Effects of dietary protein concentration and corticosterone injections on energy and nitrogen balances and fat deposition in broiler chicks

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Abstract
Reducing dietary protein concentration in isocaloric diets consistently decreased N and energy excretion, and increased dry matter (DM), nonprotein DM (NPDM), energy retention and fatness. There were significant correlations, negative between dietary energy-to-protein (E:P) ratio and N excretion and positive between the E:P ratio and the retention of DM, NPDM and energy. N excretion was correlated with energy excretion, and NPDM retention with energy retention. Corticosterone injections increased fatness despite significantly increasing N and energy excretion. A positive relationship was observed in corticosterone-treated birds between N excretion and the retention of DM and NPDM. Increased food intake because of a low dietary protein concentration was not evident when a sorghum-based diet was used. Corticosterone injections did not increase the food intake or the fatness of chicks fed this diet. Decreasing the protein concentration in diets containing identical metabolizable energy (ME) levels slightly, but significantly increased apparent ME values in 3 of 4 experiments. Corticosterone injection did not affect this variable.