Comparison and Estimation of Digestible Threonine Requirements of Broiler Chickens of Ross 308 and Cobb 500

Two experiments were conducted to determine threonine (Thr) requirements of two commercial strains, Ross 308 and Cobb 500, based on standardized ileal digestibility from 15 to 28d of age. Basal diet consisted of wheat, triticale, and corn gluten meal was formulated according to Quick Chick (2006) to be adequate in all nutrients except for Thr. Incremental levels of supplemental Thr were added to the basal diet generating six experimental treatments in a range from 0.46 to 0.81% digestible Thr. Birds were randomized across 48 floor pens (replicates and 12 birds in each replicate) in a completely randomized design and each pen was fed one of the six amino acid levels from 15 to 28d of age. Body weight gain, feed intake, feed conversion ratio, and digestible Thr intake were measured during the experimental period. Performance traits responded quadratically to graded levels of digestible Thr, and interaction effects between Thr and strain were significant for all performance traits except for body weight gain. Strain did not affect the feed conversion ratio and feed efficiency was the same in two strains. Digestible Thr requirements were estimated using broken-line linear and broken-line quadratic models. In Ross 308, digestible Thr requirements for body weight gain and feed conversion ratio were estimated at 0.69 and 0.76% of diet, respectively, by broken-line linear model. Digestible Thr need for body weight gain was optimized at 0.79% of diet using broken-line quadratic model. In Cobb 500, Thr requirements for body weight gain and feed conversion ratio were estimated at 0.69 and 0.71% of diet, respectively, by broken-line linear model. Digestible Thr need for body weight gain and feed conversion ratio were optimized at 0.7 and 0.81% of diet using broken-line quadratic model. Regarding response criterion and statistical model to estimate the requirements, no differences were observed in Thr requirements of two strains during the grower period.