# Anaerobic Digestion and Sustainability of Agriculture in Brittany: Research at the Crossroads of Geography and Agro-Economics for Assessing the Impact of French Energy Policies

Isaline Réguer

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Isaline Réguer. Anaerobic Digestion and Sustainability of Agriculture in Brittany: Research at the Crossroads of Geography and Agro-Economics for Assessing the Impact of French Energy Policies. Meyer T.; Weber F. Energy Geographies: Negotiating the French-German Interface, Springer Nature Switzerland, pp.219-242, 2024, Regionale Geographien | Regional Geographies, 978-3-031-69797-5. 10.1007/978-3-031-69797-5_11. hal-05299725

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This is an Accepted Manuscript version of the following chapter: Réguer I., *Anaerobic Digestion and Sustainability of Agriculture in Brittany: Research at the Crossroads of Geography and Agro-Economics for Assessing the Impact of French Energy Policies*, published in Energy Geographies: Negotiating the French-German Interface, edited by Teva Meyer and Florian Weber, 2024, Springer VS. This version of the manuscript has been accepted for publication, after final editorial is complete, but is not the Version of Record and does not reflect post-acceptance improvements (such as copyediting or typesetting), or any corrections. The final authenticated version is available online at: https://doi.org/10.1007/978-3-031-69797-5_11. Use of this Accepted Manuscript version is subject to the publisher's Accepted Manuscript terms of use: https://www.springernature.com/gp/open-research/policies/accepted-manuscript-terms
# Anaerobic digestion and sustainability of agriculture in Brittany: research at the crossroads of geography and agro-economics for assessing the impact of French energy policies

Isaline Réguer

UMR PRODIG, AgroParisTech, CNRS, IRD, Université Paris 1 Panthéon-Sorbonne, Université de Paris, Sorbonne Université, 75005, Paris, France

## E-mail address

isaline.reguer@agroparistech.fr

## Professional address

E5-422. Bât. E
UFR Agriculture Comparée
22 place de l'Agronomie
CS 20040
91123 PALAISEAU CEDEX

## Abstract

Since the 2010s, farmers in Brittany have developed Biogas units thanks to French authorities' support for developing agricultural anaerobic digestion. Although anaerobic digestion has been studied for years, few studies have focused on the factors behind its differentiated development on a regional scale. Additionally, while most French studies have focused on farms with a biogas unit, these studies typically do not place them in their socioeconomic and environmental context or assess their impact on other farms. Our paper addresses these issues by proposing a *Comparative Agriculture* approach, a transdisciplinary approach at the crossroads of geography and agroeconomics, which allows us to study agricultural development and its consequences. Our approach is spatially and diachronically comparative, systemic and multiscalar. First, results show that anaerobic digestion is developing in agricultural districts with the most significant agronomic potential and on farms that have historically been the best endowed in terms of access to resources, land, and farm equipment. Second, our results reveal that anaerobic digestion can have substantial effects on farms, depending on the size of the unit, down to plot and herd levels. Finally, results indicate that while anaerobic digestion increases the income of project owners, it also leads to widening inequalities between farmers.

## Keywords

energy transition; biogas; anaerobic digestion; Comparative Agriculture; transdisciplinarity; multiscalar approach

## 1 Introduction

*Agricultural biogas units* are technologies in which an anaerobic digestion process transforms agricultural biomass into two products: biogas, the energy source, and a liquid residue called digestate that farmers can use as fertiliser on crops. This agricultural biomass can be of animal
origin (slurry, manure) or crop origin, with main crops such as maize and intermediate energy crops (IEC) planted between two main crops. Some units transform the biogas into heat and electricity, known as combined heat and power (CHP) plants, while other units purify the biogas and inject biomethane into the natural gas grid, known as biomethane plants.

Agricultural biogas units have received increasing attention in France through the last decades, guided by the hindsight of Germany, which turned to biogas production ten years earlier. The French plan *Energie Méthanisation Autonomie Azote* (EMAA) declared in 2013 that "agricultural anaerobic digestion contributes to meeting [France's] environmental and energy challenges"<fnref n="1" /> (ministère de l'Ecologie 2013, p. 3).

In France, these challenges are particularly significant in two regions: the Grand-Est, which benefited very early on from German technologies and expertise due to its border location, and the region of Brittany. This region has the second most biogas units in the country. Brittany is also distinguished as France's leading region for livestock production, notably dairy, pig, and poultry farming. The livestock production concentrates large volumes of manure that is potentially available to feed biogas units.

Farmers in Brittany have invested in Anaerobic Digestion (AD) in several stages following new public policies. In 2011, the first French regulations concerning AD were implemented, ensuring the application of subsidised feed-in tariffs guaranteed for 15 years. In 2013, the EMAA plan provided a blueprint for the development of AD: the plan called for the construction of 1,000 biogas units by 2020, compared with 90 at the time. This plan presented biogas units as tools to facilitate the management of livestock manure in surplus regions like Brittany, which would ensure greater energy autonomy for farms and improve farmers' income thanks to subsidised energy prices. In 2015, the *Loi pour la Transition Ecologique et la Croissance Verte* (LTECV – French law on ecological transition and green growth) strengthened this planning mechanism. The law also imposed a maximum threshold of 15% for the supply of biogas plants with main crops in order to limit the overuse of maize for energy purposes. The main biogas units to emerge in Brittany throughout this initial period were CHP units, with electrical power mainly between 100 and 500 kW.

Since 2019, new political incentives have led to the gas grid upgrade and have facilitated AD's investments (e.g., easier loans and calls for tender) to increase national energy autonomy. These incentives led to the emergence of biomethane units because these units have higher energy production capacities than CHP units and are, therefore, more expensive. Since then, new biogas plants in Brittany have mainly been biomethane units with outputs of between 50 and 150 Nm3/hour (normal cubic meters per hour) as well as small CHP units (known as micro-units) with electrical power strictly below 100kW. The latter are much less costly and, therefore, accessible to a more significant number of farmers.

The recent evolution of biogas units raises the question of the impact of French energy policies on the sustainability of agriculture in Brittany. This paper aims to analyse the environmental and socio-economic context under which biogas plants develop and their effects in two agricultural districts emblematic of agriculture in Brittany.

Our objective is to assess the impact of AD public policies on the sustainability of agriculture in Brittany. Considering the lack of studies that analyse farms with biogas plants in their local socio-economic and environmental context (section 2), we will apply the conceptual framework of Comparative Agriculture to analyse the environmental and socio-economic context under which biogas plants develop and their effects on farms (section 3). Using a comparative,

<footnote n="1">¹ « La méthanisation à la ferme contribue à répondre aux enjeux environnementaux et énergétiques de notre pays. »</footnote>
systemic and nested-multiscalar approach (section 4), the transdisciplinary framework of Comparative Agriculture will be conducted on the case of two agricultural districts in Brittany to assess the impact of biogas units on agriculture (section 5).

# 2 Theoretical and analytical framework: assessing the impact of AD public policies on the sustainability of agriculture in Brittany

## 2.1 Theoretical framework: the question of scales of analysis

Most French studies related to anaerobic digestion's sustainability assessment have been carried out at the scale of western France (Le Guen and Damiano 2013) or the whole of France (Dziebowski et al. 2023; Laboubee et al. 2020). However, these studies are mainly technical-economic studies focusing on farms with a biogas unit. They do not study these farms in their local socio-economic and environmental context nor do they assess their impact on neighbouring farms. Nor do these studies assess levels of equity in access to this technology. Other studies have involved either agronomic and environmental analysis (Launay et al. 2022; Malet et al. 2023), mainly to assess the impact of AD on soils (Bodilis et al. 2015; Thomsen et al. 2013; Wentzel et al. 2015), or economic analysis (Grouiez et al. 2023), both of which focus on farms' scale with anaerobic digestion plants. Therefore, no transdisciplinary studies deal with the socio-economic and environmental effects of energy production by anaerobic digestion on a regional scale. This paper addresses this issue by proposing a methodology to analyse the environmental and socio-economic context under which biogas plants develop and their effects on agriculture in Brittany.

## 2.2 Comparative Agriculture: A transdisciplinary approach to study agrarian and production systems

To study agrarian dynamics related to AD in Brittany successfully, we have chosen the theoretical framework of Comparative Agriculture, as this approach aims to study the social and economic conditions under which agrarian dynamics are implemented and their consequences "for humans, society and the environment"<fnref n="2" /> (Cochet et al. 2007, p. 94). The framework enables us to study agricultural development as a “general long-term agricultural transformation process” (Cochet 2016, p. 5). Comparative Agriculture is the fruit of the enrichment of several approaches: from rural geographies as early as the 1950s (Cholley 1946) and the rural economics that followed to peasant studies (Bernstein and Byres 2001) and structuralism in the 1970s (Cochet 2011). This enrichment makes it a resolutely transdisciplinary approach.

The scale chosen for this study is the agricultural district (Asdonk et al. 1954) understood as a district with a “minimum dimension” to perceive all the “practice/ecosystem relations founding one or several modes of exploitation of the specific environment, differentiation mechanisms internal to the system, social relations and regulation modes” (Cochet 2016, p. 90). Thus, this scale allows us to analyse the agrarian dynamics and farm differentiation induced by AD development thanks to homogeneous agroecological and socio-economic conditions. Therefore, we selected two agricultural districts emblematic of agriculture in Brittany, i.e. strongly oriented toward livestock production (dairy or pig farming) and concentrating biogas units. We studied i) the western part of the Rennes basin, where dairy farming predominates

<footnote n="2">² « Pour l'homme, la société et l'environnement ».</footnote>
and where biogas units are concentrated, in particular for biomethane units, and ii) the eastern part of the Gouessant basin, where pig farming predominates and where, to date, only CHP units are present.

These agricultural districts allow us to perceive and study the local agrarian systems. In agriculture, ‘systems’ research took off in the 1980s and 1990s in both French- and English-speaking studies (Bonneval 1993; Brossier et al. 1993; Fresco 1984) and led to the emergence of the concepts of agrarian system and production, livestock and cropping systems. The agrarian system is defined at the scale of the agricultural district. It is characterised by a historically constituted mode of exploitation of the environment, by social relations of production and exchange, by a finite set of production systems, and by a set of ideas and institutions that ensure social reproduction (Cochet 2011; Mazoyer 1987; Mazoyer and Roudart 2002). The production system qualifies a set of farms with access to the same resources (e.g., land, labour, equipment, water resources), and the farms implement the same combination of cropping and livestock systems (Cochet and Devienne 2006; Reboul 1976). Then, a cropping system is used to model a set of plots characterised by the same crop rotations and similar technical itineraries, thus ensuring the same level of fertility reproduction (Sébillotte 1977, 1982). A livestock system is used to model herds with similar characteristics (e.g., sex ratio, genetics, production orientation), which are managed in the same way (e.g., for reproduction, prophylaxis, and feeding) (Gibon et al. 1999; Landais 1992; Lhoste 1984;). However, production systems are not fixed in time, so modelling them involves characterising their functioning, organisation, and historical evolution.

Applying these concepts to the two agricultural districts of study will allow us to identify and understand the impact of biogas plant development on local agriculture, from the district to the herd and plot scale.

## 2.3 Agrarian diagnoses to assess the impact of anaerobic digestion in Brittany

To perform this study, we conducted an agrarian diagnosis in the two agricultural districts. French agronomists have developed such diagnosis in this framework of Comparative Agriculture (Cochet 2011; Cochet and Devienne 2006; Devienne and Wybrecht 2002; FAO 1999). The diagnosis is based on three main stages:

1) The first stage of this study is a landscape analysis. This stage aims to identify the different agroecological zones and resources farmers have access to. This approach is part of an agro-geographical analysis.

2) The second step is a historical analysis. After identifying the various agroecological zones to which farmers have access in each agricultural district, we identify the different historical trajectories that have led to the current farms, including those with biogas plants. To do so, we adopt a historical and agronomic analysis.

3) The final stage is a characterisation of these farms' technical and economic functioning down to herd and plot. This step of the process involves agronomic, zootechnical and economic analysis.

The diagnosis is based on in-depth fieldwork. Fieldwork enables agricultural trajectories to be better understood, thus avoiding the pitfall of ahistoricity in development studies (Scott and Bhatt 2001) and elucidating the agricultural practices implemented, which are sometimes ignored or misunderstood (Beaud and Weber 2003).
## 3 Methods and data

As shown in Figure 1, our methodology is based on several steps, using comparative, systemic and multiscalar approaches.

<figure><img src="image_2.png" /><figcaption>Figure 1: Main steps in the methodology</figcaption></figure>
## 3.1 Comparison: to reveal the factors behind the deployment of anaerobic digestion in Brittany

The comparison appears necessary to “recognise the value of generality<fnref n="3" />” (Ramognino 2013, p. 3) and identify similarities in agricultural development processes linked to AD in the various districts of Brittany. To identify these similarities, one possible way to compare is to multiply observation sites (Ramognino 2013). We, therefore, proposed to compare the different physical environments where agricultural AD is developing at the regional scale. We analysed and compared Brittany’s soils to identify the biogas plants’ locations according to soil potential. Then, in each agricultural district studied, landscape analysis helped us confirm these regional observations by identifying the various local agroecological zones and locating the biogas plants within them. Indeed, the researcher can see the landscape as an “information medium”<fnref n="4" /> (Deffontaines 1973, p. 6) that he needs to understand. Observing the landscape in detail and methodically makes it possible to delineate its various zones, describe them, and then make assumptions about how farmers use the environment within them (Cochet and Devienne 2006).

A comparison approach can also help model production systems over time. Raynaud (1984) states that comparison is becoming a necessary part of the modelling process for simplifying the model while maintaining a high level of detail. The approach aims to constantly compare the cases identified with the pre-constructed models systematically and iteratively in order to help the modelling evolve. To this end, we conducted in-depth interviews with retired farmers who have experienced the agricultural changes of the last decades. Retired farmers were identified and compared systematically in each agroecological zone to reveal their differentiated development trajectory according to available resources and means. The information from these interviews helped construct a historicised pre-typology of current production systems.

The sampling was based on our spatial analysis and was purposive in order to represent all the diversity of farmers. The interviews were qualitative and semi-directive, giving the respondents time to develop their ideas despite the interviewer's guidance. These interviews were guided by the need to understand how the production system (and cropping and livestock systems) worked and how it has evolved (Cochet and Devienne 2006). There was no pre-established interview guide, and the interviewer progressively enriched their inquiries in a hypothetico-deductive approach as the model was built up. We conducted 30 historical interviews in the two agricultural districts studied (i.e., 60 in all).

This spatial and diachronic comparison helped us better understand the environmental and historical factors behind the development of biogas units in Brittany.

## 3.2 A systemic approach: to reveal the cascading effects of biogas plants on farms

The systemic approach makes it possible to study an object in all its complexity, mechanisms, and interactions with its environment, bringing to light elements that are not visible when only the sum of its parts is considered. A systemic approach is essential to understand the cascading effects caused by the introduction of a biogas unit on farms, down to plot and herd level, but also to identify the effects that this development may have on the scale of the agricultural district as a whole (e.g., on other farms and other economic agents) (Devienne 2023). Studying the agrarian system as a whole avoids the pitfall of studying only the farms involved in an anaerobic digestion project.

<footnote n="3">³ « Reconnaître la valeur de généralité »</footnote>

<footnote n="4">⁴ « Support d'information »</footnote>
In order to understand and characterise the technical functioning of these farms down to herd and plot level, we conducted in-depth interviews with active farmers thanks to a purposive sampling approach. Indeed, our sampling was based on the spatialised and historicised typology. Therefore, this sampling was not statistically representative but enabled the diversity of production systems to be represented, including those with biogas plants. The semi-structured interviews were guided by the various stages of the modelling process, i.e. location in the agroecological zones, the farm's history, and its technical and economic functioning down to the plot and herd levels. We continued our iterative and hypothetico-deductive approach: each interview allowed us to confirm or refute the hypotheses established during the previous stages (Blanchet and Gotman 2007; Cochet and Devienne 2006). We conducted 75 interviews in the Gouessant basin and 110 in the Rennes basin: 80 in 2020 (Garambois et al. 2022; Reguer 2020) and 30 new ones in 2024 to update the data.

### 3.3 Interlocking scales of analysis: to reveal territorial inequalities

The study of agrarian systems makes it possible to continually move back and forth between different nested scales of analysis. Table 1 illustrates the notion of interlocking scales of analysis and shows that this approach helps reveal elements that are not observable if we focus solely on a single scale of analysis.

<table><caption>Tab. 1 - Representation of the interlocking scales of analysis used</caption><thead><tr><th rowspan="3">Concept</th><th colspan="3">Agrarian system</th></tr><tr><th colspan="2">Production system</th><th rowspan="2">Agricultural district</th></tr><tr><th>Cropping and livestock systems</th><th>Farm</th></tr></thead><tbody><tr><th>Analysis scale</th><td>Herd and plot</td><td>Farm</td><td>Agricultural district</td></tr><tr><th>Type of analysis</th><td>Agronomy and ecology</td><td>Agronomy and economy</td><td>Geography, history, and socioeconomics</td></tr><tr><th>Sub-question of study</th><td>Do biogas units modify cropping and livestock management?</td><td>What is the effect of biogas units on farm incomes?</td><td>What is the effect of AD's development on the district?</td></tr></tbody></table>

Sources: Cochet, 2011, Cochet & Devienne, 2006.

Adapted by: Author, 2024.

The final step in the methodological process is an economic analysis. Based on our modelling of the technical functioning of the various production systems, our interviews also enable us to assess their economic results (Cochet and Devienne 2006). To do this, we measure income levels per family worker before payment of social security contributions and taxes. The value added produced is calculated based on the gross product (i.e., sum of production sold), less intermediate consumption (i.e., annual costs necessary for production, linked to the purchase of fertilisers, livestock feed, fuel, etc.) and capital depreciation (i.e., multi-year costs linked to the use of materials and equipment). Farm income is then obtained by subtracting from the value-added costs associated with land (i.e., property taxes, rent), labour (i.e., wages) and loan repayments and adding any aid. This economic modelling is based on in-depth discussions with farmers, notably concerning prices and their inter- and intra-annual variations, as well as on economic data recorded by French agricultural price monitoring organisations (e.g., average prices paid to producers, average costs).
We can carry out an economic modelling for each production system. This work is necessary to compare the highest and lowest farm incomes, revealing the maximum levels of inequality between farmers in the two agricultural districts studied. Only this complete approach makes it possible to assess the economic impact of anaerobic digestion not just for farmers with a plant but for farmers as a whole.

# 4 Results: A highly ambivalent environmental and socioeconomic assessment of AD on agriculture

## 4.1 Anaerobic digestion is developing in agricultural districts with high biomass production potential

At the regional scale, the map in Figure 2 shows that most biogas plants are on plateaus with little dissection by watercourses, where aeolian silts have remained in place. Biogas plants are, therefore, located where soil potential is highest. Indeed, on the silty plateau of the Rennes basin, where there are many biogas plants, wheat yields average between 90 and 100 quintals per hectare (qt/ha). In contrast, they are between 65 and 75 qt/ha on sandier and sandy-clay soils formed on granite or sandstone. In addition, the map shows that, in the case of biomethane units, the location of the gas grid must be added to this first explanatory factor to understand the spatial expansion of these units.

<figure><img src="image_3.png" /><figcaption>Figure 2: Map showing the location of biogas plants in Brittany according to soil substrate type and gas grid</figcaption></figure>

At the agricultural district scale, Figure 3 illustrates two different zones within the Gouessant basin: a hydromorphic valley zone with permanent grasslands and a residual bocage network
on the slopes (a) and a silty plateau with an open landscape where there is a biogas unit (b). The 'reading' of the landscape within the two districts studied confirms that biogas plants are primarily located in the heart of plateaus where soil potential is high and not at the bottom of valleys where soil potential is limited.

<figure><img src="image_4.png" /><figcaption>b) Openfield landscape</figcaption></figure>

<figure><img src="image_5.png" /></figure>

Photos: Author, 2023. Réalisation : Author, 2024

<caption>Figure 3: Comparison of two landscapes in the Gouessant basin</caption>
Our results show that biogas units are present in agricultural districts with the highest agricultural potential and most often in the heart of silty plateaus: in other words, in zones with the highest potential for plant biomass production.

## 4.2 Anaerobic digestion fits in with the largest historical farms

Our interviews with retired farmers revealed that the agricultural revolution of the twentieth century left its mark on the two agricultural districts studied. A continuous increase in volumes produced per worker (labour productivity) was made possible by farms' strong specialisations in livestock production. This process relied on significant use of inputs (e.g., fertilisers, phytosanitary products) and increasingly high-performance equipment. However, our interviews revealed that not all farmers have always had the resources and means to commit to these dynamics rapidly. To illustrate these differences, Table 2 shows the main characteristics of two typical trajectories modelled in the Rennes basin over time.

<table><caption>Tab.2: Comparison over time of typical production systems' main evolutions in the Rennes basin</caption><thead><tr><th></th><th>BR 1</th><th>BR 2</th></tr></thead><tbody><tr><th scope="row">Location</th><td>Heart of the plateaus<br>(loam soils)</td><td>Hillsides and valley bottoms<br>(clayey soils)</td></tr><tr><th scope="row">Post-war period</th><td>25-30 hectares<br>75% of arable lands<br>15-30 cows<br>2500 liters/cow/year</td><td>5-10 hectares<br>40% of arable lands<br>5-10 cows<br>2000 liters/cow/year</td></tr><tr><th scope="row">1970's – 1980's</th><td>50 hectares<br>50 cows<br>Corn silage all year round:<br>6000 liters/cow/year<br>+ fattening of pigs</td><td>25-35 hectares<br>30-40 cows<br>Corn silage only during<br>winter: 4500 liters/cow/year</td></tr><tr><th scope="row">Current period</th><td>200 hectares<br>190 cows, zero-grazing, 3<br>milking robots: 11.000<br>liters/cow/year<br>+ poultry</td><td>60-70 hectares<br>40-50 cows, grazing 5<br>months per year, milking<br>parlor: 8000 liters/cow/year</td></tr></tbody></table>

In the post-war period, farm families located in the heart of the plateau (BR1) had 25 to 30 hectares of land, 75% of which was arable (and therefore had significant fodder production potential), and had 15 to 30 cows that could produce 2,500 litres of milk per year. Farms on the slopes and valley bottoms (BR2), on the other hand, had 5 to 10 hectares, 5 to 10 cows, with only 40% arable land; the BR2 farms therefore had a more significant proportion of permanent grassland (60%) and a less rich fodder than BR1, thus explaining milk yields of approximately 2,000 litres per cow per year. Later in the century, the 1970s and 1980s were marked by a 'fodder revolution', with the introduction of maize into rotations and the possibility of producing higher nutritional value fodder (maize silage, grass silage) balanced with purchased concentrates (soybean meal). Our interviews revealed that the larger farms on the plateaus reached up to 50 hectares with 50 cows and a supplementary agricultural income source, usually
pig fattening or poultry. These families could afford to produce large quantities of maize silage, which fed the cows all year round, and the cows only grazed in spring, giving a milk yield of 6,000 litres per year. In contrast, farms on the slopes and valley bottoms reached 25-35 hectares with 30 to 40 cows. These families did not have the resources to produce large volumes of maize silage, so the cows did not get it all year round, and their milk yield was approximately 4,500 litres a year.

The 2000s were marked by the agricultural revolution of the twenty-first century, which continued the previous movement but relied on new technologies (e.g., precision farming, GPS, robotisation). Our interviews show that large farms (BR1) have the means to participate in this agrarian dynamic. Today, they can cover up to 200 hectares with 190 cows and a supplementary agricultural income source, usually poultry. Two to three milking robots and sometimes a feeding robot automates herd management. The cows no longer graze and produce up to 11,000 litres of milk annually. In contrast, the farms on the slopes and valley bottoms (BR2) are now 60 to 70 hectares in size, with 40 to 50 cows. These cows still graze 4 to 5 months a year, and no robotisation systems exist on these farms. Thus, they produce milk yields of 8,000 litres annually.

While dairy productivity (volume of milk produced per worker) has increased by a factor of 9 on average since the post-war period in this agricultural district, not all farms have had the resources and means to commit to such a rapid increase in productivity.

In the Gouessant basin, the same dynamics occurred on pig farms, with an increase in the number of sows per worker of 4 since the beginning of farms' specialisation in the 1970s. Although pig farms predominate in this agricultural district, several large dairy farms also exist in the heart of the plateaus and have followed dynamics similar to the large farms in the Rennes basin.

This historical modelling enabled us to situate agricultural anaerobic digestion within these trajectories. Our results reveal that, in both districts, biogas plants are mainly found on the largest historical farms and mainly dairy farms located on plateaus.

## 4.3 Cascading effects of biogas plants on farms: Toward a food competition

Our interviews led to model 21 production systems in the Rennes basin, including 5 with an anaerobic digestion unit, and 19 in the Gouessant basin, including 1 with an anaerobic digestion unit.

Our interviews and modelling reveal the dynamics shared by all farms with anaerobic digestion units: In terms of volume, these farms all give pride of place to livestock manure, which always accounts for more than 45% of the biomass introduced into the anaerobic digesters. Then, in compliance with current French legislation, corn volumes are often between 10% and 15%, while Intermediate Energy Crops never represent more than 30% of volumes. However, the contribution of this biomass to biogas production depends not only on the volumes introduced but also on their composition and capacity to produce methane, i.e., their methanogenic potential. The methanogenic potential of liquid manure is around ten times lower than that of maize, and the potential of maize is 1.5 to 2 times lower than that of IEC, usually rye. These results suggest that this plant biomass (i.e., rye, maize) is a significant concern for these farmers and raises the question of the impact of their implementation on these farms.
Figure 4 shows the changes in rotations and work schedules brought about by installing a biogas unit and cropping IEC.

<figure><img src="image_6.png" /><figcaption>Figure 4: Modification of crop rotations and distribution of work peaks with the introduction of a biogas unit</figcaption></figure>

While the prominent work peaks traditionally occur in late summer and early autumn (corn silage, tillage, sowing of winter grains) and in summer with the harvesting of winter grains, introducing IEC into rotations leads to a significant bottleneck at the end of winter. Indeed, farmers must not only ensile their fodder catch crops but also ensile all the IED – and not only bury the winter cover crops – and then sow the corn. The other work peaks are maintained and even intensify when the farmer increases his maize acreage to feed the biogas plant.

In addition, farms with biogas plants no longer spread slurry or manure on their crops but a digestate. The properties of digestate vary greatly depending on the biomass introduced into the anaerobic digester (Houot and Levavasseur 2020; Recous et al. 2016). Nevertheless, it generally has a high fertilising value, thanks to large fractions of mineral nitrogen in ammoniacal form.

However, since ammonia is highly volatile, the risks of air pollution and loss of fertilising value are high if farmers do not deposit the digestate as close as possible to the crop's root system. As a result, these farmers are often obliged to equip themselves with drag hose spreaders to deposit the digestate on the top of the soil or with slurry incorporators to bury the digestate in the soil.

As the latter equipment is very costly (up to 120,000 euros), farmers do not invest directly but outsource the work to Cooperatives for the Use of Agricultural Equipment<fnref n="5" /> (CUMA) or agricultural contractors. Our interviews reveal that the use of digestate can reduce the purchase of synthetic nitrogen fertilisers by around 50% and enable the widespread use of this equipment,

<footnote n="5">⁵ Coopératives d'Utilisation de Matériel Agricole (CUMA).</footnote>
which limits the volatilisation of organic manure (also used to spread any other liquid manure). Nevertheless, the amending value of digestate, i.e., its ability to maintain long-term organic matter levels in soils, remains more uncertain. Farmers themselves sometimes raise questions, and studies reveal that carbon availability to soil microorganisms is significantly reduced by using digestates (Wentzel et al. 2015). The long-term impact of digestates on soils seems to be, therefore, poorly understood.

Our interviews also reveal differentiated effects of anaerobic digestion on production systems. Table 3 summarises these main effects according to the unit's type, energy production capacity, and the production system it fits.

<table><caption>Tab.3: Description of the different production systems with a biogas unit and its cascade effects</caption><thead><tr><th>Production system name</th><th>SP a</th><th>SP b</th><th>SP c</th><th>SP d</th></tr></thead><tbody><tr><th>Location</th><td>Rennes basin</td><td>Rennes basin</td><td>Gouessant basin</td><td>Rennes basin</td></tr><tr><th>Type of biogas plant and capacity</th><td>Micro-unit<br>33 kW</td><td>CHP<br>150 kW</td><td>CHP<br>250 kW</td><td>Biomethane unit<br>80 Nm3/h</td></tr><tr><th>Year of commissioning</th><td>Since 2019</td><td>Mainly 2015-2019</td><td>2013-2019</td><td>Since 2019</td></tr><tr><th>Potential annual energy production</th><td>280 MWh</td><td>1300 MWh</td><td>2100 MWh</td><td>7800 MWh</td></tr><tr><th>Production system</th><td>3 workers<br>120-150 hectares<br>120-150 cows<br>10.000 liters/cow</td><td>4 workers<br>180-230 hectares<br>60-80 cows<br>11.000 liters/cow<br>+ poultry 2000m2 of buildings</td><td>5,5 workers<br>260-310 hectares<br>170-200 cows<br>11.000 liters/cow<br>+ 1000 pigs/year</td><td>3 workers<br>150-180 hectares<br>150-180 cows<br>9.500 liters/cow</td></tr><tr><th>Modification of the cropping system</th><td>-</td><td>Introduction of Intermediate Energy Crops<br>+ 30% of maize in crop rotation</td><td>Introduction of Intermediate Energy Crops<br>+ 70% of maize in crop rotation</td><td>Introduction of Intermediate Energy Crops<br>+ 40% of maize in crop rotation<br>External biomass purchasing</td></tr><tr><th>Modification of the livestock system</th><td>-</td><td>-</td><td>Stop spring grazing and switch to zero-grazing</td><td>Stop spring grazing and switch to zero-grazing</td></tr><tr><th>Equipment modification</th><td>-</td><td>Upgrade from 140cc to 200cc front tractor.<br>New slurry tanker</td><td>Upgrade from 280cc to 330cc lead tractor.<br>Crop sprayer, trailer, and slurry tanker renewed</td><td>Upgrading from a 200cc to a 250cc lead tractor<br>Trailer and slurry tanker renewed.</td></tr></tbody></table>

The minor biogas units (electrical power under 100 kW) have been developing in the Rennes basin since 2019 and can produce around 280 MWh of electricity annually. They are often installed on farms of between 120 and 150 hectares, with 120 to 150 cows (SPa). These units
are fed solely with livestock manure and corn rejects from the herd and, therefore, have little impact on production systems.

The CHP units, with an average electrical power of 150kW in the Rennes basin (SPb) and 250kW in the Gouessant basin (SPc), have an annual energy production potential of 1,300MWh and 2,100MWh, respectively. The units in the Rennes basin were set up mainly between 2015 and 2019 (some as early as 2011) on farms with 180 to 230 hectares, 60 to 80 cows, and a broiler fattening building. The cropping systems of these farms are being modified, with the introduction of Intermediate Energy Crops in rotations instead of winter cover crops and a 30% increase in corn acreage. This plant biomass and livestock manure are sufficient to feed the unit, so farmers do not need to change their livestock system. In contrast, the units in the Gouessant basin were set up between 2013 and 2019 on farms with 260 to 310 hectares, 170 to 200 cows, and 1,000 pigs fattened annually. Farmers introduced Intermediate Energy Crops and ensured a 70% increase in maize acreage. In addition, to provide sufficient biomass collection, these farmers no longer graze their livestock to keep it in a zero-grazing system. In all cases, the heat produced by the unit is used to heat the livestock buildings (e.g., broiler fattening buildings, fattening buildings for butchering pigs) and to power a barn dryer for in-building fodder drying.

Biomethane units are particularly disruptive to production systems. These units, present only in the Rennes basin, average 80Nm³/h and have an energy production potential of 7,400 MWh. This energy potential means they need much biomass to work properly. They are set up on farms of between 150 and 180 hectares with 150 to 180 cows (SPd). The construction of a biomethane plant systematically generates an increase in maize acreage (40%) and the introduction of IEC, to the detriment of winter grains and grassland acreage. In addition, farmers keep their cows on zero grazing to collect as much livestock manure as possible. Despite this, the farm's biomass is insufficient to keep these units running at total capacity, so farmers purchase part of the biomass off-farm (from other farmers or agri-food companies).

Our results reveal that the more powerful the unit, the more influential the cascading effects, resulting in ambivalent environmental impact.

Our interviews reveal three main effects of anaerobic digestion for other farms. The first effect concerns biomass exchanges between farms. For example, the farmer with a unit may purchase maize from another farm. In this situation, part of the other farm's crop rotation, previously devoted to maize sold as grain, is converted to maize for energy purposes. In this case, a contract is signed to reserve the land and set the purchase price: between €1,800 and €1,900 per standing hectare in 2024. The neighbouring farm provides the labour and covers the cultivation costs, while the farm with the biogas plant provides the harvesting and silage services. The same type of agreement exists for Intermediate Energy Crops.

Secondly, farms equipped with a biogas unit tend to store corn silage. As studied, they buy this corn from neighbouring farmers at a good price. This dynamic raises two questions: the risk of rising prices for local corn silage and the difficulty of accessing this fodder on the market for other farmers in drought.

A third type of effect of anaerobic digestion is at the CUMAs level: the need for more powerful equipment – in order to work faster – on farms with biogas plants weighs heavily on discussions concerning purchasing new equipment. These discussions often concern equipment of a different type and power and, therefore, at a different price compared with the expectations of other farmers who sometimes have different economic capacities. Moreover, some farmers
report that when work peaks and demand for equipment is high, priority is often given more readily to farmers with a biogas unit, as they are very regular customers.

Our results reveal ambivalent cascading social and economic effects for farms indirectly affected by anaerobic digestion.

## 4.4 Biogas units: A systematic increase in income levels, widening socio-economic inequalities between farms

The results of our economic modelling show that the installation of a biogas unit leads to an increase in gross income in all cases (Figure 5): from 20% for micro-units in the west of the Rennes basin to over 35% for CHP units in the east of the Gouessant basin. However, farms with a biomethane unit recorded the most significant income increases: the income per family worker is multiplied by 2 to 3.

These results show that the economic impact of anaerobic digestion is significant at the farm level. However, more than this first level of result is needed to understand the economic effects of this development on the whole agricultural district.

Our results show that the sharp rise in incomes linked to anaerobic digestion leads to more significant farm disparities. According to our modelling, income inequalities increase from a factor of 3 to a factor of 4 in the Gouessant basin, where only CHP units have been developed (comparing dairy farms with 170 to 200 cows and a CHP unit and pig farms with a breeding and fattening system with 140 to 180 sows). On the other hand, these inequalities rise from a factor of 4 to almost a factor of 10 in the Rennes basin, where biomethane units predominate (comparing dairy farms with 150 to 180 cows and an injection unit, and small dairy farms of the district with 70 to 80 cows).

Our results reveal inequalities that cannot be observed in studies focusing solely on farms with biogas plants.
<figure><img src="image_7.png" /></figure>

(Sources: interviews, 2020, 2023, 2024. Realisation: author)

<caption>Figure 5: Comparison of income per family worker (before social contributions and taxes) for different production systems with and without biogas unit in the Rennes Basin and the Gouessant Basin</caption>

# 5 Discussion and conclusion: policy implications for the sustainability of agriculture

Results show an uneven anaerobic digestion development in Brittany and ambivalent environmental and socio-economic impacts on agriculture in Brittany. First, biogas units are present in districts and zones with the highest potential for agricultural biomass production, mainly plateaus. This raises the question of fair access to this technology. Second, these units are present in the largest and wealthiest historical farms with important investment capacities. These results align with those of Jutteau (2024) (see *supra*), who also demonstrates, at the national scale and thanks to quantitative surveys, that biogas units are mainly present in larger farms. Third, our results show cascading effects of biogas units on farms, sometimes competing with food production and resulting in ambivalent environmental and social impacts. Fourth,
biogas units always lead to a significant increase in farm income, leading to more significant farm disparities. Our economic results are again in line with those of Jutteau (2024) (see *supra*), who shows, in another French region, that anaerobic digestion reinforces the social position of farmers.

Our methodology and results call into question the relevance of French public support for agricultural anaerobic digestion since this development appears to fail to ensure the environmental and socio-economic sustainability of Breton agriculture.

However, our approach does not allow us to accurately assess and quantify the impact of anaerobic digestion on the Breton community as a whole, i.e., by considering the effects for economic agents upstream and downstream of the sectors (supply companies, dairies, slaughterhouses, etc.), particularly in terms of value creation and employment.

Additionally, developing anaerobic digestion on farms offers a possible energy transition path – currently supported by the public authorities – but other transition paths do exist. Indeed, energy conservation offers another path to highly economical and autonomous farms (Garambois and Devienne 2010). However, our methodology currently cannot compare the environmental and socio-economic effects of different development paths (e.g., anaerobic digestion vs. energy conservation) on the scale of Brittany.

That is why our future work will perform a complete socio-economic assessment of the development of anaerobic digestion on a regional scale. To do so, we will mobilise another theoretical framework: the impact assessment of agricultural development projects (Bamberger 2006; Bridier and Michaïlof 1995; Chervel 1995). This approach will enable us to study the effects of anaerobic digestion in detail while comparing them with what could be achieved by alternative public support – e.g., for energy conservation - for all agricultural economic agents in Brittany, i.e., the Brittany community.
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