Thirty-six individually penned lambs (mean live weight 32·4 (s.d. 2·27) kg) were offered maize silagead libitumand 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 nitrogen (N) g/kg DM were 22·4, 27·4 and 27·5 and of rumen undegradable N 6·6, 7·3 and 11·6 for the control, HR and FM diets respectively.

Both the HR and FM diets depressed maize silage intakes compared with the control during the first 21 days (P < 0·05) and lambs given the FM diet continued to have lower intakes than control lambs (P < 0·05) throughout the experiment. The live-weight gain of HR lambs was considerably depressed in comparison with the control and FM lambs during the first 21 days of the experiment (P < 0·05). Overall HR lambs gained weight more slowly than control and FM lambs up to 45 kg live weight but the difference was not statistically significant. Food conversion ratio was better for FM than HR (P < 0·01). There were no treatment differences in wool growth.

The results obtained provide little evidence that fish meal had any beneficial effects upon lamb growth compared with the control diet except a possible increase in the efficiency of metabolizable energy utilization.