Low-protein amino acid-supplemented diets in broiler chickens: Effects on performance, carcass characteristics, whole-body composition and efficiencies of nutrient utilisation

Abstract

Two concurrent trials were conducted to investigate the influence of low-protein amino acid-supplemented diets on the performance, carcass characteristics, whole-body composition and efficiencies of nutrient utilisation by the male broiler chicken from age 3 to 6 weeks. The first trial comprised five isoenergetic (13.0 MJ kg-1) diets containing 225 (control), 210, 190, 172 or 153 g kg-1 crude protein (CP) supplemented with essential amino acids (EAAs) to meet the minimum National Research Council recommendations. In the second trial a composite mixture of non-essential amino acids (NEAAs) was added to the lower-CP diets (ie 210-153 g kg-1) such that they became isoproteinous (N x 6.25) with the 225 g kg-1 control. Neither the lowering of dietary CP nor NEAA supplementation had any significant influence on weight gain or the relative weights of the various carcass cuts. However, chicks fed the lowest-CP diets consumed more feed (P ≤ 0.05) and had poorer (P ≤ 0.05) feed conversion efficiency (FCE). NEAA supplementation enhanced FCE to the control levels. Whole-body compositional analysis showed that lowering dietary CP increased (P ≤ 0.01) total body fat in a linear fashion (P ≤ 0.001; r = -0.72). Equalising dietary CP with the control (ie maintaining identical energy/protein ratio) by NEAA supplementation did not correct for the fat deposition. Total body protein (gkg-1) was identical with the control with or without NEAA supplementation. Dietary energy, protein retention efficiency (PRE) and protein efficiency ratio (PER) were more efficient (P ≤ 0.01) in the lower-protein diets, while NEAA supplementation significantly (P ≤ 0.01) decreased the efficiency of N utilisation. Reducing dietary CP from 225 to 153 g kg-1 decreased N excretion in a highly significant linear fashion (P ≤ 0.001; r = 0.73). The nutritional and environmental implications of the increased body fat deposition on the one hand and the decreased N excretion on the other in the low-protein-fed chickens are discussed and the need to harmonise these apparently conflicting interests is emphasised. (C) 2000 Society of Chemical Industry.