Utilizing laboratory and field studies to determine physiological responses of cattle to multiple environmental stressors

Heat stress studies are often conducted using controlled laboratory exposures or field exposures. Each approach has limitations and provides a partial understanding of complex interactions between simultaneous environmental stressors. The question is how similar the responses are in each situation. Several physiological measures of thermal status were used to compare heat stress responses of cattle in controlled chamber stress tests and fluctuating field conditions. Angus steers (N=23; 318±8 kg BW) were first placed on either endophyte-infected or -uninfected tall fescue pastures for the field exposure, followed by a controlled heat challenge, which exacerbates the condition known as fescue toxicosis. During the controlled heat challenge, steers were assigned to diets of either 0 or 40 μg ergovaline/kg/d to maintain the treatment states. Respiration rate (RR) was measured via flank counting and telemetric temperature transmitters in the rumen of each animal monitored core temperature (Trum). Linear regression fit models for RR, Trum, and air temperature (Ta) were utilized to compare relationships between field and chamber exposure. Correlation coefficients for RR were similar during both chamber (R=0.69) and field exposures (R=0.72). Respiration rate showed greater responsiveness to change in Ta under field conditions having twice the slope (4.40 versus 1.75 bpm/°C) and a lower Y-intercept (−42.14 versus +30.97 bpm) compared to the chamber run. Ruminal temperature was consistent between exposures showing a similar slope (0.04 versus 0.03 °C Trum/°C Ta) and Y-intercept (38.40 versus 39.30 °C) for its relationship with Ta. Despite respiration rate being the more sensitive indicator of heat stress, ruminal temperature proved to be the most consistent between environments.