH and THI) were estimated by the method of Pearson (P <0.05). Results show that regardless of time of year the animals are subject to situations of discomfort, being the dry season the most impressive, with AT higher (P <0.05), the RH lower (P <0.05) and THI higher (P <0.05) in this period. Shown by the results of physiological parameters in the RF, ST and RT were higher at 1:00pm (P <0.05) regardless of time of year. Revealing the results of the physiological variables were more close to normal in time of 6:00am and 6:00pm hours for the rainy season (P <0.05). The AT and THI shown strongest correlations with rectal temperature (r = 0.743 and r = 0.627, P <0.01) during the dry season, and RF (r = 0.767 and r = 0.804, P <0.01) and ST (r = 0.703 and r = 0.895, P<0.01) in the rainy season. Showing that the ambient temperature is the factor that most impactful for animal welfare. Showing that the structure of free-stall with the use of mechanical ventilators can alleviate some times of the day the intense irradiation from the sun, bringing effects more effective in the afternoon, especially during the dry season. It is concluded that the Holstein-Gir cows reared in free-stall system in the semi-arid regions in the state of Ceará are subjected to heat stress conditions throughout the year, being the dry season the most impressive, and that during the rainy season there are only moments from the day that these temperatures are milder, but still considered outside the comfort zone for those animals, which is confirmed by the changes of physiological parameters.