Work was undertaken to determine the efficiency of growing lambs offered a fixed amount of roughage and concentrate as; 1) a mixed diet (M), containing 500 g/kg DM roughage and 500 g/kg DM concentrate, 2) split feeds (S), roughage alone followed by concentrate alone, 3) a mixture changing from roughage to concentrate (R-C), or concentrate to roughage (C-R), or 4) half the animals offered roughage and half concentrate (R/C). Lamb growth was predicted from computer models based on current feeding standards. In experiment (1), lambs were offered 50 kg roughage DM and 50 kg concentrate DM by strategies M and S. Lambs on strategy S took longer to consume their food and maintained a higher EBW ^0.75 than those on strategy M. However, the final EBW and EB composition of the two groups was similar. This contrasted with predicted results. The higher overall efficiency of lambs on strategy S was explained partly by associative effects reducing the energy available to lambs on strategy M, and partly by a higher efficiency of energy utilisation and/or a lower maintainance requirement of lambs on strategy S. In experiment (2), the influence of associative effects on strategies R-C and C-R was investigated. The prediction of dry matter digestibility (DMD, Y) from concentrate proportion (CONCP, X1) and dry matter intake (X2) showed no curvilinearity. DMD was linearly related to CONCP. The equations derived for each strategy were similar. In experiment (3), lambs were offered 52.5 kg roughage DM and 52.5 kg concentrate DM by strategies M, R-C, C-R and R/C. Lambs on strategies R-C, C-R and R/C took a similar time to consume their food and achieved a similar final EBW and EB composition. Their energy retention (ER) was similar to that predicted. Lambs on strategy M consumed their food faster than those on all other strategies and achieved a higher (NS) EBW, which contained more (NS) fat. Their ER was higher than predicted. The higher overall efficiency of lambs on strategy M was explained by differences in the ratio of protein: fat deposited and maintained by lambs on each strategy. The EB composition of all slaughtered lambs was within the limits for published data. Non-carcass composition could be used to predict EB composition. The results highlight the importance of the factors mentioned in determining the efficiency of lambs on each strategy.