Response surface methodology (RSM) and 5-level-4-factor central composite design (CCD) were used to evaluate the response of broil-ers (BWG and FCR) to dietary digestible protein (dP), lysine (dLys), methionine+cysteine (dTSAA), and threonine (dThr). Eighty 4 cages of 5 birds each were assigned to feed diets contained 5 levels of dP (16, 17, 18, 19 and 20%), dLys (0.92, 0.98, 1.04, 1.10 and 1.16%), dTSAA (0.73 0.78, 0.83, 0.88 and 0.93%), and dThr (0.58, 0.63, 0.68, 0.73 and 0.78%) from 25 to 31 d of age. Diets were prepared using corn, soybean meal (45.2% CP), and corn gluten meal (48.6% CP). Protein and amino acid analyses were performed for these ingredients. All diets were formulated to have 3100 kcal/kg. The experimental results of CCD were fitted with the second-order polynomial equa-tion. A ridge analysis was utilized to compute the optimum response for maximizing BWG and minimizing FCR. The fitted second-order polynomial equations for BWG and FCR are produced as follow: BWG (g/bird/d)= 570.1 −46.7 dP +1.6 dP2 +48.2 dLys −121.4 dLys2 −96.7 dTSAA −160.9 dTSAA2 −321.5 dThr −289.4 dThr 2−8.2 dP*dLys −5.2 dP*dTSAA +4.3 dP*dThr +191.0 dLys*dTSAA +309.6 dLys*dThr +416.7 dTSAA*dThr; R 2= 0.57; Root MS error= 3.10. FCR= −4.26 +0.80 dP-0.04 dP2 −0.27 dLys +1.53 dLys2 +8.37 dTSAA −1.93 dTSAA2 −9.88 dThr +6.22 dThr2 +0.01 dP*dLys +0.18 dP*dTSAA +0.52 dP*dThr −2.92 dLys*dTSAA −0.84 dLys*dThr −8.84 dTSAA*dThr; R2= 0.66; Root MS error= 0.075. The ridge max analysis on BWG and ridge min analysis on FCR models revealed that the maximum BWG may be obtained with dP 19.9, dLys 1.05, dTSAA 0.85, and dThr 0.70% and minimum FCR may be obtained with dP 20.0, dLys 1.04, dTSAA 0.84, and dThr 0.67%. At the optimum point, the predicted BWG and FCR were 70.6 g/bird/d and 1.54, respectively. The combination of CCD and RSM techniques consider simultane-ously all investigating factors and their possible interactions. Thus, it appears that the platform may be used to describe relationship between dietary concentration of nutrients and broiler performance to achieve optimal target.

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