The experiment consisted of a 2 × 2 × 2 factorial design testing the two D+ and D− chicken lines selected for divergent digestion efficiency (fifth selection generation), xylanase (with or without) and ampicillin and collistin (with or without) supplementations. From 8 to 22 days, 144 chickens (18 birds per treatment) were fed a diet containing 55% wheat from a high-viscosity cultivar (Rialto). Effects of treatments were evaluated on individual growth performance (8 to 19 days), digestibilities of lipids and dry matter, dietary energy value (apparent metabolisable energy corrected to zero-nitrogen retention (AMEn)), digestive organ and breast sizes, and intestinal bile acids at 3 weeks of age. Individual variabilities were much lower in D+ than in D− birds for feed : gain ratios, digestibilities and AMEn values. In all cases, feed : gain ratios were lower in the D+ than in the D− line (P < 0.001), and D+ birds showed 22% to 86% higher values than in D− birds (P < 0.001) for digestibilities and AMEn. In D− birds, antibiotics but not xylanase supplementation had significant effects on lipid digestibility (P < 0.01) and AMEn (P < 0.05), whereas both supplements improved these parameters in D+ birds (P < 0.001 for both additives on lipids digestibility, P < 0.05 for xylanase and P < 0.01 for antibiotics on AMEn). Relative weights of gizzard and proventriculus, and gizzard : intestine weight ratio were higher in D+ than in D− birds, while relative weight of intestine was increased in D− birds compared with D+ birds. Antibiotics reduced intestine relative weight in D+ (P < 0.001) and D− (P < 0.01) lines. AMEn variations were efficiently predicted by the gizzard : intestine weight ratio. In conclusion, antibiotics were very efficient for improving growth performance, AMEn and digestibility values in both chicken lines. Xylanase was less efficient than antibiotics. Because of their low individual variabilities, D+ birds were much more efficient than D− ones for the detection of significant effects induced by xylanase supplementation. Differences between lines in feed : gain ratio, digestibilities and AMEn were reduced when xylanase and antibiotics were added together. Effects of xylanase supplementation and animal genetics on lipid digestibility could not be entirely explained in terms of intestinal bile acids. Other factors should be involved, especially for the lipid digestibility difference induced by animal genetics. The gizzard : intestine weight ratio was an efficient parameter for predicting AMEn variations due to animal genetics and additives.