Protein and lipid deposition rates in male broiler chickens: separate responses to amino acids and protein-free energy.

Two experiments of similar design were conducted with male broiler chickens over two body weight ranges, 200 to 800 g in Experiment 1 and 800 to 1,600 g in Experiment 2. The data were used to test the hypothesis that protein deposition rate increases (linearly) with increasing amino acid intake, until energy intake becomes limiting for protein deposition rate. Additional amino acid intake above this point would be deposited less efficiently. An increase in energy intake would increase lipid deposition rate, but should, at low amino acid intakes, not affect protein deposition rate. Each experiment consisted of 18 treatments: two levels of protein-free energy (energy(pf)) intake, combined with nine amino acid to energypf ratios. Protein was balanced for amino acid content and lysine was the first limiting amino acid in the diet. Protein deposition rate increased with additional amino acid intake. No evidence was found that energy(pf) intake limited protein deposition rate at high amino acid intake. Extra intake of energy(pf) increased lipid deposition rate, which was independent of amino acid intake. Where amino acid intake was limiting, additional intake of energy(pf) had generally no effect on protein deposition rate. The marginal efficiency of amino acid utilization for protein deposition did not depend on body weight. The facts are relevant to the modeling of the growth of broiler chickens.