The effect of continuous and cyclical heat stress on broiler growth performance, nutrient digestibility, energy and nitrogen balances was investigated. Four hundred and fifty, 21-day-old, Cobb male broilers were raised in battery cages in five treatments: 22C/AL (continuous 22°C, ad libitum feed consumption); 32C/AL (continuous 32°C, ad libitum feed consumption); 22C/PF32C (continuous 22°C, pair-fed on the daily feed intake of 32C/AL); CY/AL (cyclical-32°C for 8 h and 25°C for 12 h, ad libitum feed consumption); 22C/PFCY (continuous 22°C, pair-fed on the daily feed intake of CY/AL). Between 39 and 42 days of age, dry matter, crude protein, crude fat and AME n were analyzed in the diets and excreta to determine nutrient digestibility. Energy and nitrogen balances were evaluated through comparative slaughter (21 and 42 days of age). Growth performance was significantly lower in broilers exposed to either continuous or cyclical heat stress. However, the cyclical heat stress had a lower effect on feed intake and weight gain and no effect on the feed conversion rate. Nutrient digestibility was only influenced by continuous heat exposure, decreasing dry matter (3.9%) and protein digestibility (9.7%) in comparison to control birds. Broilers exposed to continuous heat stress increased metabolizable energy intake (20.3%) and heat production (35.5%), and decreased energy retention (20.9%) and energy efficiency (32.4%) in relation to control ones. Nitrogen intake and nitrogen retention were reduced by both forms of heat exposure, in comparison to control, but more strongly under continuous heat. Nitrogen retention was reduced by 50.4% and 20.4%, for continuous or cyclical heat stress, respectively. Nitrogen efficiency was reduced only by the continuous heat exposure (33.1%). These results revealed important differences between the effects of a continuous or a cyclical heat exposure in broiler chickens for digestibility, performance and energy and nitrogen balances.

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CO2 is a powerful respiratory stimulus for birds, however most studies do not consider the influence of gender and age in the hypercapnic ventilatory response. We evaluated blood gases and ventilatory response to hypercapnia (7% CO2 in air) of 10 (d10) and 21 (d21) days old broiler males (M) and females (F). Under resting conditions, ventilation (VE) was higher in d10 chickens compared to d21 ones, due to a higher tidal volume (VT), (M10 days: 631.3 ± 56.8 vs F10 days: 534 ± 56.9; M21 days: 84.4 ± 14 vs F21 days: 46 ± 5.3 mL kg−1 min−1). Difference in VE between M and F was observed only in d21. CO2 exposure increased total ventilation above control values, due to an increase in VT in all groups (M10 days: 1728.1 ± 289 vs F10 days: 1498 ± 213; M21 days: 240 ± 40.4 vs F21 days: 178.6 ± 36.2 mL kg−1 min−1). The CO2 response was accompanied by respiratory acidosis (pHa fell from ≈7.59 to ≈7.32) and hypercapnia (PaCO2 increased from ≈25.6 to ≈55 mmHg) in both M and F groups. Hypercapnic ventilatory response was more pronounced in d10 compared with d21, but difference between genders was observed in d21 only. Therefore, age and gender influence the normocapnic and hypercapnic ventilation in young broiler chickens. Financial Support: FAPESP, CNPq and INCT of Comparative Physiology

We evaluated cardiorespiratory responses to 7% CO2 of 10 (d10) and 21 (d21) days old broiler males (M) and females (F) after embryonic hypercapnia (1.5% CO2) and control situation (CS). Under normocapnia no difference in ventilation (VE), tidal volume, respiratory frequency, mean arterial pressure (MAP) and heart rate (HR) was observed between groups. Hypercapnia promoted a higher VE in Fd10 and Fd21 incubated in 1.5%CO2 compared with FCS (Fd10 control: 1497±213.4 vs Fd10 1.5% CO2: 1834±111.6; Fd21 control: 179±36.24 vs Fd21 1.5% CO2: 389±86.4 mLkg−1min− 1). Only Md21 incubated with 1.5% CO2 presented higher VE compared with CS (Md10 control: 1728±289.6 vs Md10 1.5% CO2: 1823±209; Md21 control: 239±40.4 vs Md21 1.5% CO2: 388±77.5 mLkg−1min−1). MAP was not different between groups, however, HR was higher in Fd10 incubated in 1.5% CO2 compared to Fd10 incubated in CS during hypercapnic exposure (Fd10 control: 351±25.1 vs Fd10 1.5% CO2: 405±14.8; Fd21 control: 324±11.8 vs Fd21 1.5% CO2: 331± 19.6 beats.min−1). Therefore, cardiorespiratory responses to hypercapnia are higher in animals incubated with 1.5% CO2 compared to control animals and these responses are more pronounced in females. Financial Support: FAPESP, CNPq and INCT of Comparative Physiology.