The aim of the present study was to evaluate the effects of dietary total sulphur amino acids (TSAA) levels and guanidinoacetic acid (GAA) supplementation on performance, carcass yield and energetic molecular metabolites in heart and breast muscle of broiler chickens fed wheat-soy diet. A total of 450, male one-day-old Ross-308 broiler chicks were assigned in a 3×3 factorial arrangement of nine treatments, five replicates and 10 birds each. Dietary treatments included three graded levels of supplementary TSAA (+0.4 g/kg per level) resulting in a deficient, adequate and plus level of recommended by strain, with three levels of GAA (0, 0.6 and 1.2 g/kg) and were fed for 35 days. Dietary TSAA levels only enhanced performance during the starter period (P<0.05). At adequate TSAA level, weight gain (WG) and feed conversion ratio (FCR) in the grower phase were positively affected by supplementing GAA at 1.2 g/kg (P<0.05). The FCR during the overall experimental period was significantly lower when feeding GAA at 0.6 and 1.2 g/kg compared to non-supplemented (P<0.05). Heart and breast muscle ATP/AMP ratio and breast muscle ADP and PCr concentration and PCr/CrN ratio in the birds fed 1.2 g/kg GAA diet were significantly higher than non-supplemented (P<0.05). It is concluded, effects of dietary GAA supplementation were influenced by dietary TSAA level, this indicates to need for proper dietary sulphur amino acid formulation by GAA supplementation. Dietary GAA supplementation could improve broiler chicken’s growth performance via regulating some aspects of energy metabolism.

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