Invited review: Cows grazing pastures containing narrow-leaved plantain have lower urine N...

Narrow-leaved plantain (Plantago lanceolata L.) has been proposed as a potential solution to subsurface drainage (i.e., leaching) losses of soluble nitrogen (N) species (primarily nitrate; NO₃^-), a critical environmental challenge in grazing systems. It has been hypothesized that plantain has properties that can lower NO₃^- leaching through animal, plant, and soil-related processes. We summarize available evidence, explore mechanistic bases, and investigate the integration of plantain into mixed forage pastures. Paddock-scale experiments support a reduction in NO₃^- leaching with the inclusion of plantain in the pasture mix. The number of paddock-scale experiments are few and the experimental methodology for measuring the amount of NO₃^- leached varies with study, making quantification of the effect difficult. But, on average, NO₃^- leaching declines by ∼45%; the effect is variable and dependent on the pasture plantain content. At the mechanistic level, there is a negative association between the proportion of plantain in the forage mixture and urinary N concentration, the primary contributor to paddock-scale NO₃^- leaching; on average, urine N concentration declines 0.4g/L or 7.2% with every 10% increase in pasture plantain content. This result is consistent across studies and supports a negative relationship between pasture plantain content and NO₃^- leaching. The effect of plantain on urine N concentration is, primarily, a result of an increase in urine volume (i.e., urinary N dilution), and a lower N intake, potentially, depending on the pasture legume content. There is also evidence for a lower N digestibility, and greater N partitioning to milk, although the quantitative impact of this on daily urine N yield is small. In addition to the reduction in urinary N concentration, a reported slower conversion of soil ammonium to NO₃^- (i.e., nitrification), due to the presence of biological nitrification inhibitory compounds in the leaves of plantain (i.e., consumed by the cow) and, potentially, exuded by the roots, and a reduction in drainage volumes from pastures containing plantain, may also contribute to reduced NO₃^- leaching. In summary, the available evidence supports the premise that the inclusion of plantain in a pasture mix reduces NO₃^- leaching in grazing systems. Agronomic challenges around establishment and persistency of plantain in permanent pastures remain areas for research as does the need to investigate the interaction between pasture plantain content and other known NO₃^- leaching mitigation strategies in grazing systems.