Intake, physiological parameters and behavior of Angus and Nellore bulls subjected to heat stress Consumo, parâmetros fisiológicos e comportamento de tourinhos Angus e Nelore submetidos a estresse por calor

Genetics differences between breeds may determine the tolerance to high temperature, effect dry matter intake and consequently cattle performance. The effect of temperature and humidity index (THI) on diurnal, nocturnal and daily intake, water intake, physiologic parameters and behavior of Nellore (B. indicus) and Angus (B. taurus) bulls were evaluated. Eight Angus and eight Nellore young bulls (337±7.4 kg and 16 months of age) were allocated in two climate-controlled rooms for 32 days. In the period 1, all bulls were housed in thermoneutral conditions (TN, THI = 72.6) for 10 days. In period 2 (10 days), four Angus and four Nellore bulls were subjected to low heat stress (LHS, THI = 76.4) in daytime, and four Angus and four Nellore bulls were subjected to high heat stress (HHS, THI = 81.5) in daytime. The diurnal and daily dry matter intake (DMI) of Nellore were not affected (P>0.05) by heat stress. However, Angus bulls decreased diurnal DMI by 24% and daily DMI decreased (P 0.05) daily DMI (31.6 and 30.2 g/kg of BW, respectively). We observed an increase (P 0.05) by heat stress. The heart rate decreased (P 0.05) in feeding behavior. Therefore, THI stress threshold should distinct for Angus and Nellore bulls. The use of feed intake information may improve the prediction of thermic discomfort on specific climate condition.