“…In the classification tree presented in Figure 3, flock age was more important in the determination of bird mortality, as it occupied the first root node. Chickens between 24 and 28 weeks of age respond differently to environmental weather conditions as older birds are heavier, and therefore, their heat exchange with environment is more difficult as compared to younger birds (Campos, 2000;Furlan & Macari, 2002). In addition to body weight, 28-w-old layers are achieving the peak of lay, according to most genetic line manuals (Dekalb, 2009;Hy-Line, 2009;Lohmman, 2009), which means that they have higher metabolic activity, and require more nutrients, particularly amino acids (Rocha, 2006;Lin et al, 2006) and energy (Rostagno et al, 2005), generating more metabolic heart, and presenting lower capacity of responding to heat stress.…”