Context Cupuassu is frequently cultivated in Brazil as well as in the warm lowlands of other tropical American countries. Cupuassu nut contains high levels of theobromine and ether extra ct, which could negatively affect the intake and digestibility of nutrients, when included in livestock diets. Aims The present study was undertaken to evaluate how replacing maize with cupuassu nut affects feed intake, efficiency of microbial synthesis and nitrogen (N) balance in Nellore bulls at pasture. Methods Five Nellore bulls with an initial bodyweight of 336.7 ± 12.8 kg (mean ± s.d.) were kept in individual paddocks of Urochloa brizantha cv. Marandu. Concentrate supplements containing equal amounts of N and different concentrations of dried whole cupuassu nut (0%, 25%, 50%, 75% or 100% replacement of maize) were fed to bulls in a 5 × 5 latin square design, with each animal allocated to each diet for a period of 20 days, including a 14-day period of adaptation. Key results Intake of all dietary components and total digestible nutrients, except ether extract, theobromine and neutral detergent fibre corrected for ash and protein, decreased linearly with the addition of cupuassu. Theobromine intake varied from 13.86 (25% cupuassu nut) to 36.67 (100% cupuassu nut) mg/kg bodyweight. The inclusion of cupuassu in the supplement led to a linear decrease in ruminal ammonia-N concentration, N balance, and efficiency of microbial synthesis. Conclusion Replacing maize with dried whole cupuassu nut in concentrate supplements reduces the intake of DM from both supplement and pasture, the efficiency of microbial synthesis, and the N balance in grazing beef cattle. Implications The replacement of maize by dried whole nut of cupuassu in concentrate supplements for grazing cattle is not recommended because it reduces supplement intake and nitrogen retention, and it is likely to decrease liveweight gain.