The purpose of this trial was to evaluate two methods of roughage treatment and two levels of treated roughage in complete rations for weaner lambs. A 3 x 2 factorial arrangement of treatments was used. Untreated, sodium hydroxide-treated, and steam-treated sugarcane bagasse were tested either at 19% or 40% inclusion levels. All low fibre rations were formulated to contain (per kg) 130 9 crude protein, 130 9 crude fibre, 8,1 9 Ca, and 3,4 9 P whereas the high-roughage rations were formulated to contain (per kg) 102 9 crude protein, 173 9 crude fibre, 8,5 9 Ca, and 2,2 9 P all on air-dry basis. Because of the different bagasse treatments, the rations within each roughage level varied in concentration of digestible energy. The trial was performed with 84 Dohne Merino lambs fed ad libitum in groups of 12. Sodium hydroxide treatment improved live average daily gain (ADG), efficiency of live feed conversion (FCR), and efficiency of carcass FCR at both low (44 g, 5,6% and 8,8%, respectively) and high (19 g, 2,3% and 5,5%, respectively) roughage inclusion levels. The response to sodium hydroxide was thus bigger in the high energy production type diet. Steam treatment of bagasse only improved live ADG, live FCR, and carcass FCR at the lower inclusion level (14 g, 2,6% and 14,1%, respectively). At the higher levels of inclusion steam treatment had a negative effect on all parameters mentioned.