Assessing the age-dependent optimal dietary branched-chain amino acid ratio in growing chicken by application of a nonlinear modeling procedure

Based on a nonlinear N utilization model, 2 N balance experiments with growing broiler chickens were conducted, investigating the ideal amino acid (AA) ratio (IAAR) of the branched-chain AA (BCAA) Leu, Ile, and Val related to Lys. In both of the experiments, the starter (I: 10-20 d of age) and grower periods (II: 25-35 d of age) utilized 36 male Ross 308 chickens each (n = 144). Nitrogen balance periods were divided into an adaptation period (5 d) and 2 consecutive collection periods (2 × 5 d). Diets of experiment 1 were based on a consistent mixture of wheat, soy protein concentrate, wheat gluten, fish meal, and crystalline AA, subsequently diluted by wheat starch to provide 8 graded CP levels (6-34% CP). Results of nonlinear regression between N intake and N deposition yielded the theoretical maximum for N deposition (NDmaxT; I: 4,593 mg of N/BWkg(0.67)/d; II: 4,302 mg of N/BWkg(0.67)/d). Furthermore, the daily N maintenance requirement (NMR) was derived (I: 113 mg of N/BWkg(0.67); II: 215 mg of N/BWkg(0.67)/d). Both the age-dependent data for NMR and NDmaxT were applied to calculate the model parameter b (protein quality, independent of N intake) and bc(-1) (efficiency of limiting AA), respectively. Five diets based on the same ingredients as in experiment 1 were formulated for experiment 2: an AA balanced basal diet and 4 diets of the same composition but lacking in supply of Lys, Leu, Ile, or Val. All experimental diets were isocaloric and isonitrogenous. For calculation of the IAAR, observed bc(-1) data of each AA diluted diet were utilized. The concluded IAAR of Lys:Leu:Ile:Val for the starter and grower period were 100:94:55:65 and 100:106:56:72, respectively. These results indicated a higher demand for Leu and Val during the grower phase and, generally, a lower IAAR for the BCAA than previously reported in comparable experiments.