Introduction
The world’s population is expected to grow by 8.4% between 2023 and 2032, leading to a food consumption growth in demand of 15% during the same period (OECD/FAO, 2023). As a consequence, agricultural greenhouse gas (GHG) emissions are expected to grow by 7.5% globally in the same period in a business-as-usual situation (OECD/FAO, 2023). This shows the impact demographic changes will have on the food production system with the requirement to feed a growing population. This requirement will need to be achieved in a sustainable way by limiting or reducing the environmental impact of food production and consumption.
Agricultural production can have an important impact on the environment and the planet’s physical boundaries. Out of the nine planetary boundaries defined by Rockström et al. (2009) (land-system change, freshwater use, biogeochemical flows, biosphere integrity, climate change, ocean acidification, stratospheric ozone depletion, atmospheric aerosol loading and introduction of novel entities), two were found to be transgressed (biosphere integrity and biogeochemical flows) and three were deemed uncertain (land-system change, freshwater use and climate change). Agriculture was identified as a major driver of the negative evolution of biosphere integrity, biogeochemical flows, land-system change and freshwater consumption boundaries. Agriculture was also identified as a significant driver of the deterioration of the climate change boundary (Campbell et al., 2017). Proteins, which are composed of amino acids and are necessary for several key human body processes such as growth, maintenance, repair of damaged tissue and enzyme production (FAO, 2022), are central to human nutrition. On average between 2020 and 2022, 37% of protein consumption globally came from animal products. In developed countries, 58% of the total protein consumed came from animal products, with 29% of the protein derived from meat consumption alone. These distributions of dietary protein sources are projected to slightly increase by 2032 to 58.1% and 29.7%, respectively (OECD/FAO, 2023). Additionally, between 2020 and 2022, 67% of the global agricultural land was used for permanent meadows and pasture (FAOSTAT, 2024), and 62% of global agricultural GHG emissions caused by livestock and crop production came from ruminants alone (OECD/FAO, 2023). Greenhouse gas emissions caused by ruminants are expected to grow by 9.8% between 2022 and 2032 (OECD/FAO, 2023). The large contribution of the livestock sector to global GHG emissions, combined with its importance in the agricultural sector and its large proportion of protein in human diets, especially in developed countries, presents challenges for the current food system to sustainably feed the growing global population.
Plant-based protein sources are an alternative to livestock-based protein. The two main dietary plant-based protein sources are cereals and legumes. However, these two categories can be very different in terms of nutritional quality, economic returns for producers and environmental impact. Moreover, cereals and legumes can have complementary characteristics when included in crop rotations, which represents a further opportunity for the agricultural sector (Nemecek et al., 2008; Preissel et al., 2015). In 2022 in the European Union (EU hereafter), emissions from livestock production (i.e. enteric fermentation and manure management) represented 66% of the total agricultural GHG emissions while agricultural soil management and carbon-based fertilisers (e.g. urea) combined represented only 32% of those emissions (EEA, 2024). In Ireland, emissions from the livestock sector represented 77% of agricultural GHG emissions while soil management and carbon-based fertilisers represented 23%. On an individual product basis, Poore & Nemecek (2018) found that lamb and mutton meat production generated four times more GHG emissions per 100 g of protein than grains (e.g. wheat, maize, etc.) and 13 times more than pulses. Similarly, cattle production generated 11 times more GHG emissions per 100 g of protein than grains and 33 times more than pulses. Crop production could then represent an opportunity to ensure food security in a more environmentally sustainable way. Moreover, legumes can fix nitrogen (N) through biological N fixation (BNF), hence reducing the need for synthetic N fertiliser inputs and their associated emissions (Fujita et al., 1992).
A comparison of a broad range of plant-based protein sources could provide valuable insights for land use decisions and policy. Such a comparison should include economic, environmental and nutritional aspects as shifts in production need to be carried out by farmers who are unlikely to do so unless it is economically attractive or at least equivalent to current activities. Yet, while much of the existing literature involves the evaluation of different protein sources in economic (Varela-Ortega et al., 2022), environmental (Detzel et al., 2022) and nutritional (Poore & Nemecek, 2018; Adhikari et al., 2022) terms, few studies combine more than one of these aspects (Bohrer, 2017; Mosnier et al., 2021; Moughan, 2021). In addition to bringing a more holistic evaluation of protein sources, this comparison of individual plant-based protein sources could help determine optimal crop rotation scenarios, given the complementary nature of different types of crops.
This research therefore examines two research questions. First, how do cereals and legumes compare in the Republic of Ireland (described as Ireland hereafter), in terms of economic, environmental and nutritional characteristics? Second, what is the optimal cereal and legume crop rotation to reduce GHG emissions while at least maintaining protein production and profit?
The paper is structured as follows; the second section provides a contextual background to the research questions and the third section presents the data and methodology used for the analysis. The penultimate section presents the results, and finally potential policy implications are discussed in the last section.
Background
The economics of plant-based protein sources
Investment in plant-based protein and alternatives to animal-based protein sources is growing rapidly. Between 2019 and 2021, global investment in plant-based proteins more than doubled, reaching $1.9 billion in 2021 (Good Food Institute, 2023). Bloomberg (2021) estimates that the market for plant-based proteins will increase 5.5-fold between 2020 and 2030 to reach $162 billion. Despite this rapid increase, the market is still projected to be seven times smaller than the animal and dairy protein market.
The production of cereals greatly exceeds that of legumes in Europe. In 2022, 271 million tonnes of cereals were produced in the EU. This contrasts with 4.7 million tonnes of dry pulses. Moreover, 24 times more land area in the EU were dedicated to cereals compared to dry pulses and protein crops (Eurostat, 2024a). Only a minority of the crops produced in the EU-27 are used for human consumption. In 2021/2022, only 17% of the total cereal supply in the region was used for direct human consumption. Most of the cereals produced (47%) were used as animal feed. Other uses included industrial use, exports and seed production (European Commission, 2023a). The disparities between cereal and legume production could however partially be explained by the fact that cereals are also used as a source of starch in diets, when legumes are mostly used as a protein source only. Moreover, introducing legumes into crop rotations presents more agronomic constraints (e.g. rotation constraints, pest management) compared to tillage systems based on cereals only.
Integrating cereal and legume production more efficiently through crop rotations tends to show economic benefits. A long-term experiment across seven sites in Europe showed that yields of both winter and spring cereals increased when produced in rotational systems compared to monoculture. Furthermore, yield losses were limited in cases of adverse weather conditions (temperature or precipitation changes) in crop rotations compared to monoculture (Marini et al., 2020). A systematic review of grain and legume rotation systems indicated that in 35 out of the 53 studies cited, gross margin was higher for rotations with legumes (Preissel et al., 2015). Some studies which included legumes in crop rotations reported an increase in gross margin of between 1.4% and 4.7% compared to rotations without legumes in four study sites across Europe (Nemecek et al., 2008). The increase in gross margin was due to a reduction in input costs, mainly fertilisers, and not due to yield increases. However, the economic benefits from crop rotation can depend on external factors and specific, regional agronomic factors (Reckling et al., 2016). The relative efficiency of crop rotations including legumes as opposed to monocultures or rotations composed exclusively of cereals also depends on the level of input used and rotation position (Forristal & Grant, 2011). Optimising the choice of the species used in the rotations and the input levels also contributes to the system’s performances.
The environmental contributions of plant-based protein sources
Plant-based protein sources have several advantages in terms of carbon footprint compared to animal-based proteins (Sabaté et al., 2014; Detzel et al., 2022). However, plant-based protein sources are not homogenous in their environmental impacts. For example, Poore & Nemecek (2018) found that grains (e.g. wheat, maize, etc.) emitted more GHG emissions per 100 g of protein than peas and other pulses (e.g. beans), but also that pulses and nuts required more land. Grains were responsible for more water eutrophication, and peas and nuts for more water scarcity.
Cereals and legumes associated in crop rotations can show complementary environmental characteristics. Integrating cereal and legume species leads to higher yields and reduced input levels due to legumes’ ability to fix N in soils (Hauggaard-Nielsen et al., 2001; Tsialtas et al., 2018). More specifically, legumes contribute to more efficient use of resources through N fixation, which improves soil productivity and decreases the need for synthetic N fertilisers (Semba et al., 2021). For example, the introduction of peas and soybean into cereal-based cropping systems in four case studies led to a decrease in yearly N fertiliser use per hectare by approximately 27.1%, and to a lower energy demand, thus reducing global warming potential (Nemecek et al., 2008). A systematic review of 53 studies introducing legumes (chickpea, faba bean, vetch and pea) into cereal crop rotations showed a decrease in N fertiliser use of between 23 and 62 kg/ha for the cereals following the legumes, compared to pure cereal rotations. The magnitude of the reduction depends on the rationale behind the introduction of the legumes, i.e. if the legumes were introduced to maximise fertiliser savings potential or yield potential (Preissel et al., 2015).
Evaluating the inclusion of legumes in predominately cereal rotations will help determine the impact on environmental and economic indicators.
Nutritional characteristics of cereals and legumes
Legumes have a higher protein content per 100 g of dry matter than cereals. For example, oats, wheat and barley contain 10.8, 12.6 and 11.3 g of protein per 100 g of dry matter, respectively, compared to 40.2, 29.5 and 23.3 g per 100 g for soya beans, faba beans and peas, respectively (INRAE/CIRAD/AFZ, 2021). In terms of amino acid composition, cereals and legumes can complement each other, as cereals are rich in methionine and cystine, which are the limiting amino acids (amino acid present in the least amount compared to reference requirements) of legumes, while legumes are rich in lysine, which is the limiting amino acid of cereals. In the context of protein quality, one of the principal metrics is the digestible indispensable amino acid score (DIAAS; FAO, 2013). The DIAAS evaluates the proportion of protein that will be utilised by the body after digestion. Legumes report a higher or at least similar DIAAS compared to cereals. For example, peas show a DIAAS of 70, against 48 for wheat or 57 for oats (Herreman et al., 2020). This means that 70% of the amino acid content of peas can be used by the body after digestion as against 48% and 57% for wheat and oats, respectively. However, associating cereals and legumes in efficient mixed diets can lead to the complementarity in amino acids, ensuring a balanced diet (Herreman et al., 2020). Moreover, studies on crop rotations across four European sites also showed that crop rotation systems including legumes gave lower gross energy yields compared to crop rotation systems composed of cereals only (between −1% and 23%), but that the raw protein yields were increased (between +1% and + 11%) (Nemecek et al., 2008).
Protein transition in Ireland
This study focuses on Ireland, a country with several interesting characteristics which make it a suitable case to address the research questions.
Despite the economic, environmental and nutritional opportunities associated with legume and brassica production, production of oilseeds, beans and peas in Ireland is lesser than that of cereals. In 2023, the production of those three crops combined was only a fifth that of spring barley alone (CSO, 2024a). Legumes in particular come with agronomic constraints that might act as a disincentive for farmers to start producing them (Notz et al., 2023). For example, they need to be integrated in rotation and cannot be grown on more than 20% of the farm area, contrary to cereals that can be grown in monocultures as well as in rotations. Legumes also present challenges related to weed and pest management (Notz et al., 2023). However, Ireland has a lower share of arable land in the total utilise agricultural area (UAA) compared to EU countries. In 2016, the share of arable land in the total UAA in Ireland was the lowest in Europe at 9.4% (Eurostat, 2024b). However, Ireland has the highest yields per hectare among the EU-27 for wheat, barley, oats, oilseed, beans and peas (European Commission, 2023a). High yields for cereals are also possible due to the extensive use of herbicides and fungicides, which represents important direct costs. Therefore, Ireland has the potential to be a highly competitive crop producer in Europe but it also faces relatively high production costs, especially compared to other EU member states. For example, Ireland had the second-highest ammonium nitrate, sulphate of ammonia and sulphate of potash prices in the EU in 2021 (Eurostat, 2023).
In terms of bio-physical attributes, Irish soils have the potential for greater production of beans and peas. Ireland’s surface area is 7.1 million hectares, 4.19 million or 59% of which are under agriculture. Among those 4.19 million hectares, 10% were under cropland in 2020 (CSO, 2022). However, according to Gardiner & Radford (1980), 29.7% of Irish soil can be considered as highly suitable or suitable for tillage, i.e. able to be used for legume production. However, legumes offer only a short sowing cycle (Peoples et al., 2019) and are highly sensitive to climate conditions (Manners et al., 2020), while peas and beans are well adapted to the Irish climatic conditions. Often qualified as cool-weather legumes, peas and beans perform well in the Irish climate, classified under the Köppen-Geiger climate classification as temperate, without dry season and with cold summer (Peel et al., 2007). In 2023, Irish beans and peas showed the highest levels of yield per hectare in EU standard humidity among the EU-27 (Eurostat, 2024a). Ireland’s suitability for bean and pea production is also expected to increase due to changes in future climate conditions (Manners et al., 2020). Hence, there is significant potential for more legume production in Ireland. Moreover, Ireland has set ambitious climate change targets, including a 51% reduction in total GHG emissions by 2030 compared to 2018 levels. The agricultural sector has been set a 25% emissions reduction target by 2030 (Government of Ireland, 2022). However, according to the latest projections, agricultural emissions are expected to decrease by only 1% compared to 2018 levels in a business-as-usual scenario and by 17% if significant additional measures are implemented (EPA, 2024a). Introducing more legume production could assist the agricultural sector reach this target.
Ireland is hence an interesting case study for the present analysis due to its proportion of national emissions generated by agriculture, the significant GHG emissions reduction target for the sector and for its physical ability to increase its crop and legume production.
The policy environment surrounding protein crops in the EU and in Ireland
An overview of the European and Irish policy context is needed to understand the current situation when it comes to plant-based protein production. The main framework determining the agricultural policy in the EU is the Common Agricultural Policy (CAP). Created in 1962, it targets farmers and the agricultural sector directly, mainly through income support and policies for rural development (European Commission, 2023b). The original actions of the CAP were targeted at market supports, but they eventually led to an excess in supply as well as budgetary and regulatory challenges in terms of compliance with international trade rules. In 1992, the so-called “MacSharry reforms” of the CAP introduced direct payments to farmers based on the area cultivated or the number of certain livestock kept. Starting in the beginning of the 2000s, payments to farmers were decoupled from production and support for rural development was introduced (European Council, 2023).
However, some of the CAP strategies and reforms are questioned for their effects on the European agricultural sector’s production structure and environmental impact (Recanati et al., 2019; Heyl et al., 2020). First, despite the progressive decoupling of farm payments, sectors suffering from structural or conjectural issues can still benefit from coupled payments under voluntary coupled support (VCS) schemes. This scheme is limited in the amount and in the sectors that can benefit from it. These sectors include cereals, oilseeds, protein crops, milk and milk products, sheep meat and goat meat, beef and veal (European Commission, 2023c). In 2022, 38.8% of the VCS was targeted at beef and veal, 21.4% at milk and only 11.3% at protein crops (European Commission, 2022). An analysis of the CAP 2014–2020 using the CAPRI model (European Commission, 2021) found that if no VCS were granted, the number of hectares under arable crops in the EU would increase by 0.9% and the number of hectares under pulses would increase by 1.6%. Conversely, ruminants and dairy herd sizes would decrease by 0.6% each, and beef and meat activities would decrease by 2.7% (European Commission, 2021).
The CAP 2013 reform introduced the concept of “greening”, according to which the CAP payments would be conditioned to sustainable agricultural practices. Crop diversification and crop rotation were listed as qualifying sustainable practices (European Council, 2019). In the CAP 2023 reform, Ireland was one of the few EU member states to include both crop rotation and crop diversification in their CAP Strategic Plan 2023–2027 (as opposed to crop rotation only for the other member states) as qualifying practices to respect the “Good agricultural and environmental conditions” on soil protection and quality (European Commission, 2023d).
In summary, the European CAP was found to have historically favoured livestock production over arable crop production. However, in the last decade, reforms aimed at promoting sustainable agricultural practices, including financial support towards more developed and diverse crop production structures, have been implemented. This can be seen as a response to the growing protein deficit in the EU. In 2022/2023, the European self-sufficiency rates for oilseeds and protein crops were only 63% and 81%, respectively (European Commission, 2023a). For example, Ireland imports 36% of its wheat and 100% of its soya (Eurostat, 2024c, 2024d).
Given the unfulfilled potential of plant-based protein production in Ireland, this analysis then aims at evaluating the performance of plant-based protein crops in Ireland, as well as investigating scenarios that would allow an optimal integration of cereals, legumes and brassicae in the Irish arable system.
Materials and methods
In this analysis, indicators are used to evaluate the economic, environmental and nutritional performances of cereals and legumes using data from Ireland. Crop rotation in Ireland is also studied under these headings to explore scenarios where legumes and oilseed rape are introduced in a crop rotation scenario. The following sections describe the approach and data used in the analysis. We base our analysis on the Teagasc National Farm Survey (NFS) data (EU Farm Accountancy Data Network collection method for Ireland), which are used to derive performance indicators to evaluate crops produced in Ireland on a stand-alone basis. We then use those indicators to develop crop rotation scenarios and assess their suitability and sustainability in the context of the Irish arable sector.
The indicators
This analysis compares cereals and legumes in terms of economic, environmental and nutritional indicators. The economic indicator is the gross margin per hectare in euros, calculated as the difference between the gross output and the direct costs. For cereals, the gross margin includes the margin derived from straw as well as grain. Subsidies and direct payments are not included in the gross margin. Gross margin for barley does not account for malting barley but only for feed and food market prices. In order to account for the differences in direct costs between tillage farms using contractors (accounted for in the direct costs) and tillage farms using their own machinery (not accounted for in the direct costs but in overhead costs), we used a contractor cost proxy for farms using their own machinery and labour based on the costs borne by farms using contractors. The proxy used was the weighted average cost of contractors per hectare by crop which can be found in Appendix A (Table A1). It was then multiplied by each crop’s surface for farms using their own machinery. The figures used to approximate the contractor costs per hectare do not account for the cost of owned machinery or labour. Given the reporting of farms’ machinery costs in the NFS, it is difficult to distinguish between activities farms carry out using their own machinery and those activities outsourced to contractors. Hence, our estimations might underestimate the machinery costs borne by tillage farms. This is however assumed not to change the relative economic efficiency of different crop types in terms of gross margin comparison. The environmental impact is restricted in this instance to GHG emissions in kilograms of CO2 equivalent using the Intergovernmental Panel on Climate Change (IPCC) methodology following Buckley & Donnellan (2022) and as recommended by the Irish Environmental Protection Agency (EPA, 2024b). The IPCC methodology relies on three tiers that represent the level of data precision, with precision increasing from Tier 1 to 2 to 3 (IPCC, 2019). The choice of Tier was determined by data availability. When data were available, Tier 2 factors were used. Otherwise, Tier 1 was the default (IPCC, 2019). We do not consider N oxide (NOx) emissions from legume N in the GHG emissions calculation for spring peas and spring beans. Legumes’ BNF was found not to be a direct source of nitrous oxide emissions (Zhong et al., 2009). Moreover, cumulative nitrous oxide emissions per hectare were found to be lower for legumes than for cereals over a 6-year period (Garnier et al., 2024).
The nutritional indicators are the gross protein yield and the digestible protein yield. The gross protein yield is obtained by multiplying the protein content of each source by the crop yield. The digestible protein yield is the gross protein yield corrected for digestibility using the DIAAS, as done by Moughan (2021). Correcting the protein yield by the DIAAS gives the amount of protein produced that will be effectively used by the human body after the digestion process, i.e. the amount of protein available to the body. The DIAAS is presented by the FAO (2013) as the favoured metric to evaluate protein quality. It “can be used as a means of defining protein equivalent intake (protein adequacy), when it is multiplied by the actual protein content or intake (i.e. measured protein intake times DIAAS)” (FAO, 2013). We focus on proteins to account for the nutritional performance of the arable sector in Ireland. Amino acids, the main component of proteins, are the building block of several physiological processes and essential amino acids cannot be stored by the body, hence they have to come from the diet (EFSA, 2012). Protein is also a segment of the food sector that requires particular attention given the environmental challenges surrounding its production and the current protein deficit observed in Europe and Ireland (European Commission, 2023a).
Results are reported by unit of product (e.g. kg) and on a per hectare basis. Greenhouse gas emissions are also expressed per 100 g of gross and digestible protein to consider fully the environmental efficiency of protein production. The cereals considered in this analysis are winter and spring wheat, winter and spring barley and winter and spring oats. The legumes considered are spring beans and spring peas. The oilseed break crop considered is winter oilseed rape. Protein sources were chosen depending on the availability of agronomic data.
The data
The economic and environmental indicators are developed using data from the Teagasc NFS from 2016 to 2021. The NFS is part of the EU Farm Accountancy Data Network. This is a network monitored by the European Commission which gathers harmonised data on the economic performance of farms across the EU. The NFS data are collected every year at the farm level on a random sample of farms by a team of trained data recorders. A weight is assigned to each sample farm based on farm size and system to make them representative of the main land-based agricultural systems across Ireland. However, pigs, poultry, horticulture and very small farms are not included in the NFS data. The results here are population weighted. Outliers were detected and removed from each crop type for the number of hectares, the yield, the gross margin and GHG emissions. This was done by removing observations below the 0.025 quantile and above the 0.975 quantile of the distribution. We only consider farms specialised in arable production as per the NFS definition of farm systems based on the standard output.1 This ensures that the farms in the final sample have soils suitable for arable cropping and limits the possibility that the primary rationale behind producing crop is producing feed for on-farm livestock. The final sample considered here of data from 2016 to 2021 consists of 30 farms producing oilseed rape over the 6-year period, 144 farms producing winter wheat, 32 producing spring wheat, 196 producing winter barley, 242 producing spring barley, 75 producing winter oats, 74 producing spring oats, 46 producing spring beans and 9 producing spring peas.
Data on the nutritional composition of the selected protein sources were taken from the INRAE/CIRAD/AFZ Feed Tables (2021) and from existing literature as shown in Table 1. The protein content of each crop is computed as follows:
Coefficients for nutrition indicators of cereals and legumes
| Oilseed rape | Oats | Wheat | Barley | Bean | Peas | |
|---|---|---|---|---|---|---|
| Dry matter (g per 100 g) 1 | 92.4 | 87.6 | 86.9 | 87.2 | 86.6 | 87.2 |
| Protein content (g per 100 g of product) 1 | 18.6 | 9.4 | 11 | 9.9 | 25.5 | 20.3 |
| Protein content (g per 100 g of dry matter) 1 | 20.1 | 10.8 | 12.6 | 11.3 | 29.5 | 23.3 |
| DIAAS (%) 2 | 67 | 57 | 48 | 47 3 | 55 | 70 |
The quantity of gross protein produced is computed as follows:
with j the farm considered.
The quantity of digestible protein produced is computed as follows:
Table 1 presents the dry matter and protein contents as well as the DIAAS coefficients used for the cereals and legumes considered in the analysis.
A sensitivity check was performed using agronomic recommendations for N input of cereal crops Ireland (Teagasc, 2017a, 2022) and the Central Statistics Office’s yield data for the period 2016–2021 (CSO, 2024a). This was undertaken to sense check the level of inputs and yield expected from different crops grown in Ireland. Complete results of the Tukey’s test performed to evaluate the statistic differences can be found in Appendix B (Table B1). For all crops, the levels of N fertilisers and yield per hectare observed in the NFS are not statistically different than the national averages for yields and the recommended values for fertiliser.
Crop rotation in Ireland
Based on the environmental efficiency of the protein sources considered, we investigated possible crop rotation strategies for tillage farms. To do so, we developed scenarios where farms producing only wheat, barley or oats over the 6-year study period would convert 20% of the area used for cereal production to legumes, and 20% of the area used for cereal production to oilseed rape. In the first scenario, the legume included was spring bean, and in the second scenario, the legume included was spring pea. Winter oilseed rape was included in both scenarios for several reasons. First, it offers further diversification of species by including a brassica. Second, studies show that introducing legumes in crop rotations that also contain oilseed rape presents higher economic and environmental benefits compared to introducing legumes in crop rotation without rapeseeds (Preissel et al., 2015; Reckling et al., 2016). Moreover, both legumes and brassicas can counter some pests and diseases affecting cereal crops, such as the Gaeumannomyces tritici fungus threatening wheat production (Hauggaard-Nielsen et al., 2001; Teagasc, 2017b). Converting 20% of land used for cereals to legume production and a further 20% of that land to brassica production also ensures compliance with the CAP 2023–2027 conditionality requirements to receive subsidy payments. Indeed, one of the requirements of the CAP focuses on crop rotation on arable land and aims at promoting crop diversity (Government of Ireland, 2023a).
Table 2 presents the number of crops grown across the NFS sample for farms over the period 2016–2021. The first section of the table (A) shows the overall crop production across all tillage farms. This includes barley, wheat, oats, rye, beans, peas, triticale, potatoes, beet, oilseed rape, linseed, rhubarb, maize, kale and rape. The second section (B) focuses on farms producing only the crops evaluated in our analysis, i.e. wheat, barley, oats, oilseed rape, peas and beans. The third section (C) focuses only on farms with the cereals included in our analysis, but excludes farms that are already growing beans, peas and oilseed rape. Considering all crops, 47.3% of farms specialised in tillage produced three or less crops between 2016 and 2021. On average, farms produced 3.97 different crops over the 6-year period 2016–2021 and 2.7 different crops per year. When considering only spring and winter wheat, spring and winter barley, spring and winter oats, winter oilseed rape, spring peas and spring beans, 42.1% of farms produced only one or two of those crops over the period 2016–2021. They produced on average a little <3 different crops over the total period and 2.3 different crops per year. When investigating wheat, barley and oats production only, 49.5% of the farms produced only one or two of those crops over those 6 years. On average, they produced 2.60 different crops over 6 years and 2.06 different crops per year. Based on the status quo between 2016 and 2021, there is potential in Ireland for introducing legumes and oilseed rape in existing cereal production systems to follow a diversification strategy.
Diversification of crop production in Ireland over the period 2016–2021
| Number of crops grown over 6 years | Number of farms | Percentage | Cumulative percentage |
|---|---|---|---|
| (A) All crops considered | |||
| 1 | 20 | 17.9 | 17.9 |
| 2 | 14 | 12.5 | 30.4 |
| 3 | 19 | 17.0 | 47.3 |
| 4 | 16 | 14.3 | 61.6 |
| 5 or + | 43 | 38.4 | 100 |
| Total | 112 | 100 | 100 |
| (B) Only considering wheat, barley, oats, oilseed rape, peas, beans | |||
| 1 | 24 | 22.4 | 22.4 |
| 2 | 21 | 19.6 | 42.1 |
| 3 | 26 | 24.3 | 66.4 |
| 4 | 17 | 15.9 | 82.2 |
| 5 or + | 19 | 17.8 | 100 |
| Total | 107 | 100 | 100 |
| (C) Removing farms already producing oilseed rape, peas or beans | |||
| 1 | 28 | 26.2 | 26.2 |
| 2 | 25 | 23.4 | 49.5 |
| 3 | 27 | 25.2 | 74.8 |
| 4 | 15 | 14.0 | 88.8 |
| 5 or + | 12 | 11.2 | 100 |
| Total | 107 | 100 | 100 |
To capture the impact of the rotation scenarios, we applied the results per hectare found in the first part of the analysis evaluating legumes and oilseed rape on a stand-alone basis to the new number of hectares allocated to the legume considered. Therefore, the first scenario presents the change in gross margin, GHG emissions and protein yield per hectare between the status quo and a rotation where 60% of the area is used for cereal production, 20% for winter oilseed rape production and 20% for spring bean production. The second scenario presents the changes, for the same variables, between the status quo and a rotation where 60% of the area is used for cereal production, 20% for winter oilseed rape production and 20% for spring pea production. The rotation position of each crop was assumed to be left at the discretion of the farmer.
Preissel et al. (2015) and Notz et al. (2023) studied the effect of incorporating legumes in existing European cropping systems on several variables including N fertiliser use and protein yield. Considering experiments conducted in the United Kingdom and Ireland, the N fertiliser use decreased by 24.5% on average across the case studies, and an average yield gain of 15% was observed over the entire crop rotation. This means that introducing one legume into the crop rotation had benefits over all other crops produced as part of the rotation system. To take a holistic view of the changes associated with legume introduction into a crop rotation, overall (protein) yields per hectare were assumed to increase by 15% and the quantity of synthetic N fertilisers decrease by 24.5% compared to the original quantity used (pro-rata reducing based on existing profile of fertilisers used). From the new amount of N fertiliser used we recalculated GHG emissions and the gross margin for cereals (considering the decrease in fertiliser costs). We assumed the fertiliser cost savings correspond to 24.5% of the original N fertiliser quantity used for cereals, multiplied by the N fertiliser price observed for the study period (2016–2021)2(CSO, 2024b).
These crop rotations might however be subject to practical constraints related to the agronomic particularities of legumes and brassicas. Such constraints include soil texture and tillage practices (Byrne et al., 2022), as well as weed management strategies (Reckling et al., 2016). However, for the context of this analysis, it was assumed all the utilisable agricultural area could sustain these crops.
Results
This section presents economic, environmental and nutritional indicators for cereals and legumes, as well as the different crop rotation scenarios considered for cereal and legume associations.
Individual efficiency of plant-based protein sources
Economic, environmental and nutrition indicators on a per hectare basis at farm—crop scale
Table 3 presents the gross margin, GHG emissions and gross and digestible protein yield per hectare for each cereal and legume considered. On a per hectare basis, winter wheat is the most profitable crop, with a gross margin of €1,125 per hectare. The difference in gross margin per hectare between winter wheat and the other crop studies is statistically significant, except for spring wheat and winter oilseed rape. Winter wheat also shows the highest level of GHG emissions per hectare with 1,930 kg of CO2 equivalent per hectare. Results indicate that legumes perform better in terms of GHG emissions than cereals. Spring beans emit 560 kg of CO2 equivalent per hectare and spring peas 599 kg. The difference in GHG emissions per hectare between legumes and other crops is statistically significant compared to winter wheat, winter barley and spring barley. Legumes do not require chemical N fertilisers, which is a factor in this deferential (Collins & Phelan, 2021). Spring bean emissions are mostly driven by lime application and energy-related emissions (e.g. diesel), while pea emissions are driven by energy-related emissions only. Indeed, lime is advised for legume growth only if the soil pH is not optimal (Teagasc, 2017c). Between 2016 and 2021, according to our dataset, 8 of the 46 farms producing spring beans and none of the farms producing spring peas had to apply lime. Legumes also have the lowest profitability with a gross margin per hectare of €328 for beans and €579 for peas, which are 3.4 and 1.9 times less than winter wheat. However, this difference is only statistically significant for spring beans, and the gross margin does not include subsidies such as protein aid. Legumes have significantly higher protein yields than most cereals in terms of gross and digestible protein. Spring peas deliver 555 kg of digestible protein per hectare, while the comparable figure for spring beans is 738 kg of digestible protein per hectare. Overall, cereals have a better economic performance per hectare than legumes, but legumes show better environmental and nutritional performances than cereals on a per hectare basis. Oilseed rape show a protein yield comparable to that of legumes, and a significantly higher gross margin per hectare. However, oilseed rape shows high levels of GHG emissions, in the range of that of cereals. Direct payments under the Protein Aid Scheme are not included in the economic analysis.
Economic, environmental and nutritional performances per hectare
| Protein source (number of observations) | Dry matter yield (T/ha) | Gross margin (€/ha) | GHG emissions (kg CO2 eq/ha) | Gross protein yield (kg/ha) | Digestible protein yield (kg/ha) |
|---|---|---|---|---|---|
| Winter wheat (144) | 8.50 (0.14) | 1,125 (43)a | 1,930 (58)a | 1,071 (18) | 514 (9)a |
| Winter OSR (30) | 4.40 (0.16)a | 1,032 (75)a,b | 1,439 (94)b,c,d,e | 884 (32)a | 592 (21)b |
| Spring wheat (32) | 5.88 (0.39)b | 930 (139)a,c,d | 1,063 (104)b,e,f | 741 (49)b,c | 356 (24)c |
| Winter oats (75) | 7.42 (0.14)c | 909 (42)b,d | 1,512 (52)e,g | 801 (15)a,c | 457 (9) |
| Winter barley (196) | 7.72 (0.08)c | 887 (36)b,c | 1,875 (42)a,d | 872 (9)a | 411 (4) |
| Spring oats (74) | 5.60 (0.21)b | 678 (49)d,e | 1,342 (102)f,g | 605 (22) | 345 (13)c |
| Spring barley (242) | 6.07 (0.08)b | 654 (24)e | 1,563 (43)c | 686 (9)b | 324 (4)c |
| Spring peas (9) | 3.40 (0.36)a | 579 (116)a,b,d,e,f | 599 (121)e,f | 793 (83)a,b | 555 (58)a,b |
| Spring beans (46) | 4.55 (0.17)a | 328 (43)f | 560 (111)f | 1,342 (51) | 738 (28) |
Results are rounded up for clarity. Standard errors are in parenthesis. Values within a column not sharing a common superscript are significantly different at P < 0.05. Statistical differences are investigated using the Tukey post hoc test.
GHG emissions efficiency of crop product
Table 4 presents the GHG emissions per 100 g of digestible protein. Spring beans are the most environmentally efficient crop when GHGs are expressed per unit of gross and digestible protein. Spring peas and spring beans emit 0.05 and 0.09 kg of CO2 equivalent per 100 g of gross protein and 0.09 and 0.13 kg of CO2 equivalent per 100 g of digestible protein respectively. Cereal emissions range between 0.28 kg of CO2 equivalent (for spring wheat) and 0.50 kg of CO2 equivalent (for spring barley) per 100 g of digestible protein. Legumes were hence found to have lower GHG emissions compared to cereals on a per hectare stand-alone basis, but conversely also have significantly lower gross margin per hectare. The environmental efficiency of legume proteins in terms of GHG emissions per 100 g of digestible protein is better than that of cereals and oilseed rape. The difference is statistically significant between beans and all cereals and is significant between peas and all other crops except spring wheat and winter oilseed rape. Hence, legumes have potential as an alternative source of protein, both from an environmental perspective (with the aim of reducing GHG emissions) as well as when considering environmental protein efficiency to consider the challenge that is feeding efficiently the world’s growing population. Oilseed rape should also be considered for inclusion in crop rotation systems, for the reasons mentioned before as well as based on their economic and nutritional performances.
Greenhouse gas emissions per unit of protein
| Protein source (number of observations) | GHG emissions (kg CO2 eq·per 100 g of gross protein) | GHG emissions (kg CO2 eq·per 100 g of digestible protein) |
|---|---|---|
| Spring barley (242) | 0.24 (0.01)a | 0.50 (0.02)a |
| Winter barley (196) | 0.22 (0.01)a,b | 0.46 (0.01)a |
| Spring oats (74) | 0.22 (0.01)a,c,d | 0.38 (0.03)b |
| Winter oats (75) | 0.19 (0.01)b,c,e | 0.34 (0.02)b,c |
| Winter wheat (144) | 0.18 (0.01)c,f | 0.38 (0.01)b |
| Winter OSR (30) | 0.17 (0.01)b,d,f | 0.25 (0.02)c,d |
| Spring Wheat (32) | 0.14 (0.01)e,f | 0.28 (0.03)b,c,d |
| Spring Peas (9) | 0.09 (0.03)f,g | 0.13 (0.04)d,e |
| Spring Beans (46) | 0.05 (0.01)g | 0.09 (0.02)e |
Results are rounded up for clarity. Standard errors are in parenthesis. Values within a column not sharing a common superscript are significantly different at P < 0.05. Statistical differences are investigated using the Tukey post hoc test.
Crop rotation scenarios
This section investigates the impact of including peas, beans and oilseed rape in different crop rotation systems on farms which currently only produce wheat, barley and oats. This sample represents farms for which there is potential for more crop rotation and legumes and brassicas are not yet incorporated in those production systems. In this section, we consider that land entirely dedicated to wheat, barley and oats production is now divided into 60% of the land dedicated to cereals, 20% dedicated to oilseed rape and 20% dedicated to legume production. This allows a 1 in 5 years rotation for both legumes and brassicas. All other crops that could be produced in the farm remain fixed.
Table 5 presents the changes in the variables per hectare and at farm scale, resulting from the introduction of winter oilseed rape and spring peas or beans into the existing farm production systems. Figure 1 presents the distribution of the results for the two scenarios on an average annual basis. Introducing 20% of legumes and 20% of winter oilseed rape in existing cereal crop rotation leads to a statistically significant reduction in GHG emissions and an increase in digestible protein yield per hectare in both scenarios. Introducing spring peas and winter oilseed rape in a crop rotation led, on average, to an increase in gross margin by 28% and a decrease in GHG emissions by 28%, while introducing spring beans and winter oilseed rape in a rotation increased gross margin by 16% and decreased GHG emissions by 29% (both statistically significant). Gross and digestible protein yield per hectare increases significantly in both scenarios, with respective increases of 26% and 48% for peas and 44% and 61% for beans. Increases in protein yield can be explained by the larger protein yield per hectare of the legumes included in the crop rotation compared to the cereals they are substituting as well as by the overall yield benefit resulting from the general introduction of legumes in crop rotations.
Changes in gross margin, GHG emissions and protein yields per hectare after the introduction of legumes and oilseed rape in a crop rotation system
| Variable | Introduction of winter oilseed rape and spring beans | Introduction of winter oilseed rape and spring peas | ||||
|---|---|---|---|---|---|---|
|
| ||||||
| Average change | Median change | s.d. | Average change | Median change | s.d. | |
| Gross margin (€/ha) | 16%* | −2% | 102% | 28%* | 6% | 122% |
| GHG emissions (kg CO2 eq/ha) | −29%*** | −42% | 83% | −28%*** | −42% | 84% |
| Gross protein yield (kg/ha) | 44%*** | 36% | 35% | 26%*** | 20% | 27% |
| Digestible protein yield (kg/ha) | 61%*** | 52% | 40% | 48%*** | 40% | 35% |
Results are rounded up for clarity. Nitrogen oxide (NOx) emissions from legume nitrogen are not considered in the analysis. Statistical differences between before and after the implementation of the scenarios are denoted in superscripts and were computed using the Wilcoxon test for paired samples. *P < 0.05, **P < 0.01, ***P < 0.001.
Distribution of the changes in gross margin, GHG emissions and protein yields per hectare after the introduction of oilseed rape and legumes in crop rotations.
Figure 1 shows the distribution of results for both scenarios. The average increase in gross margin per hectare is driven by a few farms with very high increases in gross margin after the introduction of crop rotation. For most farms, the economic effect per hectare is close to zero. A majority of farms are showing a decrease in GHG emissions per hectare after the introduction of oilseed rape and legumes in crop rotation, for both beans and peas. From a nutritional perspective, most farms are showing a change in digestible protein yield per hectare of between 0% and +75%. The median change in gross margin per hectare is negative at −2%, only +2% for the introduction of beans and the median change in gross margin for the introduction of peas is +6%. The median GHG emission per hectare reduction is −42% after introducing both beans and peas. The median increase in digestible protein yield per hectare is +52% after introducing beans and +40% after introducing peas. For all indicators, the median change is lower than the average change, indicating a positively skewed distribution.
Discussion
According to our results, legumes are more efficient than all other crops considered in terms of GHG emissions per hectare. However, they are less economically efficient in terms of gross margin per hectare. Direct payments related to protein support in Ireland are not included in the gross margin. In the context of growing pressure on public authorities to reach ambitious climate objectives, the agricultural sector in Ireland has been set a target of a 25% reduction in agricultural GHG emissions by 2030 compared to 2018 levels (EPA, 2024a). Legumes could be included on farms which are already producing cereals to improve environmental performances and resource use efficiency. Combining cereals and legumes in crop rotations presents several advantages, both in terms of farm management and environmental outcomes (Nemecek et al., 2008; Preissel et al., 2015; Notz et al., 2023). Complementing cereals with both legumes and oilseed rape show further economic and environmental gains (Preissel et al., 2015; Reckling et al., 2016). Our results show that including legumes and winter oilseed rape on existing cereal farms which are not already producing them could lead to a significant decrease in GHG emissions per hectare. We also observe a significant increase in gross margin when peas are included in a rotation with cereals and oilseed rape. This can be explained by the decrease in fertiliser costs due to the decrease in fertiliser input, as well as by the good economic performances of oilseed rape on a stand-alone basis. The decrease in GHG emissions can be explained by two factors: legume production does not require chemical N fertiliser inputs, which decreases their environmental impact compared to cereals, and secondly associating cereals and legumes decreases the need for chemical N fertiliser for subsequent cereal production. Nitrogen oxide emissions from legumes’ N fixation were found to be negligible in the literature (Zhong et al., 2009). Combining cereals, oilseed rape and legumes in crop rotations also increases the digestible protein yields per hectare, whether the legume chosen is spring bean or spring pea. Spring beans might be preferred by farmers to introduce in crop rotations, as spring peas offer a relatively short harvest window and are associated with important yield variability (Preissel et al., 2015; Peoples et al., 2019). Moreover, results on the effects of introducing legumes in crop rotations might be sensitive to the crop sequence adopted by farmers (Forristal & Grant, 2011). For example, oats can also be used as a break crop for the disease Gaeumannomyces graminis, and crop rotation performances could be affected by the position of oats and legumes in a crop rotation setting.
It should be noted that GHG emissions were calculated using the IPCC methodology as previously published (Buckley & Donnellan, 2022). This approach is limited by the data availability for emission factors associated to each emission source. Where the data were available, Tier 2 factors, i.e. country-specific emission factors, were used. However, Tier 1 factors, i.e. default factors, were used when Tier 2 factors were not available, which can sometimes differ depending on regions or production specificities (IPCC, 2019).
Both area of production and quantity of oilseed rape produced in Ireland were much lower than the area of production and quantity of cereals produced. In 2023, 97,300 tonnes of oilseed rape were produced in Ireland against 191,700 tonnes of oats, 1,303,100 tonnes of barley and 520,800 tonnes of wheat. Conversely, only 21,600 hectares were dedicated to oilseed rape production in Ireland in 2023 against, 27,200 for oats, 186,300 for barley and 55,800 for wheat (CSO, 2024a). This could be due to the high-income risk and price volatility associated with oilseed rape (European Commission, 2017). Yield variability and losses at harvest also represent an agronomic risk that risk-averse farmers might not be willing to bear (Zahoor & Forristal, 2017). Moreover, the relatively small scale at which oilseed rape is produced also translates in the absence of large-scale processing facilities in Ireland. Oilseed rape then often has to be exported to be processed (Zahoor & Forristal, 2017).
Currently, cereals dominate arable production in Ireland. In 2022, 53% of the total output at producer price of arable crops in Ireland came from barley, oats and wheat. Other crops, including potatoes, legumes, fresh fruits and vegetables, made up the remaining 47% (CSO, 2024c). Moreover, half of the farms in Ireland do not practice crop rotation, producing only one or two different crops across a period of 6 years (2016–2021). This might be due to several factors. First, a different set of skills and knowledge is required to produce cereals rather than legumes, as growing these crops involves different agronomic requirements and practices. Moreover, legumes have lower and more volatile yields per hectare and represent agronomic risks. Peas in particular are subject to various weeds and pests and need careful monitoring. They also present production challenges, especially when it comes to harvest, with a very short window for harvesting which can result in crop losses and high yield variability (Preissel et al., 2015; Teagasc, 2017d; Peoples et al., 2019). Legumes’ performance also depends on soil type, with beans performing better on medium to heavy textured, moisture-retentive clay soils, while peas perform better on light to medium textured soils (Teagasc, 2017d, 2017e). The yields of faba beans are also highly sensitive to precipitation levels and drought; for example, limited precipitations and dry climatic conditions in Ireland in 2018 led to faba bean yields per hectare to be only a third of those in 2017 (Murphy et al., 2022). Like with oilseed rape, risk-averse farmers might then be resistant to adopting legumes, in particular spring peas, into their crop rotation. However, despite the relatively low economic efficiency of legumes in terms of gross margin per hectare compared to cereals, including legumes and oilseed rape into crop rotation increased the overall gross margin per hectare over the 6-year period according to our results. While the increase in gross margin is largely explained by the reduction in fertiliser input for legume crops, which do not require chemical N fertilisers, compared to cereal crops (Collins & Phelan, 2021), we also assume that all farmers will decrease the levels of chemical N application in the subsequent crops after the introduction of legumes in the rotations. Farms including grain legumes in their crop rotations experience an increase in overall gross margin as opposed to farms practicing monocultures or crop rotations without legumes, with the extent of the change depending on agronomic conditions (Reckling et al., 2016) and input levels (Forristal & Grant, 2011). Moreover, the Russian invasion of Ukraine also casted further uncertainty on arable production costs and crop prices, which might act as a disincentive for farmers to adopt more risky production systems. Between 2021 and 2023, the agricultural output price index increased by 17%, but the input price index increased by 28%. Fertiliser and crop protection price index increased by 54% and 31%, respectively, during the same period (CSO, 2024d).
The current policy landscape in the EU supports is not very supportive of legume production compared to other sectors of production. Among the sectors benefiting from VCSs in agriculture, protein crops were the lowest recipients of supports (European Commission, 2022). However, Ireland was among the first countries supporting the production of protein crops, introducing in 2015 a Protein Aid Scheme, a direct financial support aimed at farmers producing protein crops, as part of the “greening” of the CAP payments (Department of Food, Agriculture and the Marine, 2015). Support of the protein crops sector is being further developed in the country. The Irish CAP Strategic Plan 2023–2027 proposes a revision of the Protein Aid Scheme. Eligible crops are now peas, beans, lupins, soybean and protein/cereal mix crops. The aim of the policy is to support 14,000 ha in 2023 and 20,000 ha in 2027 (Government of Ireland, 2023b). In 2023, 16,370 ha were used for legume production in Ireland, which is in line with the target (Eurostat, 2024a). Policy support could also help to reduce the protein deficit experienced in Europe and Ireland to make the region less dependent on imports for certain key protein products (Román, 2023).
Market barriers to expanding plant-based protein should also be considered. Animal feed is the predominant outlet for arable crop production in Ireland and indeed in the EU, with around 47% of the cereals produced being used as animal feed (European Commission, 2023a). In Ireland, more than 85% of the cereals and pulses consumed are used for feed (FAOSTAT, 2023). Policies could support the legume sector through the valorisation of the food market or through price support (Zander et al., 2016). A price premium could help incentivise farmers to direct their production towards the food market as well as hedge against the agronomic-based risks associated with legume production. Moreover, scaling up the processing capacity of pulses is necessary to encourage farmers to change market outlet. Processing and market scale were identified as challenges and limiting factor in the development of pulse production in Ireland (Teagasc Tillage Crop Stakeholder Consultative Group, 2012). Such policies could make farmers more confident when it comes to the economic impact of introducing legumes in their rotations.
Animal feed being the main market outlet for crops and in particular legumes also raises the question of the so-called “feed versus food debate”, i.e. the protein efficiency of crops used in the livestock systems compared to crops used for food. The predominant market outlet for cereals and pulses in Ireland is feed (FAOSTAT, 2023). Livestock can act as a converter of low-quality protein into high-quality protein. However, livestock systems are not always efficient converters of protein. For example, monogastric animals require more high-quality feed than ruminants (Mottet et al., 2017). Monogastric production systems require more human-edible protein feed than they produce (Mottet et al., 2017; Hennessy et al., 2021), whereas ruminant production systems produce more human-edible protein than they consume. This further makes the case for promoting human food as a market outlet for protein crops instead of livestock feed, which can lead to more efficient protein utilisation and conversion, which in turn could increase food security in a more environmentally efficient manner.
Legumes can also be processed into protein-rich ingredients, e.g. protein-rich flour or protein isolates, which represent an opportunity to add value to plant protein production which can potentially increase farmers’ gross margins, while enhancing the biological efficiency of legumes as well as reducing anti-nutrient levels (Multari et al., 2015; Amagliani et al., 2022).
Conclusions
On a stand-alone basis, legumes show stronger performances in terms of GHG emissions per hectare than cereals and oilseed rape but they show a lower gross margin per hectare. They also present a good protein profile compared to other protein sources. Including legumes and brassicas in crop rotation leads to increases in gross margin and digestible protein yield and a decrease in GHG emissions per hectare. It is suggested that future policies should then focus on addressing the market barriers and agronomic risks associated with legumes production. Policy support should then complement the Protein Aid Scheme already in place in Ireland since 2015. Policy incentives and farm advisory programs should be developed further to promote the introduction of legumes in crop rotations in Ireland. If implemented successfully, this could help Ireland achieve its ambitious climate targets while contributing to enhance food and protein security. Current policy schemes aimed at supporting protein crop production could be extended to help develop the role of legumes and brassicas in the Irish tillage sector, which is currently dominated by cereal production.