Poultry meat and eggs are important foods for fulfilling the dietary needs of the ever-growing human population. Efficient poultry production, however, necessitates use of pharmaceutical products, such as antibiotics, as prophylactic and curatives to ensure rapid growth and health. Nevertheless, inappropriate and non-judicious use of these drugs results in an accumulation of toxic and harmful residues in meat and eggs of treated birds which affect consumer health by triggering allergic reactions and transmitting antibiotic-resistant microbial infections. Therefore, regulatory authorities must take rigorous steps to curtail inappropriate use of numerous drugs for animal use in order to provide safe animal origin food to humans.

Background This study was conducted to investigate the optimum dietary level of tryptophan (Trp) supplementation at which broiler chickens have better growth with efficient immune system and anti‐oxidant status. Method One hundred and twenty (n = 120) 1‐day‐old broiler chicks were fed a common commercial diet from days 1 to 7. On day 7, the chicks were randomly divided in three treatment groups, that is, Trp 0.2 [national research council (NRC) recommended level of tryptophan], Trp 0.3 (tryptophan supplemented at 0.3%) and Trp 0.5 (tryptophan supplemented at 0.5%). All the experimental diets were iso‐caloric (ME; 3,000 kcal/kg) and iso‐proteic (CP; 18.5%). Weekly data on feed intake and body weight gain (BWG) were recorded to calculate feed conversion ratio (FCR). On day 19, avian tuberculin was injected to note the cellular immunity. On day 21, two birds per replicate were killed to determine carcass and visceral organ weights. Blood serum samples were collected for analysis of humoral immune response against sheep red blood cells, total oxidant and anti‐oxidants by spectrophotometric method. Results Feed intake, carcass and visceral organ weights remained unaffected by dietary treatments while BWG and FCR tended to improve (p < .05) in broiler chicks fed the Trp 0.3 and the Trp 0.5 diets. Total oxidant status was also improved (p < .05) in broiler chicks fed the Trp 0.5 diet. Likewise, broiler chicks fed the Trp 0.3 and the Trp 0.5 diets tended to have better (p < .05) total anti‐oxidant status, catalase, glutathione peroxidase, glutathione reductase and arylesterase (ARE). The overall antibodies response and IgG improved (p < .05) by the Trp 0.3 and Trp 0.5 diets compared to control. However, IgM level remained similar across the treatment. The cellular immunity against avian tuberculin improved at 24 hr post‐injection but its effect disappeared at 48 hr. Conclusion The results of present study revealed that Trp above the NRC recommended level may give better growth, immune response and anti‐oxidant status in broiler chickens.

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