Three experiments were conducted in Vietnam to measure responses in forage intake and diet digestibility to concentrate feeding in growing cattle. The basal diet in all experiments was grass (elephant grass, Pennisetum purpureum, in experiments 1 and 2; native grass in experiment 3) fed at 1.25% liveweight (DM basis) during the day and rice straw fed ad libitum at night. The concentrate in experiment 1 was cassava powder plus urea. In experiments 2 and 3, a formulated concentrate comprised of maize, rice bran, fish meal, urea and salt was used. The concentrates were offered at ~0.3, 0.7, 1.3 and 2.0% liveweight in each experiment, and a zero concentrate treatment was also included. Effects of the amount of concentrate on substitution rate of concentrate for forage and on diet neutral detergent fibre (NDF) digestibility were measured to assess interactions between feeds. Substitution rate was high (0.5–0.7 kg DM reduction in forage intake per kg DM supplement consumed) and not affected by amount of supplement fed in experiment 1. In experiments 2 and 3, there was no substitution at the lowest intake of supplement, but it subsequently increased with amount of supplement consumed, to a high of 0.3–0.5 kg DM reduction in forage intake per kg DM supplement consumed. The cassava powder supplement markedly decreased dietary NDF digestibility (from 62 to 41%), whereas the formulated concentrate only reduced NDF digestibility by 11 and 8% units in experiments 2 and 3, respectively. It was calculated that depressions in NDF digestibility would reduce the estimated metabolisable energy content of the basal forage by 1–3.6 MJ/kg DM at the higher supplement intakes. The importance of these results in making decisions on tactical feeding systems to fatten cattle in central Vietnam is discussed.