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Context: Leucine (Leu) is one of the branched-chain amino acids that is necessary for muscle protein synthesis. Basically, poultry feed ingredients contain abundant amounts of Leu. But little is known about dietary Leu deficiency in performance and health-attributing parameters of female broiler chickens, which may occur in crude protein-restricted diets. Aims: This study evaluates the effect of Leu deficiency in diets on the growth performance, body composition, immunity, and caecum microbiota of female broilers. Methods: Female broiler chicks (Cobb 500; 8 days old; n = 540) were randomly divided into six groups with six replicates (15 birds in each replicate) in a completely randomised design. The experimental diets included six concentrations of Leu (80%, 84%, 88%, 92%, 96%, and 100%) fed for 14 days. A basal Leu-deficient diet was formulated at the 80% recommended level, and the experimental diets were created by gradual replacing of crystalline L-glutamic acid with crystalline L-Leu. Average daily intake (ADI), average daily gain (ADG), feed efficiency (gain:feed), body composition, cutaneous basophil hypersensitivity (CBH) test to phytohaemagglutinin-P (PHA-P), antibody response to sheep red blood cell (SRBC) and caecum microbiota were investigated. Key results: The birds receiving the lowest Leu application level had the lowest ADI (P < 0.05) and a 20% higher fat content (P < 0.01) than did the birds fed 96% Leu. The population of Escherichia coli and coliform bacteria count was increased (P < 0.001; by 36% and 10.3% compared with the birds fed 100% Leu in the diet respectively), and Lactobacillus (but not Enterococcus) count was decreased (P < 0.001; by 26.4% compared with the birds fed 100% Leu in the diet) with decreasing concentrations of dietary Leu. Average daily gain, feed efficiency, carcass dry matter, protein and ash percentages, and immunity were not affected markedly by dietary treatments with different Leu concentrations. Conclusions: The Leu deficiency influenced the broiler performance, body composition, and caecum microbiota, but not the immune response. Implications: This study points to the importance of Leu supplementation in the diet of broilers to improve their growth performance, fat content, and intestine health. Poultry diets contain abundant amounts of leucine, a necessary feed ingredient for higher meat production. Leucine is one of the essential amino acids, whose deficiency impairs the broilers' performance, body composition, and caecum microbiota. This study points to the importance of leucine supplementation in the diet of broilers to improve their growth performance, fat content, and intestine health. [ABSTRACT FROM AUTHOR]