Effective Proportion of Plantain (Plantago lanceolata L.) in Mixed Pastures for Botanical Stability and Mitigating Nitrous Oxide Emissions from Cow Urine Patches

Plantain (Plantago lanceolata L.) is recognised for its ability to improve summer feed productivity as well as mitigate nitrous oxide (N2O) emissions from grazed pastoral soils. This study aims to determine the proportion of plantain required in perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.), or RWC, mixed swards to maintain yield stability and to quantify N2O 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-1), 30% (5.40 g N L-1) and 50% (4.40 g N L-1) plantain mixed pastures was used to measure N2O emissions (n = 5) from the pastures of their origin and to assess the impact of the plantain rhizosphere on N2O 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 N2O 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 N2O EFs.