To improve sustainability, dairy farms can reduce protein‐rich concentrate in the cows' diet providing fresh herbage produced on‐farm. This study aimed to quantify effects of increasing the percentage of fresh herbage (0%, 25%, 50%, and 75%, on a dry matter [DM] basis) in a partial mixed ration‐based diet on cow N use efficiency and excretion. The study was performed with five lactating cows, in a 4 × 4 Latin square design for four 3 week periods. Individual DM intake, milk yield, feces and urine excretions, and their N concentrations were measured daily. Dietary crude protein concentrations varied little among treatments (127 to 134 g/kg DM). DM intake and milk yield decreased linearly by 5.2 and 3.7 kg/day, respectively, while N use efficiency increased by 4.1 percentage points from 0% to 75% DM of fresh herbage in the diet. Urinary N was not influenced by the treatments, while fecal N decreased as the percentage of fresh herbage increased. This study highlights that replacing partial mixed ration with an increasing percentage of fresh herbage with slight changes in dietary N concentration increases N use efficiency and the percentage of urinary N in excreted N.