The objective of the present experiment was to study the need for the protein supplementation in the diet of growing dairy bulls (initial live weight 272 ± 28.5 kg and final live weight 666 ± 31.2 kg, on average) fed total mixed ration based on moderate digestible grass silage and barley. The experiment comprised 24 Finnish Ayrshire bulls and 8 Holstein-Friesian bulls and included four treatments. The control diet (C) consisted of moderate digestible (653 g digestible organic matter in dry matter (DM) grass silage (450 g kg-1 DM), barley grain (275) and barley fibre (275) without protein supplementation. Three isonitrogenous experimental diets included also extra protein, i.e. (1) rapeseed meal (RSM) (supplementation 530 g DM per animal day-1), (2) wet distillers’ solubles (WDS) (600 g) and (3) a mixture of barley protein (90% of fresh weight) and wet distillers’ solubles (10) (BPWDS) (480 g). In all isonitrogenous diets the crude protein content of concentrate increased from 137 to 150 g kg-1 DM (9%) compared with the C diet. All bulls were fed total mixed ration ad libitum. The energy content of all diets was 11.6 MJ kg-1 DM. The live weight gain of the bulls tended to be higher with the BPWDS diet than with the C diet (C 1214 vs. BPWDS 1301 g d-1; p = 0.10), but the treatments had no significant effect on carcass gain, feed conversion or slaughter parameters. Only the BPWDS diet differed significantly from the C diet in DM (C 9.69 vs. BPWDS 10.38 kg DM d-1; p < 0.01) and energy intake (C 112.4 vs. BPWDS 120.3 MJ d-1; p < 0.05). The apparent organic matter digestibility (OMD) was 5% higher in the BPWDS diet than in the C diet (p < 0.001), but the RSM and WDS diets did not differ from the C diet in OMD. The results indicate that the supply of protein in dairy bulls is most probably adequate with moderate digestible, well-preserved grass silage and barley-based concentrates when intake of digestible organic matter is high enough to support microbial protein synthesis in the rumen
