Methionine and lysine have been shown to be the two amino acids most limiting for milk protein synthesis in lactating dairy cows. Many studies have shown positive effects of adding protected forms of methionine to cows on maize silage-based diets supplemented with soyabean meal. The aim of this study was to evaluate the effect on milk production and composition of adding protected methionine and lysine to cows fed a grass silage diet.

24 Autumn-calved cows (2nd lactation or greater) were blocked into groups of 4 based on calving date and milk yield in the immediate pre-experimental week. Cows within each group were subjected to each of the following 4 treatments in a latin square design: (1) 7 kg of by-product concentrate per day (C), (2) C plus 4.8 g digestible methionine per day (3) C plus 4.8 g digestible methionine and 6.2 g digestible lysine per day (4) 7 kg of barley-soya concentrate + 7.7 g digestible methionine per day. Each period lasted 4 weeks and data from weeks 3 and 4 were used to compare treatment effects. The amino acids were supplied in a rumen protected form (SmartamineTM, RPAN Technology) and all figures relate to estimated intestinally digestible amino acids based on the French system of calculating digestible lysine and methionine. The cows received primary growth unwilted grass silage ad libitum (Oven Dry Matter 189 g/kg, in-vitro DMD 714 g/kg DM, Crude protein 160 g/kg DM, pH 3.9).