Black-and-White bull calves aged 14 to 91 days were fed rations differing in the type of roughage offered to appetite: hay (group H) or meadow grass silage (group S) with 2 kg 80% formic acid/ton of fresh forage. Before weaning (days 14-42 of age), the average daily intake of roughage dry matter and gross energy per kg^0.75 was higher (P≤0.05) in group H than in group S (45.0 g and 1.1 MJ, vs. 41.1 g and 1.0 MJ, respectively). However, the differences in daily weight gains (623 and 583 g in groups H and S) and feed utilization were not significant. After weaning (43 to 91 day of life), mean daily intake of nutrients and total energy was similar in both groups with daily weight gains reaching 478 and 502 g, respectively for group H and S. Mean body weight gains during the entire experimental period were the same in both groups and equalled 530 g. The system of nutrition introduced in the present experiment may be useful for the rearing calves designed for future fattening based on roughages. However, due to the low level of Mg observed in blood plasma (0.38 mmol/l) of all the calves, the ration ought to be supplemented with a mineral mixture containing magnesium.