Response of broilers to gradual dietary protein reduction with or without an adequate glycine plus serine level

Abstract

This study was aimed to probe broiler responses to the maintenance of adequate level of glycine plus serine in low crude protein (CP) diets by glycine (Gly) addition from 8 to 42 days of age. A total of 1152 7-d-old birds were divided into 8 groups and fed with one of the following diets: (I) normal CP diet; (II, III vIV V) diets with 1.5%, 3%, 4.5% and 6% points less CP than normal diet, respectively; (VI, VII, VIII) diets with 3%, 4.5% and 6% points reduced CP than the normal CP diet, respectively, and added with Gly to achieve 2.32% total Gly + Ser. Each treatment was replicated with 12 cages. Results showed that dietary CP reduction by 3% or higher compromised (p < .05) growth performance and carcase traits of broilers. Gly fortification in the diet with a 4.5% CP reduction produced similar accumulative growth performance relative to normal-CP group. When Gly fortification in the diet with a 6% CP reduction, the accumulative growth performance were improved (p < .05) but were still inferior (p < .05) to those in normal-CP group. Gly fortification supported a similar (p > .05) abdominal fat percentage of birds fed the diet containing 4.5% points less CP as compared with birds fed the normal-CP diet. Comparatively, Gly fortification in the diet with a 6% CP reduction resulted in breast muscle yield, serum UN content on d 21 and albumin content on d 42 comparable to (p > .05) those in normal-CP group. Collectively, the absence of Gly addition allowed dietary CP reduction by no more than 3% to obtain normal performance and carcase traits of broilers. Gly fortification in the diet with a 3% CP reduction induced no improvements in growth performance and carcase traits of broilers. However, maintain a level of 2.32% total Gly + Ser via Gly fortification enabled a reduction of dietary CP by 4.5% without deteriorating the accumulative growth performance and carcase traits of broilers. Highlights Gly addition that provides 2.32% total Gly + Ser allows dietary CP reduction of 4.5% without compromising the accumulative growth performance and carcase traits of broilers. Maintaining 2.32% total Gly + Ser via Gly fortification could not completely overcome the depression of growth performance in broilers fed the diet with a 6% CP reduction.