Effects of feeding reduced crude protein diets while maintaining essential amino acids and total glycine + serine concentrations on growth performance, nitrogen excretion, plasma uric acid concentrations, and carcass characteristics of broilers

Abstract

An excessive reduction of crude protein (CP) content in broiler diets has been resulting in poor growth performance. This may be due to sub-optimum concentrations of Val, Ile, Arg, and Trp. Additionally, total Gly + Ser concentration may be below adequacy when broilers were fed reduced CP diets formulated with ingredients of vegetable origin. Glycine supplementation has been reported to ameliorate poor performance of broilers fed reduced CP diets during the starter period. An experiment consisting of 2 trials was conducted to determine the effects of feeding broilers reduced CP diets while maintaining adequate essential amino acids (AA) and total Gly + Ser concentrations on growth performance from 1 to 18 (trial 1) and 1 to 21 (trial 2) d of age. In this experiment, AA (trial 1: DL-Met, L-Lys, L-Thr, L-Val, Gly, L-Ile, L-Arg, L-Trp, L-His, L-Phe, and L-Leu and trial 2: DL-Met, L-Lys, L-Thr, L-Val, Gly, L-Ile, L-Arg, L-Trp, L-His, and L-Phe) were sequentially added in the order of limitation to meet their requirements while simultaneously lowering dietary CP content. Total Gly + Ser to digestible Lys ratios and digestible Lys concentrations were formulated at 1.90 and 1.20 and 1.70 and 1.25% in trials 1 and 2, respectively. In trial 1, body weight gain of broilers was maintained when dietary CP was reduced by 4.9 percentage points, but feed conversion increased as dietary CP content was reduced beyond 1.8 percentage points. In trial 2, dietary CP was reduced by 3.9 and 2.2 percentage points from 1 to 14 and 1 to 21 d of age, respectively, without compromising body weight gain and feed conversion of broilers. These data indicated that optimal growth performance of broilers can be obtained without placing a minimum of CP concentration when proper AA ratios are implemented in diet formulation. The lack of Gly research on broilers subsequent to the starter period led to a second experiment evaluating the effects of Gly and L-Gln supplementation to reduced CP diets on growth performance and carcass characteristics of broilers during a 6 wk production period. Glycine and L-Gln (a source of nitrogen) were added to reduced CP diets to increase total Gly + Ser and CP concentrations, respectively at 33 or 66% of the difference between their concentrations in the positive and negative control diets. Glycine had a more pronounced impact than nitrogen contribution on cumulative feed conversion of broilers only when added at 33% of total Gly + Ser concentration difference between the positive and negative control diets to reduced CP diets. Glycine and L-Gln supplementation in reduced CP diets increased total breast meat weight and yield of broilers while reducing abdominal fat weight and percentage compared with birds fed 2.5 percentage points lower CP diets. Hence, the supplementation of Gly and L-Gln provided enhancements in growth performance and carcass characteristics of broilers during a 41 d production period.