To ensure sustainable and efficient production, dairy farms must reduce their environmental impacts and nitrogen losses, which are sources of pollution, while increasing their feed self-sufficiency. Grass-based dairy systems, frequently combine fresh grass with maize silage when grass is scarce or during dietary transitions. However, the effects of combining fresh grass and maize silage on cow performance and N excretion are poorly known. This study aimed to quantify the effects of increasing the proportion of maize silage in a fresh grass diet on cow N flows and metabolism, in the context of grass-based dairy systems. Four proportions of maize silage in a fresh grass diet (objectives of 0, 17, 34 and 51% DM of maize silage) were investigated. The experiment was performed in a 4 × 3 Latin square design using eight lactating cows during three 3-week periods. DM intake (DMI), milk yield, faeces and urine outputs, and their N concentrations were measured for each cow. The fresh grass CP concentration was lower than planned (106 ± 13.0 g/kg DM). This resulted in very low dietary CP concentration, which decreased from 108 to 86 g/kg DM when maize silage in the diet increased from 0 to 51% DM, respectively. DM intake and milk yield both decreased linearly by 3.3 kg/day from 0 to 51% DM of maize silage in the diet. Thus, N intake decreased linearly by 100 g/day from 0 to 51% DM of maize silage in the diet. The N concentration of milk was highest for the diet with 0% DM of maize silage. Nitrogen excreted in faeces and urine decreased linearly by 29 and 23 g/day, respectively, from 0 to 51% DM of maize silage in the diet. The low dietary N concentration resulted in low ruminal NH3-N concentrations (8 mg/L, on average) and urinary urea excretion (down to 8% urea N in urinary N). Increasing the proportion of maize silage in an unusually low-N grass diet, without protein-rich concentrates, induced highly N-deficient diets with minimal N losses in faeces and urine but large and unsustainable decreases in DMI and milk yield.