Plantain (Plantago lanceolata L.) as an Alternative Forage to Build Resilience and Reduce the Environmental Footprint of Grazing Dairy Systems in Temperate Northern Climates: A Review

Abstract

The agriculture sector is responsible for the largest proportion of greenhouse gas emissions in Northern Ireland and mitigation strategies must be introduced if the industry is to achieve the ‘Net Zero’ targets set for 2050 by the United Kingdom government. Dairy farming is a source of nitrous oxide emissions, a potent greenhouse gas with 256 times the warming potential of carbon dioxide. One potential mitigation measure is the use of alternative forage species such as Ribwort Plantain (Plantago lanceolata). Evidence would suggest that plantain has the ability to improve nitrogen use efficiency (NUE), leading to reductions in overall nitrogenous emissions from grazing dairy systems via three pathways: reducing urinary nitrogen concentration leading to lower rates of nitrogen leaching from urine patches; improving nitrogen utilisation efficiency within the dairy cow so that a lesser proportion of dietary nitrogen is excreted via the urine; and through the action of root exudates producing biological nitrification inhibition in the soil and improving soil nitrogen retention. This review summarises the current evidence supporting plantain as an alternative forage to support animal performance and forage production whilst lowering the environmental footprint of grazing dairy systems in temperate climates. This review also highlights outstanding research questions which must be addressed for farmers to confidently introduce these alternative species into their grazing platforms.

1. Introduction

Home-grown forage from productive grassland remains a key component of dairy cow diets in Northern Ireland (NI), and accounts for an estimated 58% of all feedstuffs consumed by the Northern Irish dairy herd [1]. Increasing the proportion of home-grown forages used in dairy cow diets is key to underpinning the sustainability of the Northern Irish dairy sector, with greater usage of home-grown forages associated with reduced farm variable costs and higher levels of farm profitability [2], reduced farm phosphorus balances [3] and delivery of dairy products with improved nutritional value for consumers [4]. Perennial ryegrass (Lolium perenne L., PRG) is the most common sown species for both grazing and grass silage production in NI [5], where the temperate climate typically enables the production of high annual yields of PRG as grazed or ensiled forage. However, in recent years increases in climate volatility have put pressure on this resource. For example, droughts in summer 2018, spring 2020, summer 2021 and summer 2022 resulted in annual average yield reductions of 0.8, 0.7, 1.0 and 2.7 t dry matter (DM) ha⁻¹, respectively, compared to the 10-year average annual yield of 11.1 t DM ha⁻¹ recorded for the period 2013–2023 through an established long-term plot monitoring experiment [6].

The latest climate change projections [7] suggest an overall drying of the climate in NI; however, this will be coupled with an increase in the frequency of extreme weather conditions including periods of drought and high rainfall. The use of alternative forage species that have increased tolerance to extreme weather conditions could provide a sustainable feed source for dairy cows throughout the grazing season. The agricultural industry in NI must also address the challenge of mitigating climate change and working towards ‘Net Zero’ emissions targets [8,9] through the uptake of practices which reduce greenhouse gas (GHG) emissions intensity and/or aid in carbon sequestration [10,11]. The NI agricultural industry is estimated to contribute 27% of all GHG emissions in the province [12], with dairy farming a significant source of both nitrogenous and methane emissions [13] and with deposition of animal faeces and urine being the biggest source of nitrous oxide (N₂O) per year in grassland (54%) followed by manure application (13%) and nitrogen fertiliser use (7%) [13]. Whilst N₂O emissions make up a smaller proportion of the total greenhouse gases (GHGs) from dairy farms globally, they have a significant effect on global warming, with a lifespan of ~120 years and 265 times higher radiative potential than carbon dioxide (CO₂) [13]. However, when controlling for milk yield, pasture-based dairy systems are estimated to have lower carbon footprints compared to other livestock production systems [14] and are likely to support the efforts of agriculture achieving its net zero targets. The introduction of practices that build resilience in forage production and lower dairy farm emissions in NI, including the uptake of alternative forages, must continue to support efficient and economically viable milk production in order to be considered a feasible option for dairy farmers.

Ribwort Plantain (Plantago lanceolata, PL) is a perennial herbaceous plant with a broad distribution in temperate grasslands across the globe. Although, PL has received limited use in grazing pastures, particularly in temperate northern climates, due to its persistence issues, limited weed control option and potential palatability issues [15]. However, recent research suggests the species may perform well in NI due to its ability to grow in acidic soils and in conditions similar to that required for PRG [16]. Modern cultivars with a more upright leaf stance particularly suitable for grazing are now available from multiple forage seed companies [15]. PL holds particular interest over and above other alternative forage species for its potential ability to improve nitrogen use efficiency (NUE) in dairy systems whilst providing a reliable forage base [17]. Research from the southern hemisphere has identified improved NUE and lower N₂O emissions from dairy grazing systems incorporating PL [17], and a reduced fertiliser requirement and longer growing season relative to that of PRG [18]. In addition, N leaching into groundwater has been shown to be reduced by 89%, primarily due to the lower concentration of urine-N from cows grazing swards containing plantain [15]. However, this trend is not consistent within the literature, with N leaching also reported to increase by 23% [19].

However, to date, limited research has been completed on the use of PL in grazing pastures in the northern hemisphere, with no clear evidence currently available for its potential performance on NI dairy farms as a reliable forage source to support sustainable milk production, or on the full potential of PL inclusion in dairy grazing pastures to contribute to reducing the emissions footprint of NI dairy farms.

This review will summarise existing studies, predominantly from New Zealand, and identify research gaps which should be addressed to inform the utilisation of PL as an alternative forage species to supply the feeding requirements of dairy cattle in pasture-based production systems in NI and the potential benefits in terms of reducing nitrogenous and other GHGs losses from NI dairy farms, and farms in similar temperate northern climates.

2. Materials and Methods

A literature search was conducted in November 2024 through Web of Science using the following terms as examples: TOPIC: Plantago lanceolata, grazing and TOPIC: Plantain, grazing. These terms were selected in order to narrow the focus of the search appropriately for the topic at hand. In total, 217 results were returned. In the final database, 142 publications were retained, with 75 having been manually excluded as not relevant to the focus of this review; this was because they described experiments focusing on animal performance metrics with livestock species other than dairy cattle or were not focused on forage production for ruminant production systems (

Table 1. Publications split by location, year of publication and topic.

	Number of Publications
	Relevant	Not Relevant	Total
All publications	142	75	217
Temperate Zone
Northern Hemisphere	42	37	79
Southern Hemisphere	91	37	128
Both Hemispheres (review articles)	9	1	10
Year
<2018	53	50	103
2018–2024	69	25	115
Topics
Cattle only	28	2	30
Sward only	47	11	58
Cattle, sward and emissions	65	3	68
Different Animals/Soil	3	58	61

). The remaining articles were reviewed, and the results presented are discussed in relevant subcategories below.

3. Plantain Agronomy in Temperate Climates

Limited knowledge is available on the agronomy of PL for grazing pastures in comparison to that available for PRG in the northern hemisphere; however, it has been suggested that PL has agronomic characteristics suited to NI growing conditions (Table 2). This is partly due to its ability to occur naturally across a wide range of soil acidities (pH 4.2–7.8) [20] and survive in soils suited to PRG and white clover (Trifolium repens L., WC) [21]. Research from New Zealand has noted annual productivity of PL swards of 10–20 t DM ha⁻¹ year⁻¹, with growth rates ranging between 25 and 80 kg DM ha⁻¹ day⁻¹ but peaking at 140 kg DM ha⁻¹ day⁻¹. [22]. More recently, in a plot study, [23] found that a PRG/PL/WC sward yielded around half the total amount of DM ha⁻¹ of a PRG/WC sward. In comparison to PRG, PL has been found to exhibit higher levels of cool season growth, achieving growth rates of 30 kg DM ha⁻¹ in winter months [18,24]. PL produced 1.8 t DM ha⁻¹ and 0.9 t DM ha⁻¹ more during summer and autumn than PRG [25] but was less active than WC [24]. This presents potential opportunities to increase the length of the grazing season in NI by providing additional forage when PRG yields are restricted; however, some authors [21] have indicated that PL swards are less dense and more susceptible to treading damage from grazing livestock. This may suggest that PL would be best suited to inclusion in a simple mixed-species sward alongside grasses and clovers, rather than as a monoculture. In Ireland, combinations of pasture species [26] of PL with PRG, WC, red clover (Trifolium pratense, RC), and common chicory (Cichorium intybus, CH) indicated than PRG was more productive in three different grazing seasons in mixed-species sward than in monoculture. The introduction of forage herbs PL or CH did not increase DM production, but clover increased DM for all levels of fertiliser used. Jezequel et al. (2024) [27] and Fizpatrick et al. (2024) [28] reported similar DM yields in all sward types, even if the persistency of PL and WC was more stable over time. Overall, research has shown the potential of multispecies swards containing forage herb PL to ‘produce more while using less’ fertiliser, which is a key requirement of sustainable grazing systems [29]. However, PL can become dominant in a grass-PL sward with the proportion of PL as high as 73% and 82% of the DM yield, while clover can contribute more DM yield in the absence of PL (22% and 17%) [23].

An increased frequency of extreme weather events, such as drought and waterlogging, are now to be expected in the UK under climate change projections, with temperatures increasing >1.5–3 °C in spring and summer and prolonged wet periods during winter with rainfall increasing 24% [30,31,32]. These conditions are often one of the main causes of intra-annual variation in forage yields, presenting challenges for farmers when managing annual feed budgets. Swards which show a low yield response during these challenging conditions achieve the greatest yield stability, which is important to maintain annual feed budgets. The drought tolerant characteristics of PL [21], together with their contrasting growth patterns, have led to the reported higher intra-annual yield stability [33,34].

PL plants are deep rooting, drought resistant and can usually persist for up to 5 years, which is a much shorter lifespan than PRG or WC [35]. Scientific articles suggested that PL plants can tolerate waterlogging stress, but Wilson et al. (2023) [36] reported that following 39 days of high soil water content, there was evidence of photosynthesis reduction (15%), which limited plant growth. The taproot of PL is shallower than that of CH and lucerne (Medicago sativa); however, PL will grow in a range of soil types at various levels of acidity and fertility [37]. PL is regarded as a “low fertility” plant and soil fertility often determines its competitive ability when sown with other species.

Grazing can affect the sward composition of PL-based swards and consequent plant productivity by preferential grazing, recycling of nitrogen in animal excreta and by treading [38,39]. Persistency can be improved by avoiding damage to the tap root when ground conditions are wet in autumn and winter [40]. That said, herb species persistency in general has been reported to decline over time (after two to three years) and is recognised as one of the main drawbacks of intensively grazing such swards [38,41,42]. Avoidance of over-grazing of PL along with the consequent improved survival of the tap root, should improve the persistency [40]. Merino et al. (2024) [43] reported a management strategy that can improve PL pasture resilience, determined a defoliation with a frequency of 25 cm (measured as extended leaf length) every 15 days to balance herbage production and root development, promoting long-term pasture sustainability [44].

Furthermore, PL is responsive to nitrogen fertiliser applications, promoting leaf number, shoot growth and total biomass [45,46]. In natural grasslands PL can tolerate low fertility due to its adaptation to low nutrient conditions [20]. In a farm-scale study in NZ, swards including PL were seen to produce a similar annual DM yield to comparable swards without PL (11.7 t DM ha⁻¹) receiving the same rate for fertiliser application (150 kg N ha⁻¹ yr⁻¹), but with a 25% lower loss of applied N via leaching and no decrease in farm profitability [47]. Studies conducted in Ireland found that pasture with PRG-WC-PL produced similar yields with different applications of N fertiliser, 10 t DM ha⁻¹ with zero N and 11.4 t DM ha⁻¹ with application of 200 kg N ha⁻¹ [48] or 11.5 t DM ha⁻¹ with 75 kg N ha⁻¹ [28]. This ability to grow similar amounts of herbage under lower N rates could result in a decrease in N leaching [15]. Although profitability is rarely assessed within the literature, the ability to achieve similar yields to a PRG sward, with similar rates of artificial fertiliser [18,24,25,26,27,28,29], is a promising agronomic characteristic. However, one of the major drawbacks in relation to agronomy is the shorter persistence of PL; this has the potential to lead to increased frequency of reseeding and/or sward rejuvenation [40], which will have an economic cost.

Table 2. Summary of the characteristics of sward species.

	PL a	PRG b
Soil pH	4.2–7.8	5.0–7.0
Soil temperature (°C at 10 mm)	10–12	Up to 6
Annual rainfall (mm)	Less 500	450–635
Sward persistency (years)	Up to 5	Up to 6
Potential yield (t DM ha−1 year−1)	10–20	14–20

Values shown are the optimum range. ^a Ribwort plantain citations [20,22,23,24,29,30,31,32,40]. ^b Perennial ryegrass citations [21,23,24,25,30,31,32].

Table 2. Summary of the characteristics of sward species.

	PL a	PRG b
Soil pH	4.2–7.8	5.0–7.0
Soil temperature (°C at 10 mm)	10–12	Up to 6
Annual rainfall (mm)	Less 500	450–635
Sward persistency (years)	Up to 5	Up to 6
Potential yield (t DM ha−1 year−1)	10–20	14–20

Values shown are the optimum range. ^a Ribwort plantain citations [20,22,23,24,29,30,31,32,40]. ^b Perennial ryegrass citations [21,23,24,25,30,31,32].

4. Nutritional Value and Bioactive Compounds of Plantain

This limited knowledge is available on the agronomy of PL for grazing pastures in comparison to that available for PRG in the northern hemisphere; however, it has been suggested that PL has agronomic characteristics suited to NI growing conditions. PL offers similar nutritional value to PRG for livestock (Table 3). Minneé et al. (2017 and 2019) [49,50] reported a mean DM content of 19% for PRG, while CH and PL were lower at 11% and 13%, respectively. This was in agreement with Cheng et al. (2017) [51] who reported a DM of ~19%, 14% and 13% for a PRG/WC, PL and CH sward, respectively. Herb species such as PL have been shown to have a lower crude protein (CP) (171 and 191 g kg⁻¹ DM respectively) than PRG (207 g kg⁻¹ DM) [49], RC (204 g kg⁻¹ DM) [39] and WC (214 g kg⁻¹ DM) [24]. More recently, Della Rosa et al. (2022) [52] reported similar findings in two experiments of 15% and 18% DM in PRG and 10 % and 16% DM in PL. In experiment one, CP was 24% less in PL than in PRG (13.3% vs. 17.2%), yet in experiment two the CP % was not significantly different. Though this may be related to the different stages of growth PL was harvested at during experiment one and two, vegetative and reproductive phases, respectively. Neutral detergent fibre (NDF) and acid detergent fibre (ADF) were all greater for PRG than for PL (55.7 vs. 48.6 for NDF and 35.4 vs. 33.6 for ADF) [52]. Langworthly et al. (2023) [53] found PL and/or WC had a lower fibre (NDF and ADF) content than the PRG treatment, which resulted in the cows having a lower fibre diet. In contrast, the non-structural carbohydrates (NSC) and metabolisable energy (ME) were greater for the herb species than PRG [49], although Rodriguez et al. (2020) [54] reported lower ME and NDF for PL sward in comparison to a grass-based sward. A comprehensive plot-based study [24] identified that swards containing PL had more variable nutritional value throughout the grazing season, and typically lower digestibility, in agreement with findings by Hamacher et al. (2021) [55] and Mangwe et al. (2019) [56]. DM digestibility of PL was reported to be between 68% and 85% [52]. Rahaman et al. (2024) [57] showed that cows fed PL had higher grass DM intakes and nutrient digestibility. An earlier meta-analysis [50] concluded that PRG and PL did not differ in total herbage nitrogen content and herbage digestibility, but that there were clear differences in DM and carbohydrate content. In comparison, Della Rosa et al. (2022) [52] found that nitrogen content was 60% lower in PL than in PRG.

Grasses typically have thin roots with high branching intensity and low carboxylate exudation, relying primarily on changes in root morphology to enhance their ability to capture available nutrients. In contrast, herbs generally possess thicker roots with lower branching intensity but high carboxylate exudation, enabling them to mineralise otherwise unavailable nutrients and improve nutrient uptake efficiency [58]. The different root structures can underpin their variable capacities to modify nutrient availability and acquisition in their rhizospheres, which would determine differential nutrient accumulation in their leaves [58] and a greater number of micronutrients from the soil, therefore offering a higher level of micronutrient supply to grazing livestock in comparison to PRG swards [59]. The transfer of micronutrients from the soil to plants and in turn to grazing stock is influenced by a number of factors.

Marino et al. (2024b) [60] reported that the frequency of defoliation of PL can influence its morphological traits with a decreasing carbohydrate content in the leaf, DM and capacity for soil exploration, as a result of less resource uptake from the soil. With PL, as with other grasses [61], PRG, cocksfoot grass (Dactylis glomerata) [62], and CH [63,64], a less frequent defoliation interval can increase lignin, hemicelluloses, and cellulose which consequently reduces herbage intake and digestibility. PL appears to have the ability to cope with frequent defoliation management with little consequential impact on sward density [43].

The mineral content of PL-based swards is more diverse than PRG dominant swards. For example, PL had higher concentrations of calcium (Ca), magnesium (Mg), copper (Cu), iron (Fe), sodium (Na), chloride (Cl), sulphur (S) and zinc (Zn) than PRG [65,66]. While Darch et al. (2020) [67], showed that herbs were highest in iodine (I) and selenium (Se) (two trace elements that are deficient in NI soils and livestock), grasses were higher in manganese (Mn) and legumes in Cu, cobalt (Co), Zn and Fe. Marley et al. (2021) [68] compared the yield and micronutrient composition of pure swards of PL with those of clovers or PRG. PL contained high Ca levels of 18.01 g kg⁻¹ DM compared to CH (12.78 g kg⁻¹ DM), PRG (6.42 g kg⁻¹ DM), WC, (10.25 g kg⁻¹ DM) and RC (14.28 g kg⁻¹ DM) [68]. However, in the same experiment PL was documented to have the lowest levels of Mg (1.06 g kg⁻¹ DM) compared to the other four sward species.

Nguyen et al. (2022a) [69] reported PL integrated with PRG and PRG-WC resulted with higher organic matter digestibility (OMD), ash, starch, NSC, P, S, Ca, Mg, Na, Cl, Zn, B, Co, aucubin, acteoside, and catalpol compared to pasture without PL, but a lower composition of DM %, ADF, NDF, crude fat (CF), Fe, and Mn. However, in the latest [70] report, PL-based pastures have a similar DM yield and contain higher water content, NSC, minerals, and bioactive compounds than the PRG-WC pasture. When comparing to PRG-WC [71], PL had a lower composition of DM (%), organic matter (OM), CF and NDF but contained a higher composition of NSC, K, Na, Ca and S.

A key difference between PL and PRG as forage species for livestock production is the presence of bioactive plant secondary metabolic compounds within PL which are able to confer benefits over and above the nutritional value of the plant [72,73]. The three main bioactive compounds identified in PL are two iridoid glycosides: aucubin and catalpol, and the phenylpropanoid glycoside acteoside. The mechanisms by which these compounds are thought to impact pathways to GHG and nitrogenous emissions from ruminant livestock systems through interactions within both the soil under grazing swards and within the animal are referenced in the relevant sections below.

Table 3. Summary of nutritional value and bioactive compounds of ribwort plantain (PL) and perennial ryegrass (PRG).

	PL a	PRG b
Nutritional value
Dry Matter (DM) (%)	9–20	16–20
Metabolic energy (MJ/kg DM)	10–11	11–12
Crude protein (% DM)	13–20	17–20
Soluble sugar and starch (% DM)	8–12	10–20
NDF (% DM)	30–40	40–50
ADF (% DM)	15–25	30–40
Mineral profile
Phosphorous (% DM)	0.35–0.45	0.35–0.45
Potassium (% DM)	3.10–3.15	3.50–3.70
Sulphur (% DM)	0.35–0.40	0.30–0.35
Calcium (% DM)	1.50–2.00	0.40–0.60
Magnesium (% DM)	0.15–0.20	0.10–0.15
Sodium (% DM)	0.45–0.50	0.30–0.35
Chloride (% DM)	1.70–1.80	1.10–1.20
Bioactive compound
Aucubin (g/kg DM)	6.00–7.00	-
Acteoside (g/kg DM)	8.00–9.00	-
Catalpol (g/kg DM)	0.50–1.00	-

Values quoted are the minimum and maximum within the literature. ^a Ribwort plantain citations [49,50,51,52,65,66,67,68,69,70,71,72,73]. ^b Perennial ryegrass citations [49,50,51,52,65,66,67,68,69,70,71].

Table 3. Summary of nutritional value and bioactive compounds of ribwort plantain (PL) and perennial ryegrass (PRG).

	PL a	PRG b
Nutritional value
Dry Matter (DM) (%)	9–20	16–20
Metabolic energy (MJ/kg DM)	10–11	11–12
Crude protein (% DM)	13–20	17–20
Soluble sugar and starch (% DM)	8–12	10–20
NDF (% DM)	30–40	40–50
ADF (% DM)	15–25	30–40
Mineral profile
Phosphorous (% DM)	0.35–0.45	0.35–0.45
Potassium (% DM)	3.10–3.15	3.50–3.70
Sulphur (% DM)	0.35–0.40	0.30–0.35
Calcium (% DM)	1.50–2.00	0.40–0.60
Magnesium (% DM)	0.15–0.20	0.10–0.15
Sodium (% DM)	0.45–0.50	0.30–0.35
Chloride (% DM)	1.70–1.80	1.10–1.20
Bioactive compound
Aucubin (g/kg DM)	6.00–7.00	-
Acteoside (g/kg DM)	8.00–9.00	-
Catalpol (g/kg DM)	0.50–1.00	-

Values quoted are the minimum and maximum within the literature. ^a Ribwort plantain citations [49,50,51,52,65,66,67,68,69,70,71,72,73]. ^b Perennial ryegrass citations [49,50,51,52,65,66,67,68,69,70,71].

5. The Impact of Feeding Plantain on Animal Production

This limited knowledge is available on the agronomy of PL for grazing pastures in comparison to that available for PRG in the northern hemisphere; however, it has been suggested that PL has agronomic characteristics suited to NI growing conditions. Studies on milk production response to the inclusion of PL in dairy cow diets are limited and are mostly restricted to southern hemisphere regions. However, they have shown beneficial effects on dairy cow performance [74]. For example, Box et al. (2017) [17], observed a 14% increase in daily milk production from late lactation cows grazing a 50:50 PRG: PL mixture relative to a 100% PRG sward. Similarly, Minneé et al. (2017) [49] noted a 17% and 15% increase in milk yield and milk solids production, respectively, for late lactation cows fed PRG swards containing 40% PL swards relative to pure PRG stands [75]. In a meta-analysis, [71] found a higher milk yield (1.02 kg/cow/day) in late lactation cows grazing PL pasture, in addition to a 0.07 kg /cow day increase in milk solids and 23 g in protein; however, fat milk was reduced, and no difference was detected in early lactation. The increase in milk yield was also in agreement with recent work on mixed swards with PL compared to PRG only [53].

In contrast, Mangwe et al. (2020) [76] observed no improvement in milk output but noted improvements in milk solids and beneficial changes to the fatty acid composition of milk from late-lactation cows grazing PL compared to PRG/WC swards (51%). In the latest study, Nguyen et al., 2024 [77] concluded that milk yield and milk composition were not impacted by the presence of PL in the sward. However, TMR was also offered daily which may be the reason behind the null results. On the whole, these results would suggest that milk production is positively impacted by the use of PL swards, which could lead to improved economic returns.

Additionally, it has been shown that the introduction of PL to the diet resulted in reduced milk urea nitrogen (MUN) by 29% compared to pure PRG swards [78]. Nguyen et al. (2023b) [79] reported MUN to be 13.8 mg/dL in PRG compared 10.8 mg/dL in cows grazing PL. Thus, further evidence on the impact of PL inclusion in dairy cow diets is required. In addition, the inclusion of herb species into grazing swards is likely to have a significant effect on rumen function. This may be due to differences in herbage digestibility [24,50], alterations in grazing behaviour [80] or the presence of bioactive plant secondary metabolite compounds [73]. Navarrete et al. (2016) [73] have shown that the bioactive compounds aucubin and acteoside [21], contained in the PL can reduce gas production, ammonia (NH₃) and volatile fatty acid from the antimicrobial activity in vitro, but can improve rumen fermentation and the use of energy sources. Compared to common CH, known too as alternative forage, that can improve the milk fatty acid composition [76], PL produced 40% less NH₃ over 24 h. This impact on rumen function is of particular importance when considering NUE PL in dairy cows. Moreover, some preliminary research has also identified the potential of PL to mitigate methanogenesis in the rumen [81]. If these findings were replicated in practice on-farm, they would represent not only a further reduction in GHG emissions from dairy systems, but also a significant opportunity to reduce energy loss from the rumen associated with methanogenesis (estimated to be between 2% and 12% of gross energy intake), in turn increasing the energy available to the cow for productive purposes. Della Rosa et al. (2022) [52] showed that the CH₄ production (g/d) and CH₄ yield, were, respectively, 23 and 15% less in cows fed PL than those fed PRG. Cows grazing a mix of PL, CH and WC produced 15% less CH₄ daily, on average, compared with cows grazed on PRG-based pastures [82].

In recent years, the emergence of resistance to chemical anthelmintics has forced farmers to seek alternative and more sustainable parasitic control methods. A small number of studies have demonstrated the ability of species-diverse pastures to be used as an alternative, preventative control measure for gastro-intestinal parasite burdens in grass-based systems [83]. Gastro-intestinal parasitic infections will result in reduced feed intake and negatively impact both animal health and production efficiency [84], and the reduced production efficiency associated with internal parasites can contribute to an increase in net GHG emissions [85]. The anthelmintic benefits of certain forage species are usually associated with bioactive compounds within the plant which are directly active against parasites [83]. A number of plant secondary metabolite compounds including saponins, tannins and glycosides have been identified as having anti-parasitic effects in vitro, but confirming their efficacy in vivo is less clear. Tannins, as phytochemicals, inhibit the growth and development of rumen methanogenic and protozoa which helps to reduce methane production up to 55% [86]. In consideration of PL, neither of the primary bioactive compounds present in this species are known to have anthelmintic properties, although some studies have indicated a potential association between PL feeding and reduced faecal egg counts and parasite burdens [87].

6. Nitrogen Use Efficiency and Rumen Fermentation in the Dairy Cow

Incorporation of PL into animal diets has shown positive effects on improving NUE and mitigating N losses from grazing dairy systems [74,88,89]. In some ruminant studies [90], the lower CP content of PL allowed for lower total daily nitrogen intake per cow, significantly reducing N excretion rates and in turn increasing NUE. In addition to this, studies where total N intake rates have been comparable across diets containing varying proportions of PRG-PL, animals still exhibited reduced N excretion rates and improved NUE. For example, Minnée et al. (2020) [91] observed a 41% reduction in urinary N output from dairy cows fed PRG swards containing 45% PL, relative to PRG monocultures, despite comparable N intake rates (545 g N cow day). While initial results are promising, greater investigation is needed to better understand the mechanisms that may be contributing to improvements in NUE.

The three major secondary metabolite compounds with bioactive properties found within PL have reported to have antimicrobial and anti-inflammatory properties [92]. It has also been suggested that they may influence the rumen microbiome, impacting on rumen fermentation parameters, including reducing the production of ammonia [73]. This would influence N utilisation within the animal as ammonia produced from the breakdown of rumen degradable protein (RDP) is in excess of that which can be incorporated into microbial protein in the rumen, and this excess ammonia is absorbed into the bloodstream and ultimately largely excreted as urine urea [93]. The increased mineral content of PL combined with the known diuretic effects of iridoid glycosides has also been indicated to alter N-excretion patterns by increasing total daily urination volume and urination frequency [56], resulting in decreased urine-N concentrations applied to urine patches on pasture, which can lead to significantly reduced N losses due to lower N-loading [89].

Nguyen et al. (2022b) [71] found that UN excretion reduced by 22% due to a reduction in UN concentration and an increase in daily urine volume [94]. Nguyen et al. (2024) [79] confirmed that incorporating 17–28% of Pl in a PRG-WC pasture resulted in a reduced risk of nitrogen losses, with UN concentration reduced by 15–27%, UN excretion reduced by 4–9% and urine volume increased by 20–40%. Eady et al., 2024 [19] reported that dietary PL levels of less than 40% were not effective in reducing N leaching. However, there are numerous studies demonstrating a reduction in N leaching from mixed swards where PL made up 30% of the sward and 21% of the diet [95]. Pinxterhuis et al. (2024) [15] also confirmed that N leaching was 20–60% lower when PL comprised 30–40% of the DM of PL-PRG-WC grazed pastures.

In addition to concerns surrounding nitrogen losses, ruminant livestock production is also a significant source of anthropogenic methane emissions [90,93]. Methane is produced as a by-product of rumen fermentation, representing energy loss from dietary intakes, and is also a potent GHG. There is some evidence that the bioactive compounds present in PL may act to reduce methane production during in vitro fermentations [56,73] and from a small number of in vivo studies in dairy cattle [8,96] and dairy heifers [97] where PL was included in mixed swards (up to six forage species) in comparison to PRG/WC. If PL is confirmed to consistently mitigate methane emissions from grazing dairy cows this would amplify the implied ecological benefits already suggested through decreased nitrogen losses, whilst reducing the feed energy loss represented by methane production in the rumen.

Nitrogen, forming the basis of protein supply to ruminant animals via both rumen-undegradable and microbial protein (synthesised from rumen-degradable protein and non-protein nitrogen), is an essential component of the diet for ruminant livestock. However, ruminant animals are notoriously inefficient in terms of their utilisation of ingested nitrogen, leading to a typically low NUE. With dairy cattle, approximately 75% of ingested nitrogen is ultimately excreted in faeces and urine and only around 25% retained for productive purposes (growth/maintenance, reproduction and milk production) [98]. Whilst faecal N is relatively stable, the majority of nitrogen excretion from dairy cattle is in urine, which is much more susceptible to rapid N losses. Urine urea is readily converted to ammonium (NH⁻₄) and NH₃ which is easily volatilised and contributes to air pollution, particulate formation and eutrophication, nitrate (NO⁻₃) which is prone to being lost via leaching, and N₂O, nitric oxide (NO) and N₂ emissions [99]. Of particular concern are N₂O emissions, with over 60% of global anthropogenic emissions of this potent GHG attributed to agriculture [99]. Eady et al. (2024) [19] found no effect of PL on N₂O emissions, but other authors [100,101] have shown contrasting results of a reduction of up to 39% in N₂O emissions.

Reducing N losses from agricultural systems is therefore crucial to mitigating the harmful environmental impacts of N₂O emissions and other nitrogenous pollutants. However, whilst agricultural systems and specifically dairy farming are under increasing pressure to mitigate N losses and GHG emissions, targets for reducing the environmental impact of dairy farming must be met whilst maintaining high levels of productivity to provide food for a growing global population, with increasing demands for meat and milk products in particular [102].

For dairy cattle in particular, where production demands are high, it is predominantly DM and energy intake which have a limiting effect on both production and the transformation of ingested CP to microbial protein in the rumen [93]. CP (and specifically RDP) is rapidly broken down into ammonia; if it is in excess of what can be incorporated to microbial protein, ammonia then accumulates in the rumen before it is absorbed via the rumen wall into the bloodstream. At this point, it is detoxified in the liver to form urea and ultimately excreted. As total N intake, measured as dietary CP (N*6.25), is normally supplied in excess of the requirements for dairy cows, research studies thus far have largely focused on methods to mitigate N losses and improve NUE by reducing the total dietary N intake [103], and it has been established that there is a strong linear relationship between N intake and total N excretion where dietary CP is reduced to as low as 15% [104]. Furthermore, Doran et al. (2023) [105] declared that in grazing late lactation dairy cows a 5% CP reduction in the concentrate feed resulted in reduced N in urine (0.9%), in milk (9.8%) and an increased N in faeces (14%).

PL has been suggested to mitigate N losses from cattle via multiple pathways. In some studies, the CP content of PL has been considerably lower than PRG [91], but overall dietary N intakes between treatments have been comparable [72], and the effects of PL on altering NUE/N excretion patterns must be attributed to other factors, including the action of bioactive compounds found within PL [106].

7. The Environmental Impact of Introducing Plantain to Grazing Swards

There is significant pressure to optimise farming systems to reduce environmental impacts. Ruminant livestock, in particular cattle, are often criticised for the production of GHGs such as methane (CH₄) and N₂O [107,108]. The nutritional makeup of the diet is one factor that will influence the level of emissions produced by ruminants [109,110]. Cattle excreta deposited on grazed grassland is a major source of direct and indirect emissions of N₂O, which has a global warming potential 298 times greater than CO₂ over a 100-yr time horizon [111,112]. Emissions can vary greatly depending on the type of excreta (dung or urine), soil type, timing of application and weather conditions (particularly moisture and temperature). The N₂O emission factor for urine patches was recorded at 0.77% for wet climates and 0.32% for dry climates with an average of 0.67%, but more studies need to be made across sites/regions differing in climate and soil to improve the quantification of N₂O emissions at the national scales [113]. In fact, Ding et al. (2024) [114] observed a reduction in N₂O emissions with the use of PL monoculture swards in the well-draining allophanic soil but not in the poorly draining gley soil. PL inclusion in grass-clover swards reduced drainage water volume by 1–16 %, potentially contributing to nitrate-N loss mitigation in a range of different soils [115]. Earl-Goulet et al. (2023) [116] reported no significant effect on the soil type but did for the timing of fertiliser application. February recorded the highest N₂O-N emissions which were attributed to cooler conditions reducing microbial activity, subsequently reducing nitrification and denitrification rates.

In a study at three sites in Ireland, Krol et al. (2016) [117], found that N₂O emission factors vary seasonally with the highest emissions in autumn (particularly at sites with imperfectly drained soil). Animal diet is also very important, with 70–95% of N ingested by ruminant livestock being returned onto pasture as dung and urine due to low NUE [118,119]. Emissions of N₂O arising from these returns comprise over 40% of the N₂O associated with animal production systems [118]. Therefore, N intake is a principal driver of N losses from ruminant livestock returns [105,120].

In a recent study from New Zealand, [105,121] showed that increased PL content of swards and grazing animal diet reduced the N concentration in urine, which in turn can reduce N₂O emissions. Similarly, Pijlman et al. (2020) [122] noted 26% lower N₂O emissions following fertiliser application to PL swards relative to PRG on Dutch peat soils [123]. Simon et al. (2019) [124] suggested that the efficacy of PL as an N₂O mitigation option is due to both a reduction in urinary N excretion and a plant effect. The latter could be due to biological nitrification inhibition caused by the release of root exudates and/or changes in the soil microclimate. Indeed, recent research in New Zealand is evaluating the potential application of aucubin (secondary compound found in PL) as a nitrification inhibitor to reduce N₂O emissions [15,125,126].

Furthermore, there are only a limited number of reports in the literature on the potential benefits of PL regarding emissions from grazing ruminants (Figure 1), some findings are contradictory, and studies have used an array of methodologies which makes comparison of results more challenging. That withstanding, PL-based swards and other multi-species swards have aroused much interest as it is thought that the deep rooting species can break up compacted soils, place carbon deeper into the soil profile and improve soil organic carbon, leading to improved soil carbon sequestration. Indeed, previous work in the USA compared one, four, eight and sixteen grassland savanna plots over a 12-year period and demonstrated that monoculture plots lost carbon, while carbon content increased in more species-diverse plots [127].

8. Research Gaps and Applications in Temperate Northern Climates

In order to establish the suitability of growing PL as a forage crop within a simple mixed species grazing sward, and its ability to meet the demands of forage production for Northern Irish farms, more research trials are needed. Existing research from the southern hemisphere indicated that PL is capable of producing DM yields to rival PRG [22], and has a favourable growth pattern, with growth rates within the grazing season and grazing tolerance [18] likely to suit dairy production systems in NI. However, optimal establishment strategies and its persistency and growth rates under the Northern Irish climate require evaluation before it can be recommended as a suitable forage for local farms. If PL is to be recommended as a mitigation strategy for reducing the environmental footprint of pasture-based dairy production, its environmental credentials must also be fully evaluated under NI conditions beforehand.

Research is still required globally to ascertain the optimal proportion of PL, grasses and legumes in grazing swards for animal performance and to maximise the associated environmental benefits. Clarity is also required on the most appropriate nutrient and land management strategies to support swards containing PL, and any impacts on soil carbon stocks following the introduction of PL into grazing swards. Greater clarity is also required on the nutritive value and mineral content of PL grown in different soil types, and how these values may vary through the grazing season in order to inform accurate ration development for the supplementary feeding of high-yielding dairy cows grazing pastures containing PL.

The impact of including PL in grazing swards on both animal intakes and grazing behaviour is not yet well understood and research is needed alongside animal productivity measures [77]. There also remains a large knowledge gap relating to rumen function and how PL and its bioactive compounds may influence the structure and function of the rumen microbial community. Rumen sampling and analysis will begin to address this question and likely shed light on the mechanisms by which PL has been shown to impact methane enteric emissions and rumen ammonia levels reported in previous studies [15,52,95].

9. Conclusions

There is clear potential for the inclusion of PL in grazing swards to reduce the environmental footprint of dairy production in NI. In the southern hemisphere cows grazing PL have been shown to have reduced urinary N concentrations, leading to lower nitrogenous losses from urine patches on soils and reducing nitrate leaching [114], whilst animal productivity is maintained or improved. There is now a need to confirm these advantages can be replicated in NI, and to develop optimised protocols for the management of swards containing PL throughout the growing season.