L-carnitine as well as lysine and methionine are amino acids of important nutritional and nutraceutical interest and are used in nutritional strategies as dietary supplements to improve feed quality characteristics in animals and broiler chicken in particular. This study investigated the effect of different levels of L-carnitine and extra levels of lysine-methionine on growth performance, carcass characteristics, and some immune system markers. Two hundred seventy male Ross 308 broilers were a fed control diet (C) and eight different diets supplemented with an excess of amino acids. In the experimental diets, identified as D1, D2, D3, D4, D5, D6, D7, and D8, extra L-carnitine, lysine, and methionine were added in excess with respect to the American National Research Council (NRC) recommendations: L-carnitine equal to NRC (D1), control diet supplemented with lysine at 30% in excess of NRC, methionine at 30% in excess of NRC, and L-carnitine equal to NRC (D2), control diet supplemented with lysine equal to NRC, methionine equal to NRC, and L-carnitine at 15% in excess of NRC (D3), control diet supplemented control diet supplemented with lysine at 15% in excess of NRC, methionine at 15% in excess of NRC, and L-carnitine at 15% in excess of NRC (D4), control diet supplemented lysine at 30% in excess of NRC, methionine at 30% in excess of NRC, and L-carnitine at 15% in excess of NRC (D5), control diet supplemented with lysine equal to NRC recommendations, methionine equal to NRC recommendations, and L-carnitine at 75% in excess of NRC (D6), control diet supplemented with lysine at 15% in excess of NRC, methionine at 15% in excess of NRC, and L-carnitine at 75% in excess of NRC (D7); and control diet supplemented with lysine at 30% in excess of NRC, methionine at 30% in excess of NRC, and L-carnitine at 75% in excess of NRC (D8). During the starter and growth phases, feed intake was not affected by dietary treatment (p > 0.05). By contrast, body weight and FCR were both affected (p < 0.01) during the starter period. During the finisher phase, feed consumption was affected (p < 0.05) by dietary treatment. Feed intake of broilers fed on C, D3, D6, and D7 were statistically similar (p > 0.05) (1851.90, 1862.00, 1945.10, and 1872.80 g/pen/day, respectively) and were higher (p < 0.05) than 1564.40 g/pen/day (D5). With the exception of drumsticks, neck, back thoracic vertebrae, and proventriculus weights, the economical carcass segments were not affected (p > 0.05) by the dietary supplementation of amino acids. Duodenum and ileum weights and lengths decreased with amino acid supplementation (p < 0.05). IgT and IgG titers against Sheep Red Blood Cells (SRBC) for both primary and secondary responses were not affected by dietary treatments (p > 0.05). Dietary amino acids supplementation did not affect IgM titer after the secondary challenge (p > 0.05) and had a significant effect (p < 0.05) on serum antibody titers in broilers vaccinated against Newcastle disease (NCD) and Gumboro 's disease at the 27th and 30th days, respectively.
Keywords: amino acids; broiler; dietary supplementation; growth performance; humoral immunity.