Sustainable clover dominant swards have been developed for upland UK sheep grazing conditions by sowing a mixture of tetraploid perennial ryegrass cv Condesa and small leaved clover cv Aberystwyth S184. These pastures have produced equal output to Condesa only swards given 150-160 kg N/ha/year. Lamb liveweight gain from April to August was increased by 50 g/day on the grass/clover vs grass swards. A comparison of the suitability of these grass/clover and N-fertilised grass only swards for silage was undertaken in order to assess silage yield potential and silage feeding value. The enhanced animal performance of lambs on clover has been partly attributed to the higher levels of dietary amino acid clover has been observed to provide. In order to quantify this effect sufficient supplementary fishmeal (60 g/day) to fully meet tissue N needs was fed to lambs on the grass/clover and grass silages and responses in lamb performance compared.