PS4-3. Effects of Heat Stress on Respiration Rate and Behaviors During Different Times of Day

The objective was to evaluate the effects of time of day during a heat stress event on cattle respiration rates and behaviors. Angus and Angus × Simmental beef heifers (n = 140; BW = 249 ± 24 kg; BCS = 5.1 ± 0.33) and cows (n = 49; BW = 450 ± 29 kg; BCS = 5.8 ± 0.43) were observed for nine days and three different time periods during two heat stress events. Four pens of heifers (32, 34, 36, and 38 heifers/pen) and two pens of cows (24 and 25 cows/pen) were observed in the same sequence three times within a day (0900, 1200, and 1500). Pastures averaged approximately 2 – 2.25 hectares and were composed of predominately endophyte-infected tall fescue. Respiration rate and behavior observations (grazing, ruminating, standing, lying, and under shade) were utilized to determine potential changes in cattle activity. Behaviors are shown as percentages and are not mutually exclusive. Active rumination was determined by cattle standing or laying and actively ruminating. A third of the animals within a pen were selected to provide a representative sample for a pen respiration rate. Breaths were counted by two observers utilizing binoculars for 15 seconds. The respiration rate from each observer was multiplied by four to calculate breaths/minute and averaged. Data were analyzed using the MIXED procedure of SAS 9.4. There was a time-of-day effect on respiration rate (P = 0.01). Respiration rates increased from 91 breaths/min at 0900 to 109 breaths/min at both 1200 and 1500, indicating a physiological response to rising ambient temperature and humidity throughout the day. There was no time-of-day effect (P ≥ 0.14) on grazing, ruminating, lying or standing behaviors. There was a difference (P = 0.01) in time of day on the percentage of cattle under shade. The percentage of cattle seeking shade was greater at 1200 and 1500 compared to 0900. As the day progressed and environmental conditions worsened, respiration rates and the percentage of cattle seeking shade increased which identified the physiological and behavioral adaptations utilized to potentially minimize the effects from the heat stress event.