Previous studies have demonstrated that protein sources which are primarily degraded in the rumen such as rapeseed meal or soyabean meal may be as effective as a less rumen degradable protein source such as fishmeal in supporting higher levels of animal performance in young steers fed on grass silage. However, the response to type of protein supplement is likely to be influenced by the composition of the basal diet. This study has examined the effect of supplementing silages prepared from early (EH) and late (LH) harvested grass with two protein sources of differing rumen degradability, rapeseed meal (RSM) and fishmeal (FM) or a mixture of the two (M), thus creating eight experimental diets of LH, LHRSM, LHFM, LHM, EH, EHRSM, EHFM and EHM. Silage was offered ad libitum and supplements were included at 100 g fresh weight/kg silage DM intake. The RSM and M diets were made isonitrogenous with FM diets by the addition of urea. Animals remained on diets for 18 weeks and liveweights were monitored for a further 13 weeks while the animals were at pasture. Dry matter (DM) intakes and liveweight gains were higher with EH than with LH silage (P < 0·001). Response to type of protein supplement was dependent on silage quality. On LH silage, higher intakes were noted on FM and M (82·4 and 82·8 g DM/unit metabolic liveweight/day, respectively) relative to silage only (75·8 g DM/unit metabolic liveweight/day) and this contributed towards higher liveweight gains (P < 0·01). Liveweight gains tended to be higher on LH silage supplemented with FM compared to RSM (0·76 v. 0·67 kg/day, respectively, P = 0·08). In comparison, on EH silage, relative to EH unsupplemented, the intake was highest on M (89·2 v. 96·6 g DM/unit metabolic liveweight/day), but liveweight gains were not significantly different between supplements. On turnout to pasture, those animals fed on silage alone exhibited compensatory growth (P < 0·025) with the result that those fed on LH silage only grew faster and achieved the same liveweight after 13 weeks at grass as those supplemented with RSM or M. There was a tendency for those fed on FM to maintain an advantage in liveweight after the period at pasture. On EH silage, at the end of the grazing period no significant differences in liveweight existed between the different supplements and on average were 23 kg heavier than EH silage unsupplemented. In conclusion, silage type (stage of harvest and quality) and protein supplementation influenced animal performance. On late harvest and poorer digestibility silage, there was some indication that feeding fishmeal was better than rapeseed but this was less evident on early harvest and higher digestibility silage. However, considering the price differential between these two supplements and small difference in animal performance it is concluded that rapeseed meal is as effective as fishmeal when used as a protein supplement for growing cattle fed on grass silage.