Two bioassays were conducted to determine the ideal ratio of threonine (Thr) relative to lysine (Lys) in straight-run Cobb 500 broiler chickens from 15 to 28 d of age. Broiler chickens were fed wheat-corn gluten meal-based diet fortified with synthetic amino acids. A Lys-deficient basal diet was formulated to contain 0.60% standardised ileal digestible Lys (SID Lys) and subsequent treatments had 0.12% Lys added incrementally until a level of 1.2% SID Lys was reached for a total of six treatments. A Thr-deficient basal diet was formulated to contain 0.46% of standardised ileal digestible Thr (SID Thr) and subsequent treatments had 0.07% Thr added incrementally until a level of 0.81% SID Thr was reached for a total of six treatments. Both male and female birds were randomised across 48 floor pens (4 replicate pens, and 12 birds per pen) and fed one of the six levels of each of the test amino acid from 15 to 28 d of age. Body weight gain (BWG) and feed conversion ratio (FCR) responded quadratically (P < 0.0001) to graded levels of Thr (0.46–0.81%); however, these responses were linear (P < 0.0001) to different levels of Lys (0.60–1.20%). Using quadratic broken-line model, Lys requirements were estimated at 0.99 and 1.14% of diet for BWG and FCR, respectively. Lys requirements for BWG and FCR were estimated at 0.89 and 0.98% of diet with linear broken-line model. Thr requirement for BWG and FCR was estimated at 0.79 and 0.81% of diet with quadratic broken-line model, and 0.69 and 0.71% of diet with the linear broken-line model, respectively. Considering FCR as a response, the ideal ratios of SID Thr:SID Lys were 72 and 71% using the linear broken-line and quadratic broken line models, respectively.

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