Fixed amounts of roughage and concentrate can be allocated to growing ruminants in a number of different ways e.g. a) a mixed diet containing equal parts of roughage and concentrate b) split feeds, roughage being given alone until exhausted, then concentrate alone c) a mixture changing . progressively from all roughage to all concentrate d) half the animals given roughage and half concentrate. The results of these alternatives can be predicted from computer models based on current feeding standards. Recent evidence (Ørskov et al, 1981a; Williams et al, 1984) suggests that in cattle carcasses of similar weight and composition can be obtained by giving a fixed amount of roughage and concentrate as either a mixed diet, or as separate feeds, (i.e. alternatives (a) and (b), above)even though animals on (b) took significantly longer. (Ørskov et al (1981a) concluded that improved efficiency of energy utilisation in the split group could be accounted for by a decrease in the maintenance requirement of the animals, or an increase in the efficiency of energy utilisation during realimentation following a period of reduced nutrition. The results could also be explained by differences in carcass composition of the animals, since no measurement of the protein and fat content of the carcasses was made. Williams et al (1984) found no significant difference in carcass composition between the two groups, as determined by specific gravity measurements and concluded that associative effects caused a reduction in the energy available from straw when offered with barley in a mixed diet.