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

Abstract

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.

Keywords: Amino acid balance; Broiler; Central composite design; Low-protein diet; Standardized ileal digestibility.

Fig. 1

Response surface plots illustrating the interactive effects of SID AA on BWG in broilers (22-42 d). (A) Response surface plot of dMet and dLys for BWG. DThr was held at the midpoint (P = 0.052). (B) Response surface plot of dThr and dLys for BWG, with dMet held at the midpoint (P = 0.085). (C) Response surface plot of dThr and dMet for BWG, with dLys held at the midpoint (P = 0.021).

Fig. 2

Response surface plots illustrating the interactive effects of SID AA on FCR in broilers (22-42 d). (A) Response surface plot of dMet and dLys for FCR. dThr was held at the midpoint (P = 0.001). (B) Response surface plot of dThr and dLys for FCR, with dMet held at the midpoint (P = 0.868). (C) Response surface plot of dThr and dMet for FCR, with dLys held at the midpoint (P = 0.357).