The objective of this study was to determine the effects of 8 levels of threonine (0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 and 1.1%) and 2 levels of protein (17.5 and 20.5%) on growth performance, gut sizes and morphology as well as to estimate Standardized Ileal Digestible (SID) Thr requirements in Ross 308 males at 0 to 21 days of age. Chicks were randomized into 64 battery cages (5 chicks per replicate). FI was lower for broiler given the high CP diets as compared to those fed on low CP diets. BWG

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... and FCR improved up to 0.7% Thr in both CP levels. Fitted broken lines indicated break points at 0.62 and 0.66% SID Thr for weight gain at 17.5 and 20.5% crude protein, respectively. Significant interaction was found between CP and Thr on relative weight and length of duodenum and jejunum (P< 0.05). Thr supplementation significantly affected villus height, epithelial thickness, goblet cell number and crypt depth in duodenum, jejunum and ileum (P< 0.01). Low CP diets adequate in Lys, Total Sulfur Amino Acid (TSAA) supplemented with Thr may result in optimal BWG and FCR as well as in growth of intestinal length. Such parameters of gut functionality as microvilli height, crypt depth and epithelial thickness seemed to be improved with even higher levels of dietary SID Thr levels.