This experiment was conducted to evaluate the effects of caffeine and trans-cinnamaldehyde (TC) on growth performance, hematological parameters, stress hormones, immune response, and some blood parameters of broiler chickens. Three hundred fifty 1-d-old broiler chicks were randomly assigned to 7 dietary treatments (five pens/ treatment of 10 male broilers each) in a completely randomized design. A basal control corn-soybean meal diet for starter, grower and finisher periods were formulated and supplemented with appropriate levels of caffeine (0.5, 1 and 2%) or TC (0.5, 1 and 1.5%). Findings showed that caffeine at level of 2% decreased feed intake (FI), body weight (BW), body weight gain (BWG) and increased feed conversion ratio (FCR). TC supplementation had no significant effect on growth performance compare to control diet. Except caffeine which significantly decreased hemoglobin and hematocrit at level of 2% in 21 day old chicks (P<0.05), caffeine and TC had no significant effects on hematological parameters and stress hormone of broiler chickens in 21 and 42 d (P>0.05). Similar trend were observed for humeral and cell mediated immunity. Supplementation of 2% of caffeine significantly decreased glucose level in 21 d (P<0.05) and this effect was not observed for TC levels compare with the control treatment. On the other hand all levels of caffeine and TC at level of 1% significantly decreased cholesterol levels in 21 d compared to control treatment (P<0.05). Caffeine and TC levels were not change the blood parameters in 42 d. Findings showed that supplementation of diets with caffeine specially at level of 2% decrease growth performance and neither caffeine nor TC affect the hematological, hormonal and immunity response in broiler chickens.

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