The objective of this study was to determine the physiological responses and growth performance of turkeys fed 20% whole wheat (WW) in two feeding programmes, diets A and B, consistent with the NRC (1994) and B.U.T. (2012) recommendations, respectively. In diet B, in successive feeding periods (weeks 5–18), the soyabean meal content was increased by 2.89% to 5.13%, the wheat content was decreased by 5.29% to 8.18%, and the concentrations of L-lysine, DL-methionine and L-threonine were considerably increased. A total of 896 four-week-old Big-6 male turkeys with similar initial body weights were randomly assigned to four equal dietary treatment groups, with seven replicate pens per treatment. The birds had free access to water and pelleted diets that contained no WW or 20% WW. After 14 weeks of feeding, the body weight gains (BWG) of turkeys were similar in all dietary treatments. From week 13 to 18, WW contributed to a significant (P = 0.040) increase in the feed conversion ratio (FCR), whereas the increase in FCR noted over the entire experiment was nearly significant (P = 0.065). Dietary inclusion of WW had no effect on the weights of the gizzard, small intestine, or caecum, and it enhanced fermentation in the caecum, including increasing α-glucosidase activity and the concentrations of total short-chain fatty acids, and decreasing the pH of caecal digesta (all P = 0.001). No significant differences were noted in the parameters of gastrointestinal function and BWG between turkeys fed diets A and B. Diet B did, however, significantly (P = 0.045) improve FCR over the entire experiment.