Five diets were formulated on an iso-nutrient basis (approximately 12 MJ digestible energy, 16,5% crude protein, 5% non-degradable protein, 33% neutral detergent fibre, 18% acid detergent fibre, 0,8% calcium and 0,3% phosphorus) so that lucerne hay (LH) was substituted by increasing levels of wheat-straw (WS), lupins and fish-meal. The LH content of the diets decreased from 42 to 0%, while the WS, lupin and fish-meal contents increased from 0 to 26%, 0 to 14%, and 4,0 to 6,0%, respectively. A sixth diet was composed to be similar to the diet containing 26% WS, but thermally ammoniated wheat-straw (AWS) was used instead of untreated WS. In a digestibility and N balance study with 24 SA Mutton Merino wethers, apparent digestibility coefficients and N balance were largely independent of the substitution of LH with WS. Energy and fibre digestibility on the diet containing 26% AWS were higher (P ≤0,05) than on the diet containing 26% WS. The diets were evaluated according to a completely randomized design in terms of dry-matter intake (DMI), average daily gain (ADG) and feed conversion ratio (FCR), in a growth study with 6 – 8 SA Mutton Merino lambs per diet. Mean (± SD) initial live mass of the lambs was 15,7 ± 2,1 kg. No differences in DMI, ADG or FCR were found between the diets containing either 26% WS or 26% AWS. Total DMI of lambs decreased linearly (P ≤0,01) by 80,5 (SEb = 14,0) g/ d per unit substitution of LH with WS, lupins and fish-meal. This trend accounted for 99,3% of the variance between diets. Daily gain was similarly affected, with a corresponding linear (P ≤0,05) decline of 8,3 (SEb = 3,3) g / d associated with 59,5% of the variance between diets. The substitution of LH with WS, lupins and fish-meal did not influence FCR significantly; absolute values for diets ranging between 4,5 -5,1 kg diet DMI required per kg live mass-gain. Mean dressing percentage of male lambs did not differ significantly between diets and varied between 44,3% and 45,5%. Efficiency were thus largely unaffected by the substitution of LH with WS, lupins and fish-meal. These sources may therefore be used to substitute LH provided that the lower DMI, and resultant poorer growth rate, is taken into consideration. Thermal ammoniation of the roughage portion in the diet containing 26% WS had no benefit in terms of DMI, ADG or FCR.