Background: The vegetable-based diet alone does not provide the lysine needed to maximize poultry productive performance. Objectives: This experiment aimed to evaluate the effects of dietary digestible lysine (dLys) level on productive and reproductive performance, egg quality, blood metabolites, and immune responses in breeding Japanese quails (Coturnix coturnix japonica). Methods: The experiment was conducted in a completely randomized design with six treatments, five replicates, and 15 (12 females and three meals) 10-week-old breeding Japanese quails each. A basal diet was formulated to meet nutritional requirements of breeding quails except dLys. The basal diet was supplemented with graded (+0.82 g/kg) levels of L-lysine-HCl, corresponding to dietary dLys levels of 0.690%, 0.755%, 0.820%, 0.885%, 0.950% and 1.015%. The experiment lasted for 12 weeks, which was divided into three 4-week periods. Results: Significant differences were observed for egg production (EP), egg mass (EM), and feed efficiency (FE) in response to increasing dietary dLys concentration with quadratic trends. The highest traits were observed in the birds fed with a diet containing 0.885% dLys. However, feed intake, egg quality, reproductive performance, blood metabolites, and immune responses against sheep red blood cell inoculation were not significantly affected by increasing dietary dLys concentrations. The dLys requirements during 11-14, 15-18, 19-22 and 11-22 (overall) weeks of age for optimal EP, EM and FE, based on the quadratic broken-line regression analysis, were estimated 272, 265, 250 and 266; 293, 285, 264 and 279; and 303, 294, 281 and 293 mg/bird per day, respectively. Conclusions: The dLys requirements vary depending on the egg production phase and the trait being optimized. The estimated dLys requirement for FE was higher than those for EP and EM. During the peak stage of the first laying cycle, the dietary dLys level of 0.932%, a daily intake of 303 mg dLys/bird is sufficient for optimal performance.

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