Low crude protein diets for broiler chickens aged 8 to 21 days should have a 50% essential-to-total nitrogen ratio

Abstract

The aim of this study was to evaluate the effects of essential-to-total nitrogen (eN-to-tN) ratios supplemented with nonessential amino acids (NEAAs) in low-protein diets on the performance, nitrogen retention and blood parameters of broiler chickens from 8 day to 21 day posthatch. A total of 300 male Cobb® 500 chickens were weighed and allocated to one of six dietary treatments with 10 replicate pens in a completely randomized experimental design. The treatments were: T1, a control with 222.5 g/kg crude protein (CP) and a 47 % eN-to-tN ratio; T2, 190 g/kg CP and a 56 % eN-to-tN ratio; T3, 190 g/kg CP +12.01 g/kg of a NEAA mixture and a 53 % eN-to-tN ratio; T4, 190 g/kg CP +25.16 g/kg of an NEAA mixture and a 50 % eN-to-tN ratio; T5, 190 g/kg CP +39.99 g/kg of an NEAA mixture and a 47 % eN-to-tN ratio; and T6, 190 g/kg CP +56.83 g/kg of an NEAA mixture and a 44 % eN-to-tN ratio. The NEAA mixture consisted of alanine, glycine and glutamic acid (20:20:60). Performance was measured at 21 day. From 14 day to 21 day, excreta were collected to calculate nitrogen utilization. At 21 day, blood was collected from the wing vein to quantify uric acid, total protein, albumin and globulin and to evaluate nitrogen utilization. The data were analyzed using a Dunnett test, and the growth performance and blood parameters of the five progressive treatment levels were assessed using the linear response plateau (LRP) and quadratic broken line (QBL) models to estimate requirements. Birds fed diets with a 56 % eN-to-tN ratio exhibited a lower final body weight (FBW) and feed conversion ratio (FCR) than birds fed the control diet, whereas birds fed a diet with a 53 % eN-to-tN ratio only had a lower FCR. The LRP for FBW, body weight gain (BWG), feed intake (FI) and FCR showed an Xplat very close to a 50 % eN-to-tN ratio. Despite their good performance, animals fed diets with 47 % and 44 % eN-to-tN ratios were less efficient in terms of nitrogen utilization than birds fed a diet with a 50 % eN-to-tN. Thus, to maintain broiler performance and efficient nitrogen metabolism, low CP diets should possess an eN-to-tN ratio of 50 % or less.