An investigation was made of the potential for attaining high daily live weight (LW) gain, high feeding efficiency and carcass quality in Norwegian Red (NRF) bulls fed grass silage harvested at early stages of maturity, supplemented with minimal amounts of concentrates. Roundbale silage was produced from timothy-dominated swards harvested at three stages of maturity: stages 1 and 2 were dominated by tillers in stem elongation with two and three visible nodes, respectively, and stage 3 by tillers at early heading, with visible heads, but without head stems. Crops were wilted rapidly and a formic acid-based additive applied. All silages were preserved with restricted fermentation. Silage DOMD values were 0•747, 0•70 and 0•647 for harvesting time (H) 1, 2 and 3 respectively. Silages were fed ad libitum as sole feed, or supplemented with 2-4 kg concentrate at increasing LW, to six bulls per treatment from age 7 months (288 kg) to slaughter at 575 kg. Daily LW gain for bulls fed unsupplemented silage was 1423, 1262 and 936 g, respectively, for H1, H2 and H3, and 1570, 1567 and 1357 g for supplemented bulls. For concentrate-supplemented bulls, higher energy intake increased proportions of carcass fat more for H1 (0•163) than for H2 (0•134) but the same amount of energy was retained in the carcass per MJ net energy growth intake for these two groups. It is concluded that, apart from milk and concentrate fed to calves during the first months, an intensive beef production with NRF bulls finished before 15 months of age may be based entirely on local grass resources.