Broiler responses to feeds formulated with or without minimum crude protein restrictions and using supplementall-valine andl-isoleucine

Comparisons of corn-soy-feeding programs formulated using different protein constraints were done in the present study. A total of 1,800 one-day-old, slow-feathering Cobb × Cobb 500 male broiler chicks were placed in 72 floor pens of 25 birds each with 9 birds/m². Feeding programs (PRG) were composed of prestarter (PST,1–7 d), starter (ST, 8–21 d), grower (GR, 22–35 d), and finisher (FN, 36–43 d) diets formulated as: PRG 1 = CP restricted to a minimum (22.4, 21.1, 19.8, and 18.4% for PST, ST, GR, and FN, respectively) with amino acid-to-Lys ratios only set for TSAA (0.72) and Thr (0.65); PRG 2 = CP not restricted, whereas ratios of amino acids to Lys were also extended to Val (0.77) and Ile (0.67); PRG 3 and PRG 4 were the same as PRG 2, but withl-Val added in PRG 3 andl-Val andl-Ile in PRG 4. Feeds were formulated to have digestible Lys of 1.324, 1.217, 1.095, and 1.006% for PST, ST, GR, and FN, respectively, or 5% higher. A completely randomized design was used with a 4 × 2 factorial (4 PRG and 2 digestible Lys) design with 9 replications per treatment. Overall, no interactions between PRG and digestible Lys were observed, with 1 exception observed for BW gain and FCR from 36 to 43 d. Birds fed PRG 2 showed improvements in BW gain and FCR when fed the diet 5% higher in digestible Lys. Cumulative BW gain and FCR results at 35 and 43 d, as well as in each individual feeding phase, showed broilers from PRG 2 having the best results, but without mean separation significant differences when compared with PRG 3 and 4. Feeding a dietary program with a 5% increase in digestible Lys resulted in improved cumulative FCR only when cumulatively measured from placement to 43 d. Abdominal fat, as a percentage of the eviscerated carcass at 43 d, was lowest for birds fed PRG 2 diets. Formulation of diets having traditional recommendations of digestible Lys with Val- and Ile-to-Lys ratios of 0.77 and 0.67, without restricting CP, led to the best results overall in performance and fat pad deposition; therefore, if the goal is to reduce CP, similar results should be achieved by addingl-Ile with further reductions in protein ifl-Val is also added. Data obtained in this study support the applicability of supplementation withl-Val andl-Ile when ideal AA ratios are followed.