Two experiments (Exp) were carried out to evaluate diets for broilers formulated with reducing crude protein (CP) level, supplemented with synthetic amino acids, formulated based on total amino acids (TAA) (Exp 1) or digestible AA (DAA) (Exp 2). The experiments conducted in the period from 3 to 6 weeks of age of the birds. In both experiments, the four levels of CP were: 20.8, 19.7, 18.6 and 17.5% CP. In the Experiment 2, two other treatments were added: diets containing 20.8 and 17.5% CP and high digestibility (HD), based on corn, soybean meal, corn starch and soy protein isolate. In both experiments, the feed intake was not affected by CP decreasing. In Exp1, weight gain (WG), feed conversion (FC), breast and drumstick yield were negatively affected by CP reduction, meanwhile in Exp 2 those variables were not affected. However, in both experiments, in the week from 21 to 28 days of age, WG and FC were negatively influenced by CP decrease. As the CP levels decrease, the dry matter (DM) and organic matter (OM) digestibility increased in the Exp 2, due to the smaller inclusion of soybean meal. In both experiments, the relative protein retention was greater for rations with low CP levels. In Exp 2, although of the different CP levels did not affect the performance, the Hdig diets promoted higher OM digestibility and tended to have higher DM digestibility. The rations with 17.5%CP level and HDig promoted a more positive protein balance and higher relative protein retention as compared to the standard digestibility. Considering all the results, formulation with DAA showed advantages related to TAA.
ideal protein; metabolism; poultry; soy protein isolate
