Thermoregulatory responses of cattle to controlled dehydration during heat challenge

Few studies have quantified the impact of dehydration on bovine thermal status during heat stress. Angus steers (n=8) in trial 1 were dehydrated and rehydrated over 6 days at thermoneutrality (TN; 19–21°C), with 4 month recovery. Trial 2 consisted of 5 days of cyclic heat stress (HS; 26–36°C) followed by water restriction and rehydration as in trial 1. Measurements included respiration rate (RR) and rectal temperature (Tre) taken 6 times daily, with body weight (BW), feed intake (FI), and sweat rate (SR) determined at selected times. Dehydration‐induced reductions in FI (70–75%), BW (4–9%), and RR (~15bpm) were similar in magnitude regardless of ambient temperature. FI and BW quickly returned to baselines after 1 day rehydration, while RR took 4 days. SR approached 100 g/m2h during HS, declined to 30 g/m2h with dehydration, and did not recover until after 3 days rehydration. Tre exhibited no significant change with dehydration, due to reduction in FI that counterbalanced decreases in RR and SR. Rehydration decreased Tre (~0.5°C), with slow recovery over 4 days. Surprisingly, dehydration of cattle did not result in a change in thermal status during heat stress. USDA Agreement No. 58‐6227‐3‐016