Acclimation effects on fed and fasted broiler thermobalance during thermoneutral and high ambient temperature exposure.

1. Two experiments were conducted to quantify heat stress (HS) acclimation effects on heat production (H), evaporative heat loss (E), sensible heat loss (S) and change in heat content (HC) of 24 food-deprived and precision fed broilers. 2. In experiment 1, heat stressed fasted HS acclimated birds (group 1) exhibited lower H (22.6 v. 25.5 kJ/kgW0.66 per h), E (7.5 v. 8.8 kJ/kgW0.66 per h), core body temperature (41.8 v. 42.4 C) and respiration rate (129 v. 160 breaths/min) than nonacclimated controls (group 2). 3. In contrast to the first experiment, precision fed HS acclimated birds (group 1) exhibited a higher H (29.3 v. 28.0 kJ/kgW0.66 per h) and E (10.5 v. 9.6 kJ/kgW0.66 per h) during HS and elevated H during thermoneutral periods than their non-acclimated counterparts (group 2). The elevated H became more pronounced with each successive HS exposure. 4. These results indicate that H increases when broilers are fed, that broilers preferentially dissipate heat as S when environmental conditions permit, and that food and/or energy consumption level markedly influences the bird's capacity to exhibit a HS acclimation response, and in fact has the ability to mask it.