| Summary: | Plantain (<i>Plantago lanceolata</i> L.) is recognised for its ability to improve summer feed productivity as well as mitigate nitrous oxide (N<sub>2</sub>O) emissions from grazed pastoral soils. This study aims to determine the proportion of plantain required in perennial ryegrass (<i>Lolium perenne</i> L.) and white clover (<i>Trifolium repens</i> L.), or RWC, mixed swards to maintain yield stability and to quantify N<sub>2</sub>O emission reductions. The botanical composition was monitored when plantain was sown at different rates of 0%, 30%, 50% and 70% in RWC pastures under grazing by dairy cows over 2 grazing years. Urine from cows grazing RWC (6.15 g N L<sup>−1</sup>), 30% (5.40 g N L<sup>−1</sup>) and 50% (4.40 g N L<sup>−1</sup>) plantain mixed pastures was used to measure N<sub>2</sub>O emissions (<i>n</i> = 5) from the pastures of their origin and to assess the impact of the plantain rhizosphere on N<sub>2</sub>O emissions by applying the RWC urine to the 50% plantain mixed pasture, and vice versa, in late summer–autumn using the static chambers technique. After declining in spring, the plantain content recovered in early autumn and reached a peak of 40% in the 30% plantain mixed pasture and around 50% in the 50% and 70% plantain mixed pastures in winter. A lower N content in urine, and therefore a lower urine N-loading rate from cows grazing in the 50% plantain mixed pasture, resulted in 39% lower total N<sub>2</sub>O emissions compared to RWC urine treatment. In conclusion, 30% to 50% plantain in mixed pastures was stable throughout the 2 years, and it not only reduced the urinary N concentration in grazing cattle but also contributed to reduced N<sub>2</sub>O EFs.
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