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A study was conducted to evaluate the effects of dietary α-amylase and β-xylanase, in single or combined supplementation, on growth performance of broiler chickens fed maize-soy diets. A total of 1800 slow feathering, Cobb × Cobb 500 male broilers were randomly distributed to 8 treatments with 9 replicates of 25 birds each. Broilers were fed starter and finisher diets respectively from 1 to 21 and 22 to 40 d Broilers were fed starter and finisher diets respectively from 1 to 21 and 22 to 40 d. A negative control (NC) diet was formulated to contain 12.35 MJ/kg and 12.85 MJ/kg in the starter and finisher phases, respectively. A further four control diets were formulated with orthogonal energy density increases in increments of 0.21 MJ/kg to a maximum of 13.17 MJ/kg (starter) and 13.67 MJ/kg (finisher). All control diets were formulated to be equivalent in amino acids and minerals. The NC diet was supplemented with α-amylase (80 kilo-Novo α-amylase units/kg), β-xylanase (100 fungal β-xylanase units/kg), or both enzymes combined at the same supplementation levels. Increasing AME n resulted in a linear increase (P < 0.01) in BWG and a linear decrease (P < 0.01) in FCR from 1 to 40 d. Corresponding AME n estimates for BWG and FCR from 1 to 40 d were, respectively 0.41, 0.35, and 0.57 and 0.18, 0.11, and 0.21 MJ/kg for amylase, β-xylanase and amylase + β-xylanase. In conclusion, supplementing maize-soy diets with α-amylase and β-xylanase led to increased estimated energy yields. A marked difference occurred in favor of α-amylase when compared to β-xylanase supplementation whereas adding both enzymes in the same feed generated similar AME n to the single addition of α-amylase. Growth performance responses corresponded to findings in the AME n assay. [ABSTRACT FROM AUTHOR]