The aim of the present study was to investigate the effects of reduced dietary protein levels on metabolism and performance in growing-finishing broilers under thermoneutral temperature or thermal stress. Two experiments were performed using male Cobb 500™ broilers (n = 720) aged, 22–42 d. For both experiments, the experimental design involved four treatments (220, 200, 180 and 160 g/kg CP), with six replicates of 15 birds each in a climate-controlled room; Experiment 1: 21–25 °C; 58–62 % relative humidity during 24 h/day and Experiment 2: 30–34 °C; 58–62 % relative humidity during 8 h/day and 16 h of thermoneutrality, (21–25 °C/ 58–62 % relative humidity). The diets were isocaloric (13.18 MJ/kg), following amino acid corrections. The analyzed variables were: protein digestibility; ether extract; dry matter; apparent metabolizable energy; nitrogen corrected apparent metabolizable energy; net energy and excreted nitrogen. In addition, performance and economic parameters were analyzed: relative body weight, feed intake, weight gain, feed conversation ratio, viability, European index of production and production costs.
For both experiments, reduction from 220 to 160 g/kg CP impaired digestibility of protein and ether extract, decreasing the availability of net energy/bird. The amino acid correction used with 160 g/kg crude protein did not achieve the results for birds receiving 220 g/kg protein. However, the level of 200 g/kg crude protein in the diet is ideal for maximizing broiler performance in thermoneutral temperature and cyclic heat stress, besides providing lower nitrogen excretion in the excreta in relation to the diet of 220 g/kg CP.