The spotlight has been put on plantain lately with the recent publishing of a scientific journal article by Eady et al. titled ‘An examination of the ability of plantain (Plantago lanceolata L.) to mitigate nitrogen leaching from pasture systems’. The paper sought to focus on evaluating the scientific evidence relating to current recommendations which suggest including plantain in pastoral dairy systems as a method to mitigate nitrogen leaching. The results of their review have caused some disruption and led to greater questions to be raised around the role of plantain in mitigating nitrogen leaching from pasture-based agriculture in New Zealand. However, there have been two responses to this article from other scientists in New Zealand that add wider points of discussion.
It’s great that these conversations are being had and this blog aims to simplify the key takeaways from the review paper and the responses to it, as well as highlight what great work is being done to generate greater knowledge around the use of plantain as a nitrogen leaching mitigation tool.
What has been the messaging around plantain and its impact on N leaching?
The messaging to date around the ways in which plantain can reduce N leaching has been focused around five key mechanisms (Figure 1):
- Urine dilution
- Nitrogen partitioning
- Nitrogen retention due to root exudates
- Nitrogen retention due to urine
- Reduced drainage
These can further be broken down into those that are due to effects from animal consumption of plantain, and those effects that occur in the soil.
Urine dilution occurs as a result of animals eating plantain. Plantain has a high water content (low dry matter) which leads to an increase in urine output by animals. The result of this is frequent urination with diluted N content and means that there is some mitigation before the urine even enters the soil.
Nitrogen partitioning refers to the proportion of N that is lost via dung, urine and milk. It is assumed that milk N is an output through product, whereas N from dung does not leach as quickly as that of N from urine. Plantain has significantly less soluble and degradable N than that of perennial ryegrass, as well as higher rumen undegradable protein. It also has a higher ratio of non-structural carbohydrates to that of dietary N. Animal consumption of plantain therefore leads to less N making its way into urine but instead being transferred to dung or to milk.
Nitrogen retention refers to more N being taken up by plants and/or soil, resulting in less N available to leach. Current messaging is that this can occur in two ways:
- The first is related to the plantain plant roots and the release of secondary compounds. It is thought that these compounds may restrict the bacteria that undertake nitrification, which is the process that transforms the form of N from ammonium to the leachable form of nitrate. Any slowing down of the nitrification process gives plants a greater chance of taking up N.
- The second method is similar to the first in that there is a restriction in the nitrification process due to the secondary plant compounds of plantain. The difference is that this is attributed to the break down of plant material in the animal and excretion of these compounds back into the soil via urine.
Reduced drainage of water below the root zone from pastures containing plantain is thought to occur due the plants root structure and the need to take up more water to maintain its high water content.
The review paper - Eady et al. 2024
The review paper by Eady et al. (2024) sought to examine the scientific evidence that underpins the current recommendations for including plantain in dairy pastures as a mitigation strategy for nitrate leaching. Scientific scrutiny was applied to various studies into the effect of plantain on N leaching, largely in relation to the mechanisms highlighted above. It also looked to scrutinise the extent to which urine patches contribute to N leaching.
Eady et al. (2024) concluded that ‘many of the research studies supporting the beneficial impact of plantain do not stand up to scientific scrutiny associated with methodology and interpretation of data‘. Some of the key conclusions were:
- Plantain clearly increases the urination of grazing animals due to the consumption of low dry matter forage.
- There is insufficient evidence that greater partitioning of N to dung and milk can impact on N leaching, especially with plantain at a level of 30% of visual assessment within pasture. It is suggested that the major drivers of N leaching from pasture are in fact those such as rainfall events, treading and bare ground.
- No significant evidence that plantain under farm conditions has a significant effect on soil-based mechanisms of nitrogen retention.
- There is a misrepresentation of the extent of N leaching in pasture from urine patches relative to other N sources. Additionally, there is a significant overestimation of the magnitude of the reductions in N leaching from plantain.
- Chicory and tetraploid hybrid ryegrasses also have high water/low dry matter content, and it was recommended that further research into these forages as alternatives to plantain is undertaken. There is a need to also consider integrated factors such as water, enteric methane, management, productivity and efficiency, to ensure that the uptake of forages for N mitigation purposes are fit for purpose for farm systems.
The conclusions of the review are really interesting considering the focus there has been on increasing plantain percentage in pasture swards in order to mitigate N leaching from cow urine patches.
Was there any consensus between the review paper and those who have responded?
What concerns have been raised about the review paper?
The response
In response to this review there have been two formal comments made by Monaghan et al. (2024) and Fransen et al. (2024) which have disputed aspects and/or conclusions of the review paper.
There is one thing that all agree on: plantain has an effect on increasing urinary volume and decreasing urinary N concentration.
Monaghan et al. (2024) raised concerns regarding several flaws and omissions in the assessment of nitrogen leaching and associated commentary in the review paper.
- The review paper suggested that the importance of urinary N in leaching has been misrepresented.
- Monaghan et al. (2024) said they were ‘puzzled’ as to why much relevant published evidence on N leaching from grazed pastures was overlooked.
- It was stressed the importance of the role of urine in N leaching is reinforced by the fact that mitigations such as a combination of reduced stocking rate and animal stand off periods have been shown to decrease N leaching by ~40% compared to standard management practices.
- An important comment was made that stated ‘the contention by Eady et al. that urine N is a less important source of leached N than indicated presents rural communities and policy makers with confusing and unhelpful consideration of the role of urinary N in NZ’s water quality challenges‘.
Fransen et al. (2024) a nine-page comment on the review papers findings. The scientists who provided this comment are all part of the Plantain Potency and Practice programme (known as the Plantain programme) which is researching plantain as a low-cost forage solution to nitrogen leaching. Some key commentary in their response to the review paper was:
- A rejection of the suggestion that urine patch N has little influence on N leaching (consistent with the response of Monaghan et al.).
- That the revised estimate of urine patch nitrogen load and leaching in the review paper of 6 kg N/ha/year represents a very low scenario, and that data at paddock scale suggests a more likely range of 11-54 kg N/ha/year. Soil type and farm management, and their complex interactions, are important factors when considering N leaching calculations.
- It is also rejected that there is no evidence for the effect of plantain on nitrogen partitioning, or on potential slowing of the nitrification rate in the urine patch.
- An important statement was made that reinforced ‘DairyNZ currently recommends regional councils account only for the dilution and partitioning mechanisms (animal effects) currently represented in OverseerFM. The direct and indirect N retention mechanisms (soil effects) are the subject of ongoing research.‘.
Where to from here for plantain as an nitrogen mitigation tool?
Many people will be wondering where to now with plantain? Well, not all is lost yet. While it may feel like there’s been a huge disruption to our current understanding of how plantain can help reduce nitrogen leaching, it is great to see that conversation continues with the responses from other NZ scientists. It’s another step in the scientific process and will hopefully ensure that robust science and discussions regarding plantain continue, and allow for scientists to come to the most-informed conclusions for New Zealand farmers.
The Plantain programme is a key research programme that will continue to develop greater understanding. The seven-year project began in 2021 with DairyNZ, and many sector delivery partners including Massey University, Lincoln University, Agricom, Lincoln Agritech, Manaaki Whenua (Landcare Research), and Plant and Food Research, with the aim of plantain being able to be a low-cost forage option for N loss mitigation.
The programme has been working on demonstrating plantains efficacy at scale so that farmers can be confident when looking to invest in pasture and practice change. With $22 million in funding, and another 3-4 years left of research, there will be plenty more information and evidence to come so that farmers can have certainty that plantain does in fact have the environmental benefits that we have come to assume.
To keep up with the Plantain programme, you can follow their quarterly progress reports at https://www.mpi.govt.nz/funding-rural-support/sustainable-food-fibre-futures/sustainable-food-and-fibre-futures-projects/?Keywords=plantain&Status=All&Sector=&FundingFrom=&FundingTo=
To read more of the review paper and the responses, you find them via the following links:
The review paper: https://www.tandfonline.com/doi/full/10.1080/00288233.2024.2373220?src=exp-la#d1e216
Response – Monaghan et al: https://www.tandfonline.com/doi/full/10.1080/00288233.2024.2398153?src=recsys
Response – Fransen et al. : https://www.tandfonline.com/doi/full/10.1080/00288233.2024.2398149?src=recsys
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