Lucerne hay (LH) was substituted by urea-ammoniated wheat straw (AWS) in four lamb-growth diets, all containing 60% roughage. The ratio of LH to AWS was 60: 0, 40: 20, 20: 40 and 0: 60 in the respective diets, which were composed on an iso-nitrogenous basis. However, cell wall constituents (CWC) increased with increased AWS levels. Results from an 84-day growth study indicated that the voluntary dry-matter intake (DMI) of the lambs declined by 164 g/ d per 20% increase in AWS inclusion (P ≤ 0,01). Growth rate (ADG) similarly declined by 64,5 g/d with higher inclusion levels of AWS (P ≤ 0,01). Feed conversion ratio (FCR) was also adversely affected, with an increase of 1,14 kg DMI required per 20% increase in AWS inclusion to give an increase of 1 kg in live mass (P ≤ 0,01). The apparent digestibility of dry matter (DM) and organic matter (OM), as well as the nitrogen balance, tended to decline with increased AWS levels, but no significant differences were recorded. The lower energy intake on the diets including increased levels of AWS was reflected by the decreased deposition of fat by the end of the growth study (P ≤ 0,01), as measured by tritium water space. The economic advantage of the substitution of a high quality roughage like LH with a cheaper source (AWS) must be balanced against the reduction in DMI and ADG, and the adverse effect of increased AWS levels on FCR. Inherent limitations of AWS, like a high CWC and resultant bulkiness, must be recognized and considered in the formulation of diets including AWS as a roughage source.