Thermoregulatory Adjustments of Cattle to Long-Term Heat Stress in a Field Environment

Few studies have examined the changes in maintenance of thermal balance in cattle during repeated daily heat exposure over a summer period. One challenge with this type of study is the continuous measurement and analysis of animal and ambient conditions over an extended period of time. An objective of this study was the development of a routine that allows for this process. Another goal was the determination of time-dependent change in several key physiological indicators of thermal status as a function of ambient condition. The present study measured thermoregulatory ability over 90 days to identify early and late periods of heat response. Crossbred Angus steers (n=36; average body weight = 284±29kg) were stratified by weight, and housed in feedlots with approximately 50% shade coverage. Ambient conditions were recorded using Hobo loggers (Onset Computer, Bourne, MA). Overall ambient temperature (Ta) range was 12.2 to 36.6°, and calculated temperature humidity index (THI) was 54.4 to 85.3. A corn-based diet and water were provided ad libitum, and core temperature (Tcore) measured hourly using intraruminal telemetric boluses (Smartstock, Pawnee, OK). Respiration rate (RR) was measured at 0800 and 1700h on selected days to record daily low and high periods of performance, respectively. Relationships between RR, Tcore, and Ta were examined using linear regression and ANOVA procedures (JMP Statistical Software; SAS Institute; Cary, NC), and focused on early (Period 1; Days 1-24) and late (Period 2; Days 70-90) responses to heat. The purpose of deriving these relationships was to determine effector response (i.e., RR) to Ta, thermal status (i.e., Tcore) shift with Ta, and the RR relationship to Tcore. Mean daily Ta values showed a slight increase in heat stress level (P=0.005) from Period 1 (28.0±0.7¨¬C) to 2 (31.2±0.7¨¬C). Mean daily THI values were nearly identical (P=0.12) for Periods 1 (76.8±1.0) and 2 (79.0±1.0). Maximum Ta and THI values for the two periods ranged from 34.4 to 36.6¨¬C and 84.8 to 85.3, respectively. As a result of the slight increase in heat stress from Periods 1 to 2, mean RR and Tcore values increased from 69.0 to 85.4±0.8 bpm (P°Â0.0001) and 39.7 to 40.1±0.02¨¬C (P°Â0.0001), respectively. The most adaptive change was RR versus Ta, with a linear regression slope reduction from 3.70 to 2.61 bpm/° from Periods 1 to 2 (P<0.0001) to indicate a decreased sensitivity response for RR with continued heat exposure. In contrast, the slope change in the linear relationship of Tcore versus Ta from Periods 1 to 2 was insignificant (P=0.85). The slope change in the same relationship of RR versus Tcore was also insignificant (P=0.76) from Periods 1 to 2. Interestingly, correlation coefficient values for all relationships increased (P range = 0.02 to 0.08) from Periods 1 to 2 to suggest less variance in heat stress response with adaptation. The results of this study indicate that respiratory rate response to ambient temperature decreases with heat adaptation in cattle. This is possibly due to the adaptive reduction in metabolic heat production that is known to occur in controlled studies, and the concomitant decreased need for heat dissipation through avenues such as respiratory evaporation.