The aim of the study was to determine the effect of replacing barley grain or dry maize (50% by weight) in feed mixtures for calves with ensiled high-moisture maize grain on ruminal and postruminal digestibility of starch, rumen development parameters and rearing performance of calves. The experiment was carried out with 40 bull calves aged between 10 ± 3 and 90 days divided into 4 groups of 10 animals per group. The main source of dietary starch was barley in the control group (B), dry maize

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... ain in group M D , 50% barley and 50% ensiled maize grain in group BM S , and 50% dry maize grain and 50% ensiled maize grain in group M D M S . The results obtained show that rolled ensiled maize grain can be successfully used in diets for calves reared from 10 to 90 days of age to replace (50% by weight) dry rolled barley or maize grain. However, slightly better production results were obtained when feeding a barley and ensiled maize grain diet, which is probably due to the better intestinal digestibility of protein and starch, as shown by higher intestinal digestibility coefficients of protein and starch as well as lower faecal starch losses. The addition of ensiled maize grain to the diets did not cause significant changes in rumen fermentation, although there was a slight increase in total VFA concentration and proportion of butyric acid, a reduction in pH of rumen fluid, and calves tended to have higher serum 196 ENSILED MAIZE GRAIN AFFECT CALF PERFORMANCE β-hydroxybutyric acid concentrations at 10 and 12 weeks of age. The replacement of dry grains with ensiled maize grain in the diets for calves did not have an unambiguously favourable effect on rumen papillae development while improving wall thickness of the ventral ruminal sac.