A meta-analysis of published and unpublished data (998 treatment means) comprising of milk production trials in dairy cows fed mainly grass silage-based diets was conducted to examine the effects of increasing milk yield on milk composition. The data was divided into seven subsets investigating the effects of level of concentrate, protein and fat supplementation, carbohydrate composition of the supplement, silage digestibility, silage fermentation quality or replacement of grass silages with legume silages. The relationships between milk yield and milk composition varied depending on dietary management. Both positive and negative relationships between milk yield and concentration of milk components (protein, fat and lactose) were observed. Correlation of milk fat and protein concentration ranged from negative to positive. Most of the relationships can be explained by relative changes in the supply of nutrients absorbed from the digestive tract.