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We investigated effects of phosphorus (P) fertilisation on yield and forage nutrient content measured for 28 years in a long-term grassland experiment. Mountainous grassland plots were set up at constant nitrogen (N) fertilisation of 25 kg ha‑1 yr‑1 and at two levels of P: no P (P0) and 31 kg P ha‑1 yr‑1 (P31). Potassium was applied in non-limiting amounts. Here, we report biomass yield, N yield, and the nutritional status of herbage from the first cut per year, on average across years. Biomass yield was 3.74 and 4.38 Mg ha‑1 yr‑1 at P0 and P31, respectively, indicating a yield loss of 15% due to the lack of P (P<0.001). Despite this yield difference, N content did not differ between P treatments and was 18.2 g N kg‑1 DM on average. As a result, N yield was significantly enhanced by adequate P supply (P<0.001), and was 66.5 and 79.2 kg ha‑1 yr‑1 at P0 and P31, respectively. The phosphorus nutrition index (PNI) indicated P limited growth at P0 (PNI=0.6), while it confirmed adequate P supply at P31 (PNI=1.1). We conclude that adequate P fertilisation not only enhanced biomass yield but also allowed plants to exploit N sources better, leading to more N yield for a given amount of soil N.