On two organic farms, the effect of supplementation with either home-grown barley or organic commercial concentrates primarily based on local protein sources and barley, on weight gain and subclinical coccidiosis was investigated in 3 -5 months old dairy breed calves in the period after turn-out to ryegrass/clover pastures. The calves (mean liveweight9SD, 1249 24 kg and 1339 24 kg on Farm I and II respectively) were supplemented daily with 3 kg energy and protein-rich commercial concentrates (group EP), 3 kg barley (group E) or 1 2 kg barley (group Eres) for 8 weeks following turn-out on pasture.Daily weight gains in the 8 weeks were for Farm I: 1216 and 1042 g/day for group EP and E (P B0.01) and for Farm II: 1071, 671 and 770 g/day for group EP, E and Eres respectively (PB 0.001). However, liveweights were similar between groups at housing, although group EP had significantly higher liveweight at housing on Farm I (PB 0.01). Initial liveweight had effects on daily gain in the grazing season, but supplementation with energy and protein reduced this effect on Farm I. No clinical signs of coccidiosis were observed, but markedly higher levels of oocysts per gram faeces (opg) were observed on Farm II, using pastures previously grazed by cattle compared to Farm I. Calves having a maximum oocyst count above 5 000 opg were subclinically affected by coccidiosis, as indicated by low faecal dry matter and reduction in daily gain of 222 g/day (PB 0.05). It is concluded that liveweight can be increased when supplementing calves with organic concentrates rich in energy and protein in the period following turn-out compared to supplementation with barley alone. However the overall gain at the end of the grazing season is limited due to compensatory growth in the grazing period following supplementation.