Two experiments were conducted to evaluate adaptation length (AL) and composition of reference diets on nitrogen (N)-corrected apparent metabolizable energy (AMEn) in 22-d-old broilers. Birds were allocated to nine treatments (n = 6) consisting of wheat – soybean meal (SBM) (reference diet), corn–wheat–SBM, and wheat middlings (WM)–wheat–SBM (exp. 1), or oats–SBM (reference diet), corn–oats–SBM, and WM–oats–SBM (exp. 2) in conjunction with three AL (12, 8, and 4 d) in a factorial arrangement of treatments (3 × 3). Dry matter (DM), N, energy (En) utilization, and AMEn of corn and WM were determined using the difference method. In exp. 1, birds on the WM–wheat–SBM-based diet had the lowest (P < 0.05) DM, N, and En utilization, as well as AMEn compared with the other two diets. Additionally, AMEn for corn was higher (P < 0.05) compared with that of WM. In exp. 2, N utilization in birds on the corn–oats–SBM-based diet was lower (P < 0.05) compared with birds on the oats–SBM-based diet; however, AMEn of corn and WM was not different. In both experiments, AL was not significantly different. Based on these results, the composition of the reference diet could influence AMEn values of corn and WM in 22-d-old broilers.