Two independent experiments were conducted with male Cobb × Cobb 500 broilers to determine the optimal
valine-to-digestible-
lysine ratio for broiler development. We conducted a randomized block experiment with 7 treatments, each with 8 replicates of 25 starter birds (8 to 21 d of age) and 20 finisher (30 to 43 d of age) birds. To prevent any excess of digestible
lysine, 93% of the recommended level of digestible
lysine was used to evaluate the
valine-to-
lysine ratio. The utilized levels of dietary digestible
lysine were 10.7 and 9.40 g/kg for the starting and growing phases, respectively. A control diet with 100% of the recommended level of
lysine and an adequate
valine-to-
lysine ratio was also used. The feed intake,
weight gain, feed conversion ratio, and carcass parameters were evaluated. The treatments had no significant effect on the feed intakes or carcass parameters in the starter and finisher phases. However, during both of the studied phases, we observed a quadratic effect on
weight gain and the feed conversion ratio. The broilers of both phases that were fed test diets with the lower
valine-to-
lysine (
Val/Lys) ratio had poorer performance compared with those broilers fed control diets. However, when higher
Val/Lys ratios were used for the starting and growing broilers that were fed test diets, the 2 groups had similar performance. During the starting phase, in broilers that were fed a higher
Val/Lys ratio,
weight gain, and the feed conversion ratio improved by 5.5% compared with broilers fed the basal diets. The broilers in the growing phase also had improved performance (by 7 to 8%) when the test diets had higher
Val/Lys ratios. Based on the analysis of the starter phase data, we concluded that the optimal digestible
Val/Lys ratio for Cobb × Cobb 500 broilers is 77%, whereas for birds in the finisher phase (30 to 43 d of age), a digestible
Val/Lys ratio of 76% is suggested.