Thirty-six individually penned lambs (mean live weight 32·4 (s.d. 2·27) kg) were offered maize silage ad libitum and one of three concentrate mixes, two of which contained extracted rapeseed meal (control and HR) and the other fish meal (FM). The concentrates were given according to live weight and in sufficient quantities to provide proportionately about 0·4 of the dry matter (DM) intake of the lambs. The dietary concentrations of the nitrogen (N) g/kg DM were 22·4, 27·4 and 27·5 and of the rumen undegradable N 6·6, 7·3 and 11·6 for the control, HR and FM diets, respectively. All lambs were slaughtered at 45 kg live weight and chemical composition of the empty body and some of the component parts determined. A further 12 lambs were slaughtered at the beginning of the experiment to establish body composition before the dietary treatments were imposed.
The HR lambs had lower fleece.free empty body (FFEB) gains than either control or FM lambs (P < 0·05). This reduced gain of HR lambs was particularly associated with a reduction in fat deposition (P < 0·01) such that the FFEBs contained less fat than control and FM lambs (P < 0·01). The efficiency of conversion of metabolizable energy for growth (kg) was worse than both the control (P < 0·01) and the FM (P < 0·001) diets. The FFEBs of HR lambs also contained more ash (P < 0·05) than the lambs receiving the other diets.
The FM diet was associated with greater gains of fat and energy in the guts compared with the control diet (P < 0·05) and FM lambs had a better kg, value than control lambs (P < 0·05). There was no evidence that FM lambs had better N retention than lambs on the other two treatments.