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Two trials were carried out to assess the effects of different ratios of standardized ileal digestible isoleucine:lysine (SID Ile:Lys) on the growth performance of broiler chickens fed low-protein diets. A total of 1,320 male chickens were distributed in each trial into 6 treatments, with 10 replicates with 22 birds each. A control diet was formulated that satisfied the nutritional requirements of the broilers, and a low-protein diet was formulated with reduced protein content, meeting broiler nutritional requirements, except for the SID Ile levels. Five SID Ile:Lys ratios (56%, 61%, 66%, 71%, and 76%) were obtained by adding l -isoleucine to the low-protein diet. The body weight (BW), body weight gain (BWG), average daily feed intake (ADFI), and feed conversion ratio (FCR) were evaluated from day 1 to day 21 in trial 1, and from day 22 to day 44 in trial 2. ANOVA was performed on the data, and the treatments were compared to the control group using Dunnett's test (P ≤ 0.05). Regression analyses were performed for modeling the variables assessed and the ratios of SID Ile:Lys. There was no significant difference between the treatments on ADFI of birds (P > 0.05). The BW, BWG, and FCR showed a quadratic effect as the SID Ile:Lys ratio increased in low-protein diets in trials 1 and 2 (P ≤ 0.05). In conclusion, the recommended ratio of SID Ile:Lys in low-protein diets for growth performance is around 66% for broiler chickens from 1 to 21 d old and is around 65% for broiler chickens from 22 to 44 d old. [ABSTRACT FROM AUTHOR]