Precision optimization of standardized ileal digestible lysine:methionine: Threonine ratios in low-protein male broiler diets using central composite design

This study aimed to determine the optimal standardized ileal digestible (SID) ratios of lysine (dLys), methionine (dMet), and threonine (dThr) in low-protein diets (17 % crude protein, CP) to optimize growth performance, feed efficiency, and economic viability in Arbor Acres Plus male broilers aged 22-42 days. Using a 3-factor, 5-level central composite design (CCD) within a response surface methodology (RSM) framework, 960 male broilers were randomly divided into 16 treatments, each comprising 6 replicates with 10 birds per replicate. The treatments included a control group (Con) fed a 19 % CP diet, and 15 dietary treatments with graded levels of dLys (0.97-1.23 %), dMet (0.43-0.53 %), and dThr (0.64-0.84 %). Results indicated that dLys had the strongest linear effect on body weight gain (BWG; P < 0.001). The optimal SID ratios were determined as dMet/dLys = 0.41 (0.468 %/1.145 %) and dThr/dLys = 0.64 (0.730 %/1.145 %) for BWG, and dMet/dLys = 0.38 (0.454 %/1.180 %) and dThr/dLys = 0.64 (0.757 %/1.180 %) for FCR. These optimizations enabled a 10.5 % reduction in dietary CP level without compromising growth performance, along with a 32.3 % decrease in nitrogen (N) excretion compared with the Con group. Furthermore, apparent total tract digestibility (ATTD) of amino acids (AA) improved significantly compared with the Con group (P < 0.001). In summary, the findings suggest that precise AA balancing, with a priority on lysine (Lys) sufficiency, could sustain broiler productivity in low-protein systems while reducing feed costs and environmental footprint.