Four concentrate supplements differing in crude protein (CP) and undegradable protein (UDP) content were offered to 16 lactating Friesian cows together with grass silage (dry matter (DM) 196 g/kg, pH 4·38, CP 160 g/kg DM, in vitro DM digestibility 0·68) ad libitum in a Latin-square trial with 3-week periods. The supplement treatments were: (1) barley 122 g CP per kg DM, degradability (dg) 0·77; (2) barley/soya-bean meal 210 g CP per kg DM, dg 0·69; (3) barley/soya-bean meal/fish meal 190 g CP per kg DM, dg 0·61; (4) barley/soya-bean meal/fish meal 219 g CP per kg DM, dg 0·59. Supplements were given at 8 kg/day. Total daily intakes of silage (kg DM), CP and UDP (g) on treatments 1 to 4 were 7·77, 2087, 375; 8·35, 2804, 655; 8·29, 2676, 717; 8·70, 2917, 826, respectively. Milk yield (kg/day) and yields of fat, protein and lactose (g/day) on the four treatments were 21·3, 791, 617, 984; 23·0, 816, 688, 1055; 23·0, 818, 696, 1050; 23·6, 813, 735, 1071 for treatments 1 to 4 respectively. Yield and concentration of protein and lactose were significantly lower on treatment 1 than on the other treatments, while the of blood metabolites indicated treatment effects on blood glucose, non-esterified fatty acids, plasma protein and urea. Digestibility of organic matter and non-ammonia nitrogen (NAN) flow to the abomasum (g/day), measured in sheep given a fixed silage/supplement ratio at maintenance, were 0·81 and 18·4, 0·81 and 20·8, 0·82 and 21·4, 0·82 and 22·4 for treatments 1 to 4 respectively. The NAN flow was significantly greater on treatment 4 than on treatment 1.