Heat tolerance in naturalized Brazilian cattle breeds

Research carried out at the Sucupira Farm of Embrapa Recursos Genéticos e Biotecnologia, in Brasilia-DF, aimed to evaluate heat tolerance in five naturalized and two exotic breeds of cattle, at different temperatures and humidity, based upon physiological and blood parameters. The data collected included heart rate (HR), rectal temperature (RT), respiratory rate (RR), and sweating rate (SR). Blood was also collected for hemogram analysis and cortisol level. The measurements were taken at 7 am and at 2 pm after the animals were exposed to the sun, with water ad libitum. This procedure was repeated for six days, three in the middle of the dry season when temperatures and humidity were lower and three at the start of the rainy season when temperatures and humidity were higher. Significant statistical differences were observed in RR and HR for day, breed and animal within breed. The analysis indicated that the Junqueira and Nellore breeds are most adapted to climatic conditions in Brasilia; while Mocho Nacional and Holstein breeds were the least adapted. Although sample size is limited, these results are important to identify which breeds are most resistant towards climatic variations observed in the Brasilia region, as well as which physiological parameters are the most indicated for use in animal breeding programs intended to select animals and breeds adapted to thermal stress conditions.