The use of mealie hay as roughage source in lamb finishing diets. The influence of supplementation of mature maize plants (6% crude protein; 11,4 MJ DE/kg) with both energy and nitrogen on production aspects of slaughter lambs was investigated. A total of 130 lambs were fed mature maize plants supplemented with different levels of energy (12,5 and 13,7 MJ DE/kg) and nitrogen (11 and 14% crude protein where 16,6 and 33,3% crude protein respectively were replaced by urea on an iso-nitrogenous basis). It was evident that mature maize plants without any supplementation were inadequate for fattening lambs. Supplementation with protein and energy resulted in improved animal performance. However, replacement of natural protein with urea in diets reduced the efficiency of food conversion, growth rate and dressing percentage. Significant (P < 0,05) interactions between urea and energy levels at the high protein concentration (14%) on growth rate and dressing percentage were evident.