Modeling of threonine requirement in fast-growing chickens, depending on age, sex, protein deposition, and dietary threonine efficiency.

In addition to dose-response studies, modeling of N utilization, depending on intake of the first limiting amino acid in the diet, is one of the tools for assessing amino acid requirements in growing animals. Based on a verified nonlinear N-utilization model and following the principles of the diet dilution technique, N-balance experiments were conducted to estimate the Thr requirement of fast-growing chickens (genotype Cobb), depending on age, sex, CP deposition. and efficiency of dietary Thr utilization. Different predictions were made for the feed intake to conclude the optimal Thr concentration in the feed. The results are based on N-balance experiments with a total of 144 male and 144 female growing chickens within 4 age periods (I: 10 to 25 d; II: 30 to 45 d; III: 50 to 65 d; IV: 70 to 85 d), using diets with graded protein supply (6.6, 13, 19.6, 25.1, 31.8, and 37.6% CP in DM) from high-protein soybean meal with a constant amino acid ratio and Thr as the first limiting amino acid (3.87 g of Thr/100 g of CP; dietary Lys:Thr = 1:0.54). The observed optimal Thr concentration (% of feed) was influenced by age, sex, level of CP deposition, dietary efficiency of Thr utilization, and predicted feed intake. For male chickens, assuming an average CP deposition (60% of the potential) and average efficiency of Thr utilization, 0.78% (10 to 25 d), 0.73% (30 to 45 d), 0.65% (50 to 65 d), and 0.55% (70 to 85 d) total dietary Thr were observed as optimal total Thr concentration in the diet (corresponding to 60, 135, 160, and 180 g of daily feed intake, respectively). Data are discussed in context with the main factors of influence like age, sex, level of daily CP deposition, efficiency of dietary Thr utilization, and predicted feed intake.