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

Two concurrent trials were conducted to investigate the influence of low-pr otein amino acid-supplemented diets on the performance, carcass characteris tics, whole-body composition and efficiencies of nutrient utilisation by th e male broiler chicken from age 3 to 6 weeks. The first trial comprised fiv e isoenergetic (13.0 MJ kg(-1)) diets containing 225 (control), 210, 190, 1 72 or 153 g kg(-1) crude protein (CP) supplemented with essential amino aci ds (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) central. Neither the loweri ng of dietary CP nor NEAA supplementation had any significant influence on weight gain or the relative weights of the various carcass cuts. However, c hicks fed the lowest-CP diets consumed more feed (P less than or equal to 0 .05) and had poorer (P less than or equal to 0.05) feed conversion efficien cy (FCE). NEAA supplementation enhanced FCE to the control levels. Whole-bo dy compositional analysis showed that lowering dietary CP increased (P less than or equal to 0.01) total body fat in a linear fashion (P less than or equal to 0.001; r = -0.72). Equalising dietary GP with the control (ie main taining identical energy/protein ratio) by NEAA supplementation did not cor rect for the fat deposition. Total body protein (g kg(-1)) was identical wi th the control with or without NEAA supplementation. Dietary energy, protei n retention efficiency (PRE) and protein efficiency Patio (FER) were more e fficient (P less than or equal to 0.01) in the lower-protein diets, while N EAA supplementation significantly (P less than or equal to 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 less tha n or equal to 0.001; r = 0.73). The nutritional and environmental implicati ons 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.