This study was performed to investigate the effects of dietary threonine (Thr). level on performance, metabolizable energy, intestinal morphology, and immune system in coccidian–infected broiler chickens. The diets contained: 88%, 100% (Non challenged (NC) and challenged control (PC)), 112%, 124%, and 136% of Thr requirement according to Cobb 500 recommendation and fed during grower (pre challenged) and finisher (post challenged) periods. On d 23 (end of grower period), each bird received 0.5 mL of distilled water or received around 24000 sporulated oocysts. On d 23 and 31, one bird per replicate was slaughtered to measure the performance criteria. Mean dietary apparent metabolizable energy corrected for nitrogen (AMEn) and digestible energy were greater in NC birds than the challenged birds fed on 88% or 100% Thr diets. Feed intake and blood parameters were not significantly influenced by increasing levels of Thr in the diet. Compared with unchallenged birds with coccidia (NC), the growth performance, morphological parameters (not crypt depth), and immune responses decreased (P< 0.05) in the birds (PC) that were challenged with coccidia, and oocyte numbers were enhanced. Growth performance, jejunal morphology, and immune responses improved and oocyte count decreased in coccidian- challenged birds fed on the diets with greater levels of Thr (P < 0.05). Feeding the challenged birds with the diet containing greater levels of Thr improved (P < 0.05) their growth performance, morphology, and immune responses and decreased oocyte number. The birds fed on the diet with 124% Thr demonstrated a similar response as the NC birds. Increased diet Thr level linearly increased average daily gain and decreased feed conversion ratio in the grower and the whole experimentation periods. The AMEn and digestibility of crude protein were enhanced linearly Thr level increased in coccidian-challenged birds. It is concluded that diets containing 124% of Thr recommendation led to the greatest efficacy on the intestinal immune response and normal growth maintenance of the birds contaminated with coccidia.

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