Abstract Context Cassava bagasse is readily available in tropical countries as an energy source for ruminant diets, but previous research has shown low cattle performance at high inclusion rates. Aims The nature of the dose–response relationship between increasing inclusion rate of cassava (Manihot esculenta Crantz) bagasse (CB), substituting for protein meals in diets of fattening cattle, was used to evaluate the reduced animal performance at high inclusion rates. Methods Thirty growing Madura (Bos indicus × javanicus) bulls were fed concentrate-based diets with five levels of CB inclusion at 20 g dry matter (DM)/kg liveweight and elephant grass (Pennesetum purpureum) separately at 5 g DM/kg liveweight. The five tested CB inclusion rates into the offered diet were 30, 40, 50, 60 and 70% of total dry matter. Lines of best fit were used to describe the relationships between animal performance and CB inclusion rate. Key results Intake of dry matter and concentrate both demonstrated a quadratic relationship with increasing CB inclusion, both maximised at 46–47% CB inclusion in the offered diet or a CB intake of ~9.7 g CB/kg liveweight. Liveweight gain and change in body condition score both demonstrated a quadratic dose–response relationship with increasing CB inclusion, with maximal performance achieved at 34–38% inclusion of CB. Although protein content at the highest level of CB inclusion was low (68 g/kg DM), modelling of predicted liveweight gain indicated that this was most constrained by energy intake (as a function of concentrate intake), rather than crude protein. Conclusions Inclusion of CB up to ~45% of the diet, substituting for protein meals, does not improve, but has no detrimental effect, on liveweight gain. The plateauing of CB intake above 50% inclusion in in the offered diet suggests that cattle were eating to a maximal CB intake. Several hypotheses for reduced intake of concentrate at high levels of CB inclusion were proposed, including low protein content and hydrogen cyanide content of the CB, none of which satisfactorily explained why intake of concentrate was depressed above 50% CB inclusion of the diet on offer. Implications Rations should be formulated to contain no more than 50% CB for maximal cattle performance. Further research is required to understand the causes of depressed intake at high inclusion rates of CB.