Research on Environmental, Economic, and Social Sustainability in Dairy Farming: A Systematic Mapping of Current Literature

Dairy cows are able to convert fibrous materials, such as grass, roughage, and by-products from the food industry, into milk and meat, which justifies their role in food production. However, modern dairy farming is associated with major sustainability challenges, including greenhouse gas emissions. In order to develop sustainable future production, it is important to implement existing knowledge and fill knowledge gaps. The aim of this study was to systematically map the scientific literature on environmental, economic, and social sustainability at farm level in dairy farming. Literature published between January 2000 and March 2020 and with the geographical focus on Europe, North America, and Australia–New Zealand was included. In total, the literature search resulted in 169 hits, but after removing duplicates and papers outside the study scope only 35 papers remained. Of these, only 11 dealt with the three dimensions of sustainability, and several of these only mentioned one or two of the dimensions or set them in relation to that/those actually studied. Overall, the selected literature did not clearly explain how aspects of sustainability are interlinked, so possible negative or positive interactions between different aspects of sustainability dimensions remain unidentified.


Introduction
In 2018, there were over 290 million dairy cows worldwide, producing almost 700 million tonnes of milk [1]. India has the greatest number of dairy cows in the world, over 50 million (17.6% of the global dairy cow population), but the United States of America is the largest producer of milk, producing over 98 million tonnes per year (14% of world milk production [1]). Comparing global regions, Europe accounts for 32% of world production, the Americas for 27%, Asia for 31%, Africa for 5%, and Oceania for 4% [1]. Cow's milk is a valuable source of nutrients, providing humans with energy, protein, essential minerals, and vitamins. In addition, dairy farming provides the main income for many farmers around the world. As such, dairy farming contributes to several of the 17 global Sustainable Development Goals (SDG) set by the United Nations, including SDG1: Zero hunger, SDG3: Good health and wellbeing, and SDG8: Decent work and economic growth [2,3]. However, dairy farming may also jeopardize other SDGs, including SDG13: Climate action and SDG15: Life on land. Global average whole-milk consumption in 2017 was 88 kg per person, with very significant differences between countries/regions. Per capita consumption in Western Europe was almost 270 kg Valentin and Spangenberg [20] highlighted a fourth dimension of sustainability, namely institutional sustainability. While social sustainability refers to human capital and capabilities, institutional sustainability can be defined as the human interactions and the rules under which they are governed [20]. However, since this fourth dimension of sustainability is rarely mentioned in other literature this paper focus on environmental, economic, and social sustainability.
To date, much research has been devoted to sustainability and different dimensions of sustainability in dairy farming. However, the process of developing more sustainable future dairy production systems would benefit from an overview of the current state of knowledge in this area of research, including how the concept of sustainability is perceived. This would serve as a useful basis for highlighting important knowledge gaps and for prioritizing future actions and research initiatives. To our knowledge, there have been no previous attempts to map current knowledge on sustainability in dairy farming in a systematic way. The aim of this study was thus to systematically map the scientific literature on farm-level sustainability in dairy production. In order to limit the scope, the analysis was restricted to studies conducted in Europe, North America, Australia, and New Zealand, which enabled comparisons of production systems located in similar economic contexts.

Materials and Methods
The work presented in this paper is part of a larger project, other findings from which have been published previously [21,22].

Definition of Fundamental Concepts within Sustainability
Sustainability is a holistic concept, but it needs to be split into different dimensions that can be analyzed empirically, which in turn provide a basis for further scientific analyses on a higher level, e.g., to investigate possible negative or positive interactions between different aspects of sustainability. When evaluating the sustainability of a specific production system, it is important to include all these different dimensions (environmental, economic, and social). In this study, the three dimensions of sustainability were defined as follows:

Environmental Sustainability
The environmental dimension of sustainability is a natural science-based aspect that, basically, includes the total impact human activities may have on the ecosystems. Life cycle analysis (LCA) is the most commonly used approach for quantification of the environmental impact. It concerns the flows of resources (energy, land, water etc.) to the production system and emissions from the system and quantifies potential impacts. All environmental impacts caused by emissions, at all levels, should be included as well as the perspective of resource availability both locally, regionally, and globally. The focus of environmental sustainability assessments should be the effect of total production and not the organizational boundaries, i.e., all flows to and from production systems needs to be accounted for, regardless of economic ownership [23]. In addition to LCA, there are other methods assessing the environmental impact. In essence, most are similar in terms of the basic principles, but may use different approaches for describing the impacts. As an example, in the ecological footprint approach, all emissions and resources used are converted into "hypothetical area used", generating a common unit for the environmental impact of single products, which makes communication and comparison of differences between systems easier [24].

Economic Sustainability
From a theoretical perspective, economic sustainability can be considered in two ways. One focuses on the sustainable use of natural resources within a defined economic system, meaning that economic sustainability is achieved when the economic activity is not undertaken at the cost of natural resources. This economic concept of external effects is useful in understanding and fully capturing costs associated with production. The other way of considering economic sustainability focuses on growth of the economic system. Basically, this implies that in order to achieve economic sustainability, there must be a return on capital invested in the firm. When it comes to evaluation of natural and economic capital, there are two different approaches: weak and strong sustainability [25,26]. In weak sustainability, the sum of those two types of capital is combined, whereas in strong sustainability natural and economic capital are evaluated separately [26]. There is no clear consensus on how best to measure economic sustainability [27]. However, the sustainability accounting standards produced by the Global Reporting Initiatives [28] provide some guidance on which aspects to consider. In terms of economic indicators, these standards include aspects such as costs, revenues, profit, and investments.

Social Sustainability
Social sustainability is the least well defined and most often overlooked dimension of sustainability [29]. Social sustainability can be defined as the development of social structures and processes that meet the needs of the present in a community at the same time as laying a foundation for the community to continue to develop in a sustainable way in the future. In this study, the social dimension of sustainability included aspects such as community development, community resilience, livelihood, social equity, health equity, and labor rights. Aspects not directly associated with human societies, such as animal health and welfare, were also included in this dimension.

Systematic Mapping Approach
For the present analysis, we applied a systematic mapping approach, also known as evidence gap mapping. Systematic mapping is a transparent, robust, and repeatable method used to identify, collect, and assort relevant literature concerning a particular research topic [30]. It provides a structured overview of the research area [31], but without aiming to synthesize or further process the study results [32]. Instead, the method serves to identify and gather the published knowledge within a given research area and to indicate knowledge gaps where future research is needed. In contrast, a systematic review aims to answer a specific research question [32] through gathering relevant literature, further synthesizing the results using an appropriate methodology, e.g., meta-analysis [31,32], and specifically identifying knowledge gaps. However, the latter was beyond the scope of the current work. In this study, the systematic mapping approach was complemented with an approach whereby we extracted data from the set of papers reviewed to describe how sustainability dimensions considered in each paper were identified. Using a template, extracted keywords were defined and categorized to get an overview of aspects studied.

Literature Search
A comprehensive search of available online literature databases was conducted to obtain an impartial selection of published literature. To identify relevant papers, the online databases Scopus, Web of Science Core Collection, and CABI (https://clarivate.com/webofsciencegroup/solutions/ webofscience-cab-abstracts/): Cab Abstracts were used. The searches were limited to papers published between January 1, 2000 and March 20, 2020.
To find relevant papers in relation to dairy farming, the following search terms were used: "('dairy cow*' Or 'dairy herd*' Or 'dairy cattle' Or (milk* W/2 herd*))". These were then combined with search terms for environmental, economic, and social sustainability (Table 1), which were defined through an iterative process by all co-authors, in collaboration with a university librarian. The co-authors have expertise in different areas and could thus identify search terms and indicators usually used in relation to the three sustainability dimensions. In Scopus, the search was made within the search field "Title, Abstract, Keywords". In Web of Science Core Collection and CABI: Cab Abstracts, the searches were made within "Topic", which includes title, abstract, and keywords. Table 1. Library search terms used to locate papers dealing with on-farm environmental, economic, and social sustainability in dairy farming.

Sustainability Dimension Search Terms 1
Environmental ("environmental impact assessment" OR (environment* W/2 assessment) OR (environment* W/2 impact) OR (environment* W/2 protection) OR (climate W/1 change*) OR biodiversity OR ecosystem* OR pollution OR deforestation OR eutrophication OR (habitat W/2 destruction) OR (land W/2 degradation) OR (ozone W/2 depletion) OR "acid deposition" OR (odour W/2 emission) OR "air quality" OR "biochemical oxygen demand*" OR "chemical oxygen demand*" OR (nitrogen W/2 balance) OR (nitrogen W/2 cycle) OR (carbon W/2 cycle) OR eco-toxicity OR "carbon footprint" OR LCA OR "life cycle assessment") Economic ((agricultur* W/2 development) OR (agricultur* W/2 production) OR (farm* W/2 comparison*) OR (farm W/2 entrant*) OR (farm W/2 result*) OR (farm W/2 development) OR production OR diversification OR intensification OR "technical efficiency" OR "economic efficiency" OR "eco-efficiency" OR profit OR econom* OR return OR "economic viability" OR "economic performance") Social ((attitude* W/2 work) OR labour OR labor OR (quality W/2 life) OR "living condition*" OR "rural welfare" OR (work* W/2 condition*) OR "rural development" OR "social welfare" OR "social security" OR "social service*" OR "social equity" OR (health W/2 service*) OR "social status" OR (women W/2 status) OR "equal right*" OR equality OR (rural W/2 employment) OR livability OR "health equity" OR "labour rights" OR "labor rights" OR "social justice" OR "social capital" OR (community W/2 development) OR (community W/2 resilience)) The results of the searches were imported into the reference management software EndNote X8 TM . For each search in the three databases, a separate library was made, and the number of references found was recorded. After the searches were complete, the libraries were merged into one single library. Any duplicates were removed using the automatic function in the EndNote X8 TM software. The retrieved library was then manually searched for papers relevant to sustainability in dairy farming. Only full-length, trial-based papers were included, i.e., literature reviews, book chapters, conference papers, and organization reports were excluded. This to ensure that only original, peer reviewed papers were included. Further, the full-length papers had to be written in English to be included. In addition to ensuring that the articles addressed sustainability (single dimensions or a combination of dimensions), a geographical limitation was also set, whereby only studies conducted in Europe, North America, and Australia-New Zealand were included, to enable a focus on studies conducted on production systems in similar economic contexts. Exclusion due to publication type, language, or geographical origin was performed manually and was not set up in the searches.
Using a template, data were extracted from the abstracts of the hits identified, in order to describe aspects of the three sustainability dimensions dealt with in each paper. In the template, keywords used in each study to describe sustainability were defined and categorized. The full text of selected papers that listed aspects of the dimensions of sustainability in the abstract was analyzed in order to assess the actual scientific content on the three dimensions, i.e., to determine whether all dimensions were empirically studied.

Results
The literature search resulted in a total of 169 hits, of which 101 (60%) originated from CABI: Cab Abstracts, 36 (21%) originated from Web of Science Core Collection, and 32 (19%) originated from Scopus. After removing duplicates and articles that did not match the scope of the study, i.e., non-peer-reviewed reports, book chapters, conference abstracts, review papers, and papers not written in English, there remained a total of 35 papers, which were categorized in terms of sustainability dimensions and analyzed in the systematic mapping process ( Table 2, Appendix A). A majority of these (28/35 papers) originated from Europe, while five originated from North America and two from Australia-New Zealand. The papers were assessed regarding environmental, economic, and social sustainability and whether one, two, or all three dimensions were covered within each article. However, during the mapping process it was found that aspects of the different sustainability dimensions were only mentioned briefly in some papers, e.g., in the abstract or introduction, without actually being part of the study. In other papers, one or two dimensions were actually measured and analyzed, and the third was discussed in relation to those studied. Only around one-third (31%) of the 35 papers in the final set actually included the three dimensions in the same study.
During the reading process, data describing aspects of the sustainability dimensions that were studied in each paper were extracted and categorized (Figure 1). Regarding environmental sustainability, the main focus in the 30 papers covering this dimension was on climate impact (GHG emissions) and nutrient balance, including manure handling and fertilization regimes. Factors studied in relation to GHG emissions were most often different feeding strategies, but genetic traits were included in one study. Energy use and renewable energy sources were also studied, but to a lesser extent. One paper reported studies on how to integrate farming with wildlife conservation.
In the 23 papers covering economic sustainability, it was often measured as labor income in different production systems, using different feeding strategies and in relation to policy changes. Production costs for grass and cereals were linked to animal productivity in some studies. In addition, some studies calculated the cost per tonnes of CO2 emissions under different scenarios. In one study, the opportunity cost of land if oat drink were to be produced instead of cow's milk was calculated.
A wide variety of social sustainability aspects were included in the 18 papers that covered this dimension. Aspects raised by several studies were labor (both requirement and efficiency), working conditions (work load, working hours), and farmers' income. Other aspects raised were food safety, nutritional composition of the milk, the attitude of society to farming, and consumer wiliness to pay for products with added benefits, e.g., milk from grazing cows. In addition, policy making and how new policies affect farm structure, management, and labor income were studied. Farmers' attitudes and the factors on which they base different decisions were mentioned. One paper dealt with antimicrobial resistance.
Six of the papers in the set included animal welfare as part of the sustainability concept. Aspects covered by these papers were grazing regimes, animal longevity, use of growth hormones, and societal concerns about animal welfare.
Almost half (49%) of the 35 papers included were based on modelling and/or scenario analyses ( Figure 2). The majority (83%) of these studies considered the present day as the time perspective, 9% were future looking, 6% compared the past with the present, and 3% considered the situation before and after a change. Regarding environmental sustainability, the main focus in the 30 papers covering this dimension was on climate impact (GHG emissions) and nutrient balance, including manure handling and fertilization regimes. Factors studied in relation to GHG emissions were most often different feeding strategies, but genetic traits were included in one study. Energy use and renewable energy sources were also studied, but to a lesser extent. One paper reported studies on how to integrate farming with wildlife conservation.
In the 23 papers covering economic sustainability, it was often measured as labor income in different production systems, using different feeding strategies and in relation to policy changes. Production costs for grass and cereals were linked to animal productivity in some studies. In addition, some studies calculated the cost per tonnes of CO 2 emissions under different scenarios. In one study, the opportunity cost of land if oat drink were to be produced instead of cow's milk was calculated.
A wide variety of social sustainability aspects were included in the 18 papers that covered this dimension. Aspects raised by several studies were labor (both requirement and efficiency), working conditions (work load, working hours), and farmers' income. Other aspects raised were food safety, nutritional composition of the milk, the attitude of society to farming, and consumer wiliness to pay for products with added benefits, e.g., milk from grazing cows. In addition, policy making and how new policies affect farm structure, management, and labor income were studied. Farmers' attitudes and the factors on which they base different decisions were mentioned. One paper dealt with antimicrobial resistance.
Six of the papers in the set included animal welfare as part of the sustainability concept. Aspects covered by these papers were grazing regimes, animal longevity, use of growth hormones, and societal concerns about animal welfare.
Almost half (49%) of the 35 papers included were based on modelling and/or scenario analyses ( Figure 2). The majority (83%) of these studies considered the present day as the time perspective, 9% were future looking, 6% compared the past with the present, and 3% considered the situation before and after a change.

Discussion and Conclusions
Using a systematic mapping approach provided a good overview of the currently available scientific literature on sustainability in dairy production, including the environmental, economic, and social dimensions of sustainable development, and of the current state of knowledge within this research area. This provided a basis for discussing knowledge gaps and priority areas for future research. We found that only 11 of the 35 papers in the final set analyzed the three sustainability dimensions within dairy production. Although sustainability has been an important topic of discussion since the Brundtland report [15] was published more than 30 years ago and although dairy farming is an important part of livestock production in many countries, there have been surprisingly few studies covering the three dimensions of sustainability simultaneously. However, papers covering one or two sustainability dimensions were more common. Dairy farming today is associated with major sustainability challenges, mostly in relation to the environment due to the GHG emissions. This was reflected in the number of papers dealing with environmental sustainability, as 30 of the 35 papers analyzed studied aspects relating to environmental impact. Economic sustainability (23/35 papers) was studied to a larger extent than social sustainability (18/35 papers). Social sustainability was also more commonly covered or mentioned in association with one or both of the other two dimensions rather than on its own.
In the systematic mapping approach, the search terms are carefully defined in an iterative process to enable maximum coverage of the published scientific literature within the area of the study. However, even though we included search terms for the three dimensions of sustainability in our searches, we found that only a small proportion of the search hits actually dealt with environmental, economic, and social sustainability in the same study. There may of course be additional studies that have covered these issues, but which did not appear among the hits as they may not have contained the exact words used in our search strings. Creating a search string that is specific enough to locate relevant papers and that is also fully inclusive is a challenge, and hence no literature search is complete. Thus, there may have been relevant papers that were not located in our searches. However, since the search strings used can be considered comprehensive, the searches most likely identified the majority of studies dealing with sustainability within dairy production.
Considering the definitions of sustainability in dairy farming, a major challenge was the very wide range of aspects used in the different papers in relation to the sustainability dimensions. For example, the term "environmental sustainability" was used in different papers to describe studies covering everything from full-scale LCA to anaerobic reactor performance when digesting dairy manure. Regarding social sustainability, there is no uniform definition of this dimension, which was

Discussion and Conclusions
Using a systematic mapping approach provided a good overview of the currently available scientific literature on sustainability in dairy production, including the environmental, economic, and social dimensions of sustainable development, and of the current state of knowledge within this research area. This provided a basis for discussing knowledge gaps and priority areas for future research. We found that only 11 of the 35 papers in the final set analyzed the three sustainability dimensions within dairy production. Although sustainability has been an important topic of discussion since the Brundtland report [15] was published more than 30 years ago and although dairy farming is an important part of livestock production in many countries, there have been surprisingly few studies covering the three dimensions of sustainability simultaneously. However, papers covering one or two sustainability dimensions were more common. Dairy farming today is associated with major sustainability challenges, mostly in relation to the environment due to the GHG emissions. This was reflected in the number of papers dealing with environmental sustainability, as 30 of the 35 papers analyzed studied aspects relating to environmental impact. Economic sustainability (23/35 papers) was studied to a larger extent than social sustainability (18/35 papers). Social sustainability was also more commonly covered or mentioned in association with one or both of the other two dimensions rather than on its own.
In the systematic mapping approach, the search terms are carefully defined in an iterative process to enable maximum coverage of the published scientific literature within the area of the study. However, even though we included search terms for the three dimensions of sustainability in our searches, we found that only a small proportion of the search hits actually dealt with environmental, economic, and social sustainability in the same study. There may of course be additional studies that have covered these issues, but which did not appear among the hits as they may not have contained the exact words used in our search strings. Creating a search string that is specific enough to locate relevant papers and that is also fully inclusive is a challenge, and hence no literature search is complete. Thus, there may have been relevant papers that were not located in our searches. However, since the search strings used can be considered comprehensive, the searches most likely identified the majority of studies dealing with sustainability within dairy production.
Considering the definitions of sustainability in dairy farming, a major challenge was the very wide range of aspects used in the different papers in relation to the sustainability dimensions. For example, the term "environmental sustainability" was used in different papers to describe studies covering everything from full-scale LCA to anaerobic reactor performance when digesting dairy manure. Regarding social sustainability, there is no uniform definition of this dimension, which was reflected in the even wider variety of aspects found in the papers analyzed. The variety of aspects studied in relation to the different sustainability dimensions makes it complicated to compare between studies, both due to the many different aspects and to the different approaches used in the included papers. One priority area for future research should be to develop a nomenclature to define and categorize various aspects of the sustainability dimensions. This would help to make better comparisons between studies and to draw further conclusions.
As can be seen in Table 1, animal welfare was not initially included among our search terms as an aspect of social sustainability, but it was added during the mapping process. Animal welfare is a multifaceted concept that can be argued as a part of both environmental, economic, and social sustainability. For example, improved animal health (which leads to reduced use of antibiotics) is not only important for the animal itself, but also for environmental sustainability (increased resource efficiency, reduced drug residues, and resistant bacteria in soil), economic sustainability (reduced costs and increased productivity), and social sustainability (public health, product quality, and food safety) [33,34]. It should perhaps be added as an additional dimension of sustainability. However, the public is increasingly aware of animal welfare [12,13] and is thus closely linked to the societal view of animal production. Further, as animals are living individuals, which are part of the social system on farms as well as the society as whole, the concept can be considered to relate to social sustainability. Hence, in this study we included animal welfare in the social dimension of sustainability. It is also important to bear in mind that other aspects of sustainable development may be connected to each other in similar ways. During the reading process for this study, it became apparent that the current literature does not describe or consider these interrelations or discuss possible negative or positive interactions between different aspects of sustainability dimensions. This is an important area for future research.
Institutional, or governance, sustainability has been identified as a fourth dimension of sustainability [20]. Dairy farming, including both crop production and animal husbandry, is regulated both at national and international level. Hence, it is valuable to also include the institutional or governance dimension when studying sustainability in relation to dairy farming. None of the papers included in the systematic mapping have mentioned this dimension, let alone included it in the study. However, a number of the studies have included aspects such as policies within the framework of social sustainability, although policies clearly should be included in institutional sustainability.
Almost half of the papers analyzed were based on modelling or scenario analysis. This is an efficient approach to investigate how different management regimes, both feeding regimes and fertilization regimes, will affect, e.g., the economic outcome or the environmental impact. However, it is important that the data used as input to the calculations are valid and up-to-date, so that representative estimates can be obtained. This is important both from an environmental, economic, and social perspective. In agriculture, conditions vary between countries, and between regions within countries, in terms of climate (e.g., temperature, precipitation), soil type, etc. Further, different countries have different laws and regulations that affect the dairy production in general and perhaps the economy in particular. This makes it even more important to ensure that the data used is valid for the type of production and for the conditions where the results will be applied. Otherwise, the actions based on the outcomes may not achieve their full potential in efforts to achieve sustainable dairy production.
When working on the systematic mapping, we identified a number of aspects that can provide a basis for identifying research gaps and prioritizing future actions. The aim of the study was not to create and extensive list of actions that need to be taken, which would require a systematic review of the literature, but there are three issues worth highlighting.
First, it is evident that the number of published studies that actually cover both environmental, economic, and social sustainability simultaneously on farm level is low. We found that 11 of the 35 papers included in the systematic mapping included the three dimensions of sustainability, but in several of these cases one or two of the dimensions were only mentioned in passing or in relation to that/those actually studied. In addition, almost half the papers included in final set were based on modelling or scenario analyses. This indicates a need for more holistic assessments of sustainability in dairy farming and empirical applications of such methods.
Second, during the mapping process it emerged that a very wide range of aspects are used in the literature to describe different aspects of sustainability at farm level. Further, none of the studies had included the institutional dimension of sustainability, but some of the studies included aspects related to this dimension within the social dimension instead. This makes it difficult to compare different studies and to combine results to get a broader picture. The contribution of previous research is highly valuable, but future studies should take a more comprehensive approach, i.e., include the different dimensions of sustainability within the same study and consider how different aspects of sustainability in dairy farming are best measured, based on a conceptual understanding of sustainability. A common, standardized vocabulary would allow comparison of studies to a greater extent. Further, a standardized way of measuring different aspects of sustainability could lead to a wider area of application for data obtained.
Third, it is not yet well understood how different aspects of the sustainability dimensions are interlinked, and possible positive or negative interactions between them remain unidentified. Knowledge of these interactions would be valuable when seeking to determine how individual aspects of sustainability can be promoted and how the whole system at farm level can be improved.