Actavia 0 Třináctý, J. Křížová, L. Richter, M. Černý, V. Říha, J. 2009 2009-6-30 239-248 The objective of this study was to determine the effect of supplemental lysine (Lys), methionine (Met) or both amino acids added in the form of rumen-protected (RP) tablets with copolymer coating to a diet of dairy cows on yield and composition of milk and concentration of plasma amino acids (AA). The experiment was carried out on four high-yielding lactating Holstein cows with average milk production of 33.5 kg/day in the form of Latin square design. The four treatments were as follows: C - control without AA supplementation, L - control plus supplement of RP Lys, M - control plus supplement of RP Met and ML - control plus supplement of RP Met and Lys. The experiment was divided into 4 periods. Each period (14 days) consisted of 10-day preliminary period and 4-day experimental period. Cows were fed a diet based on maize silage, lucerne hay and supplemental mixture. Average milk yield recorded in ML was 34.18 kg and was higher than that recorded in L or M (32.46 kg and 32.13 kg, respectively P < 0.05) and tended to be higher than in C (33.33 kg, P > 0.05). The content of protein and casein was higher in L and ML in comparison with C (P < 0.05) and tended to be higher than in M (P > 0.05). Protein yield in ML (1 054 g/day) was higher than that found in C, L or M (990, 998 or 968 g/day, respectively, P < 0.05). The same response was found for casein yield (P < 0.05). Although the proportion of individual casein fractions was not affected by the treatment, the yield of α- and β-casein differed (P < 0.05) while the yield of κ-casein was not affected by the treatment. Concentrations of blood metabolites, except for betahydroxybutyrate (BHB), were not changed. Plasma Met concentrations were increased (P < 0.05) in the M and ML group. Similar, but insignificant increases (P > 0.05) were also observed in plasma Lys in the L and ML group. 6 54