Modern Pig Production: Aspects of Animal Welfare, Sustainability and Circular Bioeconomy

Abstract

Modern pig production must balance efficiency, animal welfare, and environmental sustainability while embracing circular bioeconomy principles. This review critically examines the scientific literature from the past decade, focusing on the environmental impacts of pig farming, animal welfare considerations, and circular bioeconomy strategies. Key challenges include the ethical treatment of pigs, regulatory frameworks, and the sector’s contribution to climate change through emissions and resource use. Sustainable pig farming relies on innovative housing systems, welfare-oriented management practices, and legislative measures that improve animal welfare. Moreover, integrating circular bioeconomy strategies, which include manure management for biogas production, alternative feed ingredients, and wastewater recycling, enhances resource efficiency while reducing environmental footprints. Life Cycle Assessment (LCA) methodologies provide insight into the environmental impacts of different production systems, guiding policymakers and producers toward more sustainable practices. Despite these advances, further research is needed to optimize feed alternatives, improve manure treatment technologies, and explain how to improve animal welfare standards. This review highlights the importance of interdisciplinary approaches in achieving sustainable pig farming and underscores the need for continued innovation in aligning productivity and environmental aims.

1. Introduction

Pigs are similar to humans in many ways, resulting in some major advantages and some significant drawbacks in relation to the sustainability of pig farming. Pigs and humans are highly social animals, vulnerable to large predators but capable of group defense, that eat roots, fruits, nuts, seeds, and some animal material when they can obtain it. Most of the food consumed by pigs can be eaten by humans and vice versa. Hence, while pigs compete with humans for food resources, they historically coexisted with humans, often relying on human food waste. Additionally, pigs can derive nutritional benefit from human feces, a relationship that is likely to have persisted for a long time, as evidenced by parasites that specialize in alternating between the two species. Homo sapiens and Sus scrofa have lived in parallel and have had mutual benefits and evolutionary advantages because they did so. As with wolves/dogs and humans [1], it could logically be said both that pigs domesticated humans and humans domesticated pigs.

What advantages do pigs gain from their similarity to humans? The complex behavior and high level of cognitive ability of pigs [2,3] have long been recognized by those who have lived with them. Observers have noted that pigs often make direct eye contact with humans when evaluating their next action. Such abilities and actions sometimes, but not always, lead humans to have more empathy with pigs than with some other species, and this can result in better welfare for pigs, especially when the human knows the pig individually. Additionally, the dietary similarities between pigs and humans have historically resulted in pigs being kept in proximity to humans, where they were fed leftovers or even stored food.

The negative consequence for the sustainability of the similarity of pigs and humans concerns the now widespread practice of feeding pigs material that humans could eat. The grain that is fed to pigs is sometimes of a quality that would not be approved as human food, but most of an intensively farmed pig’s diet could have been eaten by humans. Even when it could not have been eaten by humans, the land could usually have been used to grow human food. The consequence of this is a negative score in one important area of sustainability: world resource usage.

The term sustainable has a broader meaning today than in the past [4,5,6], for each system should be considered taking account of a wide range of negative impacts. According to the United Nations Brundtland Commission, sustainability is defined as “meeting the needs of the present without compromising the ability of future generations to meet their own needs”. A system or procedure is sustainable if it is acceptable now, and if its expected future effects are acceptable, in particular, in relation to resource availability, consequences of functioning, and morality of action [7,8]. Decisions made by governments, other agencies, and the general public regarding sustainability are ethical evaluations that should ideally be based on high-quality information encompassing all sustainability components now and in future generations [3,9,10]. Several factors can render a food production system unsustainable, for example, adverse effects on human welfare, including health; poor welfare of production animals; inefficient usage of world resources; harmful environmental effects, such as greenhouse gas production, water pollution, low biodiversity, or insufficient conservation; unacceptable genetic modification; not being “fair trade”, in that producers in poor countries are not properly rewarded; or insufficient job satisfaction for those working in the industry and damage to rural communities [6]. The components of sustainability are now thought of as an aspect of product quality that alters purchasing behavior and subsequently drives changes in production methods [3,7,11].

There is growing awareness of the need to take account of all of the components of sustainability and to promote circularity in evaluating food production and other aspects of bioeconomy [11,12]. This approach emphasizes the effective use, reuse, and recycling of bio-based resources, as well as renewable energy, water, and soil [13,14]. Circular bioeconomy is a new economic approach centered on renewable natural resources, aiming to reduce waste and substitute for the extensive use of fossil fuel-origin, non-renewable products [15]. In recent years, concerns in some countries regarding the transmission of pathogens to humans and to other animals have led to legislation banning the use of human food waste as pig feed. Whereas traditional practices often prioritize short-term financial gains, circular bioeconomies aim to optimize the use of renewable biological resources efficiently while enhancing environmental and social outcomes [16]. Traditional swine production systems often follow a linear model that focuses on maximizing production while neglecting environmental sustainability and resource efficiency. The innovative aspects of circular bioeconomy, on the other hand, are focused on enhanced resource efficiency, waste valorization, and sustainable nutrient management [17]. The use of food waste in pig nutrition is such an example. However, in countries such as Japan, after proper treatment that eliminates pathogen risks, human food waste is permitted as pig feed. With advances in treatment methodologies, the practice of feeding treated food waste to pigs is expected to become more widespread in the future, as it represents a sustainable solution [18].

For many consumers, animal welfare is a key component of food product sustainability [5,19,20,21]. When consumers in Brazil were asked about food sustainability, they reported that the welfare of the farmed animals was the most important component of sustainability [22]. Animal welfare is a multi-dimensional concept encompassing freedom from suffering and frustration and normal biological functioning that promotes natural behavior and the satisfaction of needs [23]. Welfare can be defined as the state of an individual as regards its attempts to cope with its environment, and it is based on the ability of the individual to cope, adapt, and maintain body and mental stability in a constantly changing environment [24,25]. To date, most quantifiable welfare indicators focus on negative aspects, such as increased levels of stress hormones, exhibition of abnormal behaviors (e.g., oral stereotypies, belly-nosing, nibbling, tail-biting), aggressiveness, fear, diseases, and lameness, which provide some information about the mental state of animals [26]. However, the growing recognition that welfare is much improved when sentient animals, such as pigs, experience positive emotions has contributed to the concepts of “happiness” and “a life worth living” [27].

Despite the importance of considering the sustainability of pig production, there have been few systematic attempts to record and analyze all the scientific work on this subject that has been published. In the context of pig farming, it is imperative to develop an integrated assessment framework that places circularity, sustainability, and animal welfare on a common evaluative base. Such a holistic approach ensures that progress in resource efficiency or environmental performance does not undermine ethical obligations toward animal welfare, thereby promoting coherent and resilient agri-food systems. Therefore, the aim of the critical analysis presented here is to review the scientific work that has been carried out during the last decade concerning welfare issues and environmental impact reported on the different stages of pig production. Circular bioeconomy principles in pig production are also critically discussed. Although there are different reviews that separately examine these concepts concerning pig production, there are few data on an integrated approach of circularity, sustainability, and animal welfare in pig production.

2. Welfare During Pig Production

The welfare of pigs during production is a multifaceted issue encompassing various factors, including housing conditions, management practices, health indicators, and societal perceptions. Integrating animal welfare assessment into pig farming practices can enhance the welfare of the animal, farm productivity, and public demand for pig products. Pigs should be provided with a range of stimuli that offer positive affective engagement, i.e., control of key aspects of their environment, comfort, contentment, interest, and confidence. Thinking about this is helped by considering four welfare quality principles: appropriate nutrition that provides satiety, adequate housing, good hygiene conditions, and the opportunity for normal biological functioning and behavior display [27,28,29].

Nowadays, there are advanced technologies that include recent innovations, such as sensor-based welfare monitoring and AI-based behavioral analytics, that allow for a real-time surveillance of pigs [30]. Collection of data is carried out through image/video processing, sound analysis, or any other sensor capable of detecting the elements responsible for behavioral changes, such as RIFD, thermometers, etc. [31]. The animals’ bio-responses are used as data, and the system’s developed algorithms and dynamic mathematical models try to convert them into measurable performance, welfare, and sustainability production indicators [32]. Nowadays, this transformation could be facilitated by Artificial Intelligence (AI) methodologies, specifically, the Internet of Things (IoT) and multimodal data analysis, underscoring the compelling potential of AI technology to assess overall pig welfare status [33].

There has been a series of Directives within the European Union that provide minimum standards for the avoidance of poor welfare in pigs (

Table 1. Legislation in the European Union referring to pig animal welfare aspects.

Number	Reference to	Reference
EC Directive 91/630/EEC	Minimum standards for the protection of pigs	[34]
EC Directive 2001/88/EC	Additional welfare improvements for sows (group housing, stocking density, and floors)	[35]
EC Directive 2001/93/EC	Environmental enrichment and specific regulations concerning tail docking, teeth resection, and castration	[36]
EC Directive 2008/120/EC	Stocking density for sows and gilts, requirements for light and maximum noise levels, necessity for permanent access to fresh water and to materials for rooting and playing, introduction of higher level of training and competence on welfare issues for personnel	[37]
EC Recommendation 2016/336	Enrichment materials that can be used for the improvement of pig welfare	[38]

). The first effort was made with the Council Directive 91/630/EEC that contained general requirements for housing, equipment, surveillance, and feed and water of pigs, but also minimum space allowances for weaners, breeding sows, and fattening pigs [34]. The aforementioned Directive was replaced by EU Council Directive 2001/88/EC that set requirements for additional welfare improvements for sows, such as group housing during the majority of the gestation period and further requirements regarding their stocking density and floors [35]. Moreover, EU Council Directive 2001/93/EC highlighted the importance of environmental enrichment for pigs and made specific regulations concerning tail docking, teeth resection, and castration [36]. The above legislation was consolidated by the Council Directive 2008/120/EC that, among other effects, promotes an improved quality of flooring surfaces and increased living space available for sows and gilts, introduces higher levels of training and competence on welfare issues for personnel, sets requirements for light and maximum noise levels, provides permanent access to fresh water and to materials for rooting and playing, and lays down a minimum weaning age of four weeks for piglets [37]. Released in 2016, the Commission Recommendation (EU) 2016/336 on the application of the Council Directive 2008/120/EC further indicates the various forms of enrichment materials that can be used for the improvement of pig welfare [38].

In contrast, and surprisingly, given the near universality of consumer attitudes, formal pig welfare legislation is scarce in top pork meat-producing countries, such as the USA and China. In the USA, the Animal Welfare Act (AWA) signed in 1966 is the only federal law that regulates the treatment of animals in research, teaching, testing, exhibition, transport, and by dealers. However, although the U.S. Department of Agriculture generally protects one aspect of welfare, farm animal health, there is little federal legislation to specifically protect the welfare of farm animals during rearing [39], with minor exceptions, such as the bill “H.R. 7004: PIGS Act of 2022” that was presented in the US Congress to prohibit the confinement of pregnant pigs. Moreover, in China, animal welfare is considered as part of the food safety of animal-derived products. As a result, although there are laws regarding laboratory animals and zoo management, legislation relevant to farm animals is mainly based on food safety concerns [39]. Despite years of debate, China has not yet established a national law to promote animal welfare [40]. In Brazil, Normative Instruction 113 (16 December 2020) was established by the Ministry of Agriculture, Livestock, and Food Supply (MAPA), regulating good management and animal welfare practices in commercial pig farms in order to promote an increasingly sustainable and competitive pig industry (Ministério Da Agricultura, Pecuária E Abastecimento-Mapa). The high degree of international heterogeneity in animal welfare standards raises questions with respect to international trade and highlights the necessity for complementary policies to prevent relocation of industries and to ensure international policy alignment [41].

Animal welfare has nowadays been recognized as a worldwide target of sustainable agricultural policy [20]. The United Nations Committee on World Food Security formally denoted animal welfare as a fundamental pillar of sustainable agricultural development, food security, and human nutrition, together with the other classic domains, i.e., economics, society, and environment [42]. At the same time, there is increasing political modernization around the issue of animal welfare that has become apparent from the “One Health” or “One Welfare” concepts set by national non-governmental organizations (NGOs) and consumer groups [43,44]. As a result, the food production and retail industries have integrated animal welfare as a component of market-driven strategies in several countries [45], and a specific International Standard Organization (ISO) pertaining to animal welfare (TS 34700) [46] was established in 2016. More than 90% of EU citizens prioritize animal welfare, and more than half of them are ready to pay more for products derived from systems offering high welfare status, since they are considered healthier, safer, and environmentally friendlier [47], although their level of general knowledge about pig farm management and housing conditions is low [48,49]. The EU Commission has therefore established an Animal Welfare Platform in an effort to promote dialogue and enhance knowledge about animal welfare [50]. However, it is worth mentioning that, although consumers generally care about animal welfare, when compared with other societal issues, such as racial equality, women’s rights, poverty, peace, and security and environmental protection, many rank it lower [51].

The majority of the welfare problems observed during pig production are related to poor housing or management practices, such as high stocking density, high degree of confinement, lack of outdoor access, poor ventilation, high temperatures, use of slatted floor systems with few or no enrichment materials, and inadequate diet formulation [52]. Concrete flooring and high stocking densities significantly hinder ease of movement, while the lack of environmental enrichment and proper bedding has been associated with various welfare issues, including increased aggression and stress-related behaviors [53,54,55]. In general, considering biological function and needs and hence providing access to an outdoor area, optimum space allowance and the opportunity to display normal behavior appear as key priorities for sustainable pig production [46,52,56]. The intensification of pig farming and the increase in pig farms, in terms of animal number, raises concerns for the welfare conditions in pig farms in relation to their size. A study assessing intensive fattening pig farms in Germany using the Welfare Quality^® protocol found no significant differences in welfare outcomes across varying farm sizes. Indicators such as feeding, housing, health, and behavior showed comparable results between small and large farms. However, certain issues, like moderately soiled bodies, were more prevalent in larger farms [57]. While farm size alone may not directly determine pig welfare, associated factors such as stocking density and housing design play critical roles; larger farms seem to face more challenges ensuring adequate space and environmental enrichment to maintain and improve animal welfare standards.

Furthermore, the environmental conditions within pig housing systems, such as ventilation, temperature, and bedding quality, play a crucial role in the overall welfare of pigs. Poor ventilation can lead to high ammonia levels, which are detrimental to respiratory health and overall welfare [58]. Insufficient bedding can exacerbate discomfort and stress, resulting in worse welfare [59]. Pigs exposed to heat stress often exhibit decreased voluntary feed intake as an adaptation to reduce metabolic heat production, which could otherwise lead to weight loss and decreased growth rates [60]. Additionally, high temperatures can impair reproductive performance in sows, with studies indicating that elevated ambient temperatures around farrowing negatively influence feed intake and, consequently, the weaning weight of piglets [61,62]. This is particularly concerning, as it can have long-term implications for the health, productivity, and sustainability of the whole operation system. Moreover, transportation of pigs during high temperatures poses additional welfare risks. Studies have shown that high ambient temperatures during transport can lead to increased mortality rates and stress-related behaviors [63,64]. The European Commission has established maximum temperature limits for transporting livestock, but these regulations often do not take account of humidity levels, which can significantly impact animal welfare [65].

One of the most pressing consumer concerns related to poor housing conditions is confinement of sows in the mating and gestation units. These units severely restrict movement, preventing sows from turning around or engaging in natural behaviors, which can lead to physical ailments such as pressure sores, ulcers, and increased stress levels [66]. Sows housed in gestation crates exhibit frustration, higher levels of stereotypic behaviors, minimization of interaction with the piglets, increased heart rate, changes in stress hormone secretion, and impaired thermoregulation that is naturally achieved through posture changes and wallowing [52,67]. Another significant welfare issue is the restricted feeding regime for obesity prevention, which, together with confinement, can result in oral stereotypies, such as bar-biting and sham- or vacuum-chewing [68]. The European Parliament is also discussing a ban on the use of farrowing crates by 2027, which is supported by 170 non-governmental organizations (NGOs) across Europe and citizens’ initiatives like “End the Cage Age” [69]. As a result, housing systems that offer increased freedom of movement for the sow, such as loose farrowing systems, are being proposed, as they support nest-building activity and tactile contact with the piglets [70]. Moreover, outdoor farrowing systems are considered superior, since they allow free movement and the display of a series of natural behaviors, such as nesting, rooting, and grazing [71].

Several mutilations of neonate piglets, such as castration, tail docking, and teeth resection, are also implemented in the farrowing unit and are related to distress, frustration, and welfare impairment, since they cause acute and chronic pain [72]. Tail docking as a routine treatment is generally forbidden in Europe and should only be implemented if assessment of tail-biting risks and application of improvement measures, such as enrichment materials and improved practices, have not led to a sufficient reduction of tail biting [73,74]. At the same time, surgical castration is performed for the removal of an unpleasant odor from pork known as ‘boar taint’ but also for the prevention of undesired sexual and aggressive behavior in pigs. However, this practice has raised significant animal welfare concern in recent years, since scientific evidence proves that this surgical procedure inflicts pain, even on very young pigs. An effort has been made by the EU to avoid surgical castration of pigs by presenting the best practices for raising entire males or vaccinating them, causing immunocastration [75]. Finally, the rationale for teeth clipping is to reduce the prevalence of facial injuries to littermates and udder lesions in sows. However, increased incidences of hemorrhages and abscesses in the mouths of piglets are observed, causing acute and long-term pain [76].

Early weaning at 3–4 weeks of age is one of the most significant piglet welfare issues as a result of numerous stressors: separation from the sow, abrupt transition from milk to solid feed, alteration of physical environment and removal to the weaner unit, exposure to pathogens, and social distress caused as a result of regrouping [77]. An alleviating strategy is to maintain weaned pigs in the farrowing pen at least until they have recovered from the nutritional stress [52]. The occurrence of undesired behaviors displayed by weaner, grower, and finisher pigs, such as belly-nosing, aggression, and tail-biting, is also a result of their frustration and increases injuries and skin damage incidence. It seems that the absence of materials for foraging, rooting, and exploring with their snout directs pigs’ attention to the other individuals in the pen.

3. Sustainability of Pig Production

In recent decades, sustainability has become a central issue in the growing livestock sector. The livestock industry faces significant challenges, including environmental pollution, limited natural resources, and food safety [78,79,80]. As a result, the need to develop sustainable livestock production systems has gained great importance in order to satisfy increased demand for animal products while at the same time shifting towards more environmentally friendly production processes. According to the Brundtland Commission [81], sustainability encompasses three main dimensions: environmental, economic, and social. Additionally, animal welfare is widely regarded as a key component: the fourth dimension of sustainable animal production [3,10]. Sustainable livestock systems are thus described as production systems that are ethical, “economically viable, ecologically sound and socially acceptable, both now and in the future” [82]. This concept also applies to pig production, which is frequently analyzed in the context of specific issues such as harmful emissions and other adverse environmental impacts, animal welfare including health, farm income, and, embracing all of these, societal acceptance [11,83].

All of these aspects therefore play a crucial role in developing and assessing truly sustainable animal production. However, due to the complexity of sustainability and the interdependencies between its dimensions, environmental impacts are more frequently analyzed than economic or social issues. Studies of environmental sustainability in pig production have often been primarily associated with the management of hazardous waste, excess nutrients, and emissions generated by various production processes but should also include all of the impacts of feed production [80]. Another key environmental concern is the maintenance of biological biodiversity, while the exhaustive use of limited natural resources, such as land and water use, is also a major environmental aspect frequently studied in connection with pig production [84]. Some systems, such as semi-intensive silvopastoral systems used mainly for ruminants at present, can be used to contribute to pig feed and are much higher in biodiversity than most crop production systems [8].

Over the years, legislation, policies, and guidelines that expect to mitigate negative impacts of livestock production have been developed. For example, the Sustainability Assessment of Food and Agriculture (SAFA) framework [85] provides guidelines for consistent and reliable assessments of sustainable systems, while the One Welfare concept [44] offers a holistic approach supporting sustainability and emphasizing the links among animal welfare, biodiversity, and the environment [79,86]. Moreover, there are several European Directives to control pig production [87]. Directive 2010/75/EU of the European Parliament on industrial emissions (IEDs) introduces the construction of intensive rearing facilities for pigs, with a view to reducing emissions to air, soil, and water while also promoting resource quality enhancement by implementing the best available techniques. Furthermore, pig production is also subject to the National Emissions Ceilings (NEC) Directive 2001/81/EC, addressing the important air emissions of SO₂, NOx, VOCs, and NH₃. Moreover, Nitrate Council Directive 91/676/EEC offers the main framework for the protection of ground and surface waters against the excess of nitrates from agricultural and farming practices. Last but not least, pig production as an economic activity causing GHG emissions should be in line with the Paris Agreement (Council Decision (EU) 2016/1841) and its key objectives.

Table 2. Summary of sustainability assessment studies on pig production [79,80,82,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115].

Authors			McAuliffe et al.	I. Noya et al.	Pexas et al.	Lamnatou et al.	García-Gudiño et al.	Rudolph et al.	Reckmann et al.	Zira	Gunnarsson et al.	Winkler et al.	Dolman et al.	Noya et al.	Sagastume Gutierrez et al.	Gutierrez et al.	Monteiro et al.	Reyes et al.	Makara et al.	Bandekar et al.	Bav.	Pirlo et al.	Villavicencio-Gutiérrez et al.	Wu et al.	Liu et al.	Giraldi-Díaz et al.	Savian et al.	Pazmiño et al.	Zira et al.	Treml et al.	Santos et al.	Hietala et al.	Thoma et al.	Sun et al.
Year			2016	2017	2020	2016	2020	2018	2013	2020	2020	2016	2012	2017	2016	2018	2016	2019	2019	2019	2017	2016	2022	2024	2021	2021	2023	2021	2021	2025	2025	2024	2024	2025
Geographical context				Galicia (Northwest Spain)	Denmark	Spain	Spain	8 European countries	Germany	Sweden	Europe, North America, Australia, and New Zealand	Austria	Netherlands	Spain	Cuba	Mexico	Brazil, France	Cuba	Poland	United States	Italy	Italy	Mexico	China	China	Mexico	Brazil	Ecuador	Sweden	Germany	Portugal	Finland	America	China
Pig production—Stages	Feed production		x	x		x	x	x	x	x			x		x		x	x	x	x	x		x	x	x	x	x	x	x	x	x	x	x	x
	Energy and transport					x	x	x	x			x	x		x	x	x	x		x	x	x	x	x		x	x	x	x	x	x	x		x
	Animal growth	Gestation		x			x	x	x								x	x				x	x			x	x	x	x	x	x	x	x	x
		Farrowing		x			x	x	x								x	x				x	x	x		x	x	x	x	x	x	x	x	x
		Weaning		x			x	x	x								x	x				x	x	x		x	x	x	x	x	x	x	x	x
		Fattening		x			x	x	x								x	x				x	x	x		x	x	x	x	x	x	x	x	x
		Finishing					x	x	x								x	x					x				x	x	x	x	x	x	x	x
	Slaughtering								x	x														x	x			x	x	x		x	x
	Pig housing				x			x	x				x		x			x	x							x	x			x		x
	Waste/Manure management		x		x		x	x	x				x		x	x	x	x	x	x	x	x	x	x	x	x	x	x	x	x	x	x	x	x
	Whole-system pig production		x							x		x	x	x	x	x				x	x				x					x	x			x
Environmental Sustainability Assessment	GWP		x	x	x	x	x	x	x		x	x	x	x	x	x	x	x	x	x	x	x	x	x	x	x	x	x	x	x	x	x	x	x
	AP		x		x		x	x	x		x	x	x		x		x		x		x	x		x	x		x	x	x	x	x		x	x
	EP		x	x	x		x		x		x	x	x		x		x				x	x		x	x		x	x	x	x	x		x	x
	EC				x	x	x		x		x		x				x				x			x
	LO			x			x				x	x			x		x		x		x		x	x
	Other *		-	FD, FE, ME, TA	NREU, NRRU	CED, -	CED	NREU	-	-	-		NREU	FD, FE, ME, TA, WD	ADF, HT, POF, TE	FE, ME, PMF, POF, TA, WD	CED, TE	FE, ME, TA, TE	FD, FE, FET, HT-CE, HT-NCE, IR-E, IR-HH, ME, OD, PMF, POF, TE, WD	CED, CWC	ADF, NREU, OD, TE	ADR, PO	ALO, CC, FD, FE, FET, HT, IR, MD, ME, MET, MD, NLT, OD, PMF, PO, TA, TE, ULO, WD	WU	FE, FET, HTP. GWP, TA, TE	CF, EF, WF	-	FDP, GWP, MEP, ODP, PMFP, POPF, TAP	BDP, FDP, FE, FET, GWP, HTP, ME, MET, SCL, TAP, TE	-	FE, FET, FRS, HT-CE, HT-NCE, IR, LU ME, MET, MRS, OD, OF-HH, OF-TE, PMF, TA, TE, WU	WD	-	-

* The following abbreviations have been used concerning the environmental sustainability indicators—ADF: abiotic depletion of fossil fuel; ADR: abiotic depletion of resources; ALO: agricultural land occupation; AP: acidification potential; BDP: biodiversity damage potential; CC: climate change; CED: cumulative energy demand; CF: carbon footprint; CWC: cumulative water consumption; EC: energy consumption; EF: energy footprint; EP: eutrophication potential; FD: fossil depletion; FDP: fossil depletion potential; FE: freshwater eutrophication; FET: freshwater ecotoxicity; FRS: fossil resource scarcity; GWP: global warming potential; HT: human toxicity; HT-CE: human toxicity cancer effect; HT-NCE: human toxicity non-cancer effect; HTP: human carcinogenic toxicity; IR: ionizing radiation; IR-E: ionizing radiation effect; IR-HH: ionizing radiation human health effect; LO: land occupation; LU: land use; MD: metal depletion; ME: marine eutrophication; MEP: marine eutrophication potential; MET: marine ecotoxicity; NLT: natural land transformation; NREU: non-renewable energy use; NRRU: non-renewable resource use; OD: ozone depletion; ODP: ozone depletion potential; OF-HH: ozone formation human health; OF-TE: ozone formation terrestrial ecosystem; PMF: particular matter formation; PMFP: particulate material formation potential; PO: photochemical ozone formation; POF: photochemical oxidant formation; POPF: photochemical oxidant formation potential; MRS: minerals resource scarcity; SCL: soil carbon loss; TA: terrestrial acidification; TAP: terrestrial acidification potential; TE: terrestrial ecotoxicity; ULO: urban land occupation; WD: water depletion; WF: water footprint; WU: water use.

provides a brief visualization of publications, in terms of impact recorded, research topic, year of publication, and spatial distribution of the studies. To present these publications in a more descriptive manner, they have been mapped according to their relevance to pig production stages and sustainability assessments. Therefore, content-related criteria have been applied to ensure that publications covering unrelated research topics were excluded. To maintain a reliable and high-quality selection of papers, this review is limited to publications in peer-reviewed scientific journals. Consequently, conference proceedings, PhD or masters’ theses, diploma dissertations, working papers, and textbooks were excluded, as the availability of such sources is virtually unlimited. Only papers published in English-language journals were included. The analysis covers papers published between 2010 and 2025. Despite these limitations, the authors strongly believe that the selected publications represent the majority of relevant research in this field.

Due to its associated environmental concerns, and in order to contribute to the achievement of the sustainability goals set by the current legislative framework, pig production is regularly evaluated through a Life Cycle Analysis (LCA) approach. In this light, environmental and socio-economic impacts can be identified and, therefore, mitigation options be implemented in order to improve the integrated sustainability of pig production [97]. Several LCA studies have been implemented to measure the potential environmental performance of various pig farming systems, such as conventional, adapted conventional, traditional, and organic [116]. The pig production sector is a highly complex system including not only all the animal growth stages and processes involved in the production chain up to the cutting room but also activities related to feed production, land transformation, transportation, energy consumed, and waste management [88,104]. Thus, both on-farm and off-farm production stages and activities are evaluated under the scope and boundaries of the LCA studies. More specifically, on-farm activities refer to breeding and fattening of pigs. In addition to animal growth, the on-farm production stage includes the housing of fattening animals, manure storage, and the management of associated emissions [79]. As regards off-farm activities, they mainly involve the production of feed, including the use of pesticides and fertilizers, as well as the energy and raw materials required for processing and transport [95]. Even though most of the studies are considered cradle-to-farm-gate LCAs, slaughterhouse and cutting stage are rarely encompassed in the system boundaries [85,87]. Likewise, although the common practice is to apply LCA methodologies considering pig production as a whole system, due to the system’s complexity, several researchers focus on environmental impacts arising from separately key stages, such as feed production, pig housing, animal growth, or manure management strategies [88,90].

The impacts of sustainable pig farming are both global and local. On a global scale, apart from increased GHG emissions, the pig sector is also known as one of the most important contributors to the acidification of ecosystems, as well as to the eutrophication of freshwater systems [90]. Thus, as regards environmental issues, the most representative impact categories addressed through LCA analysis are global warming (GW), acidification (AP), and eutrophication (EP) [88,92,108,111,114,115,117,118]. Furthermore, attention is often given to the assessment of other common impact categories, such as depletion of abiotic resources (AD) [37,50,73], photochemical ozone formation (PO) [86,97], Non-Renewable Energy Use (NREU), and Non-Renewable Resource Use (NRRU) [79,90,93]. Evidence suggests that feed production is the farm activity with the highest contribution to all impact categories, followed by manure management [105,106,109,111,113,119], while the fattening stage has the highest impact of the animal growth activities [107,112].

As explained in the introduction to this paper, an important sustainability topic that is often not addressed in LCA studies is the extent to which farm animal feed could have been utilized by humans directly or, if it could not be thus used, the possibilities for using land for other crops that humans could eat [10,20]. Pig and poultry production are of less value to humans because human food production would be more efficient if the grain was eaten by humans rather than fed to pigs or poultry and then these animals eaten. Ideally, pig food would be something that humans could not eat or would be treated human food waste. For this reason—and since ruminants can eat the leaves of grasses, other forage plants, shrubs, and trees, and since about 42% of the land that can be used for human food production is not suitable for arable farming—the current trend towards increasing pig and poultry production and decreasing ruminant production may well be reversed. At least in Africa and Asia, small ruminant production is increasing at rates similar to pork, suggesting a shift in production dynamics [120]. Whenever sustainability is evaluated, all components should be taken into account, and, for each farmed species, the best systems for sustainability can be very much better than the worst systems [10].

Considering the diverse scenarios analyzed in the various studies mentioned above, the direct comparison of results is difficult and in some cases impossible [88,108]. In particular, differences in system boundaries; systems’ specific characteristics, like feed composition, waste management practices, and the production period; as well as the studies’ aim and objectives, such as focusing on the assessment of feeding strategies, housing systems, or manure management practices, impede the generalization of results. Furthermore, discrepancies in modeling assumptions and software calculations further complicate a comparative analysis across the various studies. Moreover, it is highlighted that the heterogeneity, both in the assessment methodologies applied and the way the results are reported in the LCAs conducted, significantly challenges comparison across different publications, making it difficult to draw meaningful conclusions. Therefore, it is recommended that each study should be analyzed individually [121].

Even though the majority of the environmental LCAs were conducted following the ISO 14040/14044 standards series [122], the selection of different impact assessment methods significantly influence Life Cycle Impact Assessment (LCIA) results. This effect is further strengthened by the variation in system boundaries definitions. Thus, to ensure comprehensive LCA outcomes of the pig production sector, it is important to include all relevant impact categories within a standardized assessment framework. In this regard, the adoption of common Product Environmental Footprint (PEF) guidelines [123] and category rules (PEFCRs) could play a vital role in standardizing methodologies. Consequently, comparability between different studies will be enhanced, especially in terms of system boundaries, emission modeling, LCIA methods, and background datasets [113,121]. Meanwhile, the FAO Livestock Environmental Assessment and Performance (LEAP) guidelines [124] can support practitioners by offering a useful base for methodological alignment, serving as a temporary solution until more specific standards are established and implemented within the sector.

With respect to the other important aspects of sustainability, socio-economic subjects in pig production farming have been less well addressed [75,77,91,105,110,125]. Data limitations, as well as the complexity of social issues, make the assessment of these aspects challenging [126,127]. The net farming income (NFI) indicator [79], feed conversion ratio (FCR), and feed costs (FC) [128] have been used in order to evaluate the economic impact of the livestock systems. Where that social performance is considered, societal impacts are quantified based on antibiotic usage and mortality rates [79,88]. However, key social impacts, such as labor conditions, community well-being, equity in technology access, and consumer trust, although being difficult to measure, are essential for comprehensive sustainability evaluations. Recent studies highlight the importance of integrating the dimension of technological aspects into sustainability assessment. Technological innovations, such as precision livestock farming (PLF) technologies, smart monitoring systems, automated management tools, and real-time welfare assessment, can offer significant potential to enhance efficiency, improve animal welfare, and mitigate environmental impacts [129,130]. Nonetheless, the implementation of such technologies also brings complex social implications, which should not be overlooked [131]. For instance, the adoption of digital tools can widen socio-economic disparities among farmers, particularly in terms of access to infrastructure, technical expertise, and financial resources [132,133]. Thus, in order not to pose risks to social sustainability, technological innovation in pig production must be evaluated not only for its efficiency gains but also for its broader economic, social, and ethical impacts.

4. Circular Bioeconomy Aspects in Pig Production

The integration of circular bioeconomy principles in pig production is increasingly recognized as a crucial strategy for enhancing sustainability and resource efficiency within the agricultural sector. Circular bioeconomy emphasizes the utilization of biological resources while minimizing waste and environmental impact, thereby creating a regenerative system that benefits both the economy and the ecosystem. The implementation of circular economy principles can lead to significant economic benefits for pig producers. By reducing waste and improving resource efficiency, farms can lower operational costs and enhance profitability [134]. The transition towards a circular food system not only addresses environmental challenges but also creates new business opportunities and promotes resilience within the swine sector [13].

The transformation of pig production through the lens of circular bioeconomy strategies showcases innovative technologies and methods that significantly enhance the sustainability, efficiency, and resilience of operations compared to traditional practices. Emerging technologies, such as anaerobic digestion and bioconversion processes, allow pig farms to convert manure into biogas and organic fertilizers, thereby closing nutrient loops and reducing reliance on chemical fertilizers [135]. Furthermore, alternative feeding strategies that incorporate circular bioeconomy principles utilize locally sourced ingredients and by-products to optimize resource use, compared with traditional feeding methods that may overlook the dynamic dietary needs of pigs during their growth phases, potentially resulting in excess nutrient excretion and increased feed costs [136].

Biomethane is a renewable energy carrier derived from biogas, representing a purified form of natural gas. It is generated by upgrading biogas—a gaseous mixture produced via anaerobic digestion of organic substrates—through the removal of carbon dioxide and other impurities, thereby increasing methane content to a minimum of 95% [137,138]. Various technologies are available for this upgrading process. Among them, biomethanation is particularly promising, wherein CO₂ is biologically converted to CH₄ through hydrogen injection, either directly within the anaerobic digester (in situ) or in a subsequent reactor (ex situ), achieving methane concentrations of up to 98–99% [138,139]. Photosynthetic upgrading using microalgae in dedicated bioreactors is another viable approach. Additional techniques include cryogenic separation, pressure swing adsorption, and water scrubbing, each presenting unique benefits and constraints [137,139]. While the literature on upgrading biogas derived specifically from pig farm wastewater remains limited [140], full-scale plants processing pig slurry have been operational for over a decade using feedstock-independent upgrading technologies. For instance, the Köckte plant in Germany has produced around 350 Nm³/h of biomethane from pig and cattle slurry since 2013 [141]. Similarly, a facility in Liffré, France, operational since 2015, injects approximately 50 Nm³/h of biomethane into the grid, treating agricultural residues, intermediate crops, and mixed animal manures including pig slurry [141].

Recent advances concerning the biogas upgrade to biomethane place emphasis on algae-based biogas upgrades, the use of novel solutions for CO₂ capture, and removal or novel materials for CO₂ adsorption. Handayani et al. investigated the upgrading of biogas using Chlorella vulgaris microalgae in laboratory-scale photobioreactors composed of two columns separated by a membrane, achieving 98% v/v CO₂ removal and thereby enhancing biogas quality [142]. In a separate study [143], novel Deep Eutectic Solvents (DES) were evaluated as promising absorbents for CO₂ capture. The maximum CO₂ absorption capacity reached 0.608 mol-CO₂/mol-DES, with the highest CO₂/CH₄ selectivity factor reported at 25. Lastly, Imran-Masood et al. explored the application of biochar derived from spruce sawdust for biogas upgrading, reporting a CO₂/CH₄ selectivity of approximately 3 and demonstrating successful regeneration of the biochar across 10 consecutive cycles [144]. Overall, the upgrading of biogas for biomethane production offers a viable valorization pathway for pig manure, with substantial potential to convert waste-based biogas into a high-value renewable fuel, thereby supporting circular economy principles and climate change mitigation.

One of the primary aspects of circular bioeconomy in pig production is the effective management of waste, mainly pig manure. One of the challenges associated with pig manure is its high nutrient content, particularly nitrogen (N) and phosphorus (P), which can lead to environmental pollution if not managed properly. The application of manure-based fertilizers (MBFs) has been shown to reduce nutrient losses and contribute to a circular economy by closing the nutrient cycle. By partially replacing traditional mineral fertilizers with MBFs, farmers can enhance nutrient recovery and improve soil fertility, which is crucial for food security. Furthermore, the use of MBFs allows for year-round availability of nutrients, contrasting with the seasonal availability of raw manure [145]. Moreover, the implementation of solid–liquid separation techniques allows the recovery of valuable nutrients while reducing eutrophication and the environmental footprint of manure application [146].

Anaerobic digestion is another effective method for managing pig manure within a circular bioeconomy framework. This process not only reduces the volume of manure but also generates biogas, a renewable energy source that can be utilized on-farm for heating or electricity generation, thereby reducing reliance on fossil fuels and lowering greenhouse gas emissions, or sold [147,148]. The digestate produced from anaerobic digestion is nutrient-rich and can be used as a fertilizer, thereby promoting sustainable agricultural practices [149]. Technologies such as gas-permeable membrane systems can enhance ammonia recovery from anaerobically digested manure, further improving nutrient management and reducing environmental impacts [150].

In the context of waste management and renewable energy generation, pig fat, along with other animal fats, serves as a viable feedstock for biodiesel synthesis, contributing to the reduction of waste from the meat industry while providing a substitute for, and means of reduction in, fossil fuels by integrating animal by-products into energy production [151,152]. Pig fat biodiesel has been shown to possess favorable properties for fuel applications. The density, kinematic viscosity, emissions profiles, and flash point of biodiesel derived from pig fat are higher than those of biodiesel from other sources, such as waste fish oil and poultry fat, making it a competitive option in terms of fuel performance [153,154]. Additionally, biodiesel derived from pork fat contributes significantly less to greenhouse gas emissions compared with conventional fossil fuels. The carbon released during combustion is part of the current carbon cycle, having been absorbed by the animals during their life [155]. This aligns with the goals of a circular economy, where waste materials are repurposed into valuable resources, thus minimizing environmental impact and promoting sustainability [156].

Water waste management in the pig industry is another crucial aspect of sustainability. Swine wastewater comprises swine excretions and water for cleaning facilities. It is estimated that each pig generates approximately 1300 tons of wastewater annually [157,158]. The adoption of microalgae-based wastewater treatment processes in pig farming systems represents a promising circular bioeconomy approach that addresses both nutrient management and environmental sustainability. Microalgae have been shown to be highly effective in removing nutrients from wastewater, particularly nitrogen and phosphorus, which are abundant in pig slurry, and simultaneously producing rapid-growing biomass that can be used as biofuels and animal feed [159,160]. This dual benefit of nutrient recovery and biomass production is a key advantage of microalgae-based systems over conventional wastewater treatment methods, which typically focus on nutrient removal without generating valuable by-products [161].

The integration of microalgae with bacteria in wastewater treatment processes can further enhance nutrient removal efficiency, highlighting the potential for reduced energy requirements and improved environmental sustainability [162]. This synergistic approach not only optimizes nutrient uptake but also contributes to carbon dioxide fixation, making it an environmentally friendly solution for managing pig waste. Moreover, the biomass produced from microalgae can be utilized as a high-quality feed ingredient for pigs that improves growth performance and nutrient digestibility and provides essential fatty acids and other bioactive compounds that promote health [163,164]. This not only reduces the reliance on conventional feed sources but also contributes to a more sustainable feed supply chain.

The utilization of by-products in pig nutrition represents a significant advancement in sustainable production, particularly in the context of rising feed costs and environmental concerns. By incorporating alternative feed ingredients derived from agricultural waste or industrial by-products, pig farmers can enhance the sustainability of their operations while maintaining or improving animal performance. One notable example is the partial replacement of soybean meal with alternative protein sources. The inclusion of oilseed by- and co-products, distillers’ dried grain with solubles, and microalgae in pig rations has been well documented, mainly regarding the effects on pigs’ growth, carcass yield, and meat quality [165]. A further important area of development is the use of the leaves of shrubs and trees with high protein content and low levels of toxins, for example, from semi-intensive silvopastoral systems, as a source of feed for livestock, including pigs and also fish [7,8].

Rapeseed meal is derived from the press cake that remains after oil extraction. If further processed through fermentation or other methods to improve its digestibility, rapeseed meal can be incorporated into diets for growing and finishing pigs, with a level of inclusion between 10% and 20% in most cases, without adversely affecting average daily gain or feed conversion ratios [166,167,168,169]. The use of rice by-products, such as defatted rice bran and rice distiller’s grain, in levels up to 20% in the rations without any negative effects on performance, was also reported [137]. Furthermore, the inclusion of dried microalgae at low rates (5%) has been widely investigated and emerged as a promising strategy to both improve the nutritional quality of pork and promote sustainable production [170,171,172,173,174].

Other by-products of the food industry that are considered a good source of protein for animal feed are distillers’ dried grain with solubles (DDGS). DDGS is the main co-product from the ethanol industry, produced by dry mill ethanol plants, and can be added to pig rations at levels up to 30% without compromising performance [175,176,177]. Moreover, the incorporation of former food products (FFPs) in pig rations, which include food products that are no longer intended for human consumption but are safe for animal feed, is another promising aspect of minimizing food waste and promoting circular bioeconomy in pig production. Those by-products can effectively replace a portion of conventional cereal grains in pig diets without adversely affecting growth performance or digestibility [178]. Among feed ingredients that can be used in swine ration formulations, bakery by-products can be an excellent choice due to their energy content, high palatability, low levels of anti-nutritional agents, and market availability [179,180]. When the main ingredients in the diets of growing pigs and sows are maize and soya, dried bakery meal can safely replace up to 50% of maize without any adverse effect on their health, yield, and carcass quality [181,182,183]. The inclusion of bakery meal at 15–30% w w⁻¹ caused no detrimental effects on the overall growth parameters of post-weaning piglets [184,185], while it led to significantly less water consumption and land use impact, thus decreasing the overall environmental impact of the pig production by around 5% [186]. The above findings are promising, as they suggest that bakery by-products can be a sustainable and cost-effective alternative in pig nutrition, promoting nutrient recycling and shifting to more diverse and more efficient food patterns. At the same time, the widespread use is constrained by limited availability, seasonal variability, and challenges in consistent large-scale production and supply chains [187]. The scalability of alternative feeds is hindered by regulatory barriers, logistical issues in collection and processing, and the need for standardized nutritional profiles to ensure they can reliably replace conventional feed ingredients without compromising pig health and performance. Technical issues such as proper unpackaging of packed foods and effective removal of plastic wastes from the feed material should be also considered. Finally, challenges exist in achieving cost-effective and predictable growth since alternative feeds are not readily available and usually of inconsistent nutrient profile.

Another significant potential for the development of sustainable renewable biological resources is highlighted in the research of Gaffey et al. [17] that explores the benefits of green biorefineries implemented within the pig sector. In green biorefinery systems, green biomasses, such as grasses and legumes, can be processed into multiple co-products, including leaf protein concentrate (LPC), which is suitable for monogastric animals [188]. The results from the case study that was conducted on a pig farm indicated that LPC could replace soyabean meal at a 50% displacement rate, with pigs showing positive performance in intake and weight gain. Additionally, the resulting pig slurry demonstrated higher biogas content and 26% higher biomethane potential compared with the control slurry [17].

The adoption of circular bioeconomy practices in pig production systems presents numerous economic opportunities but also faces several barriers. While all the above technologies and systems exist, their implementation often requires substantial capital investment and technical expertise, which small- and medium-sized pig farmers may lack [189]. Economic challenges include the initial investment costs and the fluctuating economic benefits that vary based on market conditions [190]. For example, the costs associated with constructing and operating anaerobic digesters may not be easily recuperated if energy prices are low or if the market for by-products does not compensate for the investment made. Furthermore, the availability of subsidies or incentives for transitioning to circular practices may differ significantly among regions, affecting farmers’ decisions to invest in these technologies [191]. Finally, regulatory frameworks can significantly impact the adoption of circular bioeconomy practices. Inconsistent regulations regarding waste management and environmental protection can create uncertainty and discourage investments in necessary technologies [191]. A lack of comprehensive policy support may hinder farmers from understanding the benefits and applications of circular principles [191].

5. Conclusions

The pig production sector faces significant challenges in balancing efficiency, animal welfare, and environmental components of sustainability. This review highlights the critical role of housing conditions, management practices, and regulatory frameworks in ensuring the good welfare of pigs while also addressing the sector’s contributions to climate change and resource depletion. The integration of circular bioeconomy principles, such as manure valorization, alternative feed sources, and wastewater treatment, presents promising solutions for improving sustainability. However, the adoption of these strategies varies widely depending on policy frameworks, technological advancements, and economic feasibility. In addressing these barrier regulations, support for technology implementation and educational initiatives will be critical for facilitating the transition towards a more sustainable and resilient pig production system. Moreover, consumer perceptions can play a pivotal role for broader adoption of circular practices; market demand is oriented to products that are perceived to be sustainably produced, which encompasses ethical animal husbandry, environmental stewardship, and local sourcing.

Regulatory measures, particularly within the European Union, have played a crucial role in improving welfare standards and reducing environmental impacts. Policies such as Council Directive 2008/120/EC on pig welfare and Directive 2010/75/EU on industrial emissions have set important benchmarks. However, global disparities in policy adoption highlight the need for harmonized international standards to ensure a more sustainable and ethical pig production system worldwide. Additionally, emerging precision livestock farming technologies can further optimize resource efficiency while enhancing welfare monitoring.

Despite advancements, several research gaps remain. Further studies are required to optimize alternative feed ingredients, particularly from food industry by-products, ensuring both nutritional adequacy and environmental benefits. However, the level of inclusion in pig rations requires further research, regarding nutrient variability, digestibility, extraction methods, and antinutritional factors. Although different studies reviewed in this article have demonstrated the effective inclusion of alternative feeds in pig diet without adverse health effects or reduced meat productivity, the long-term adoption of these practices should be carefully evaluated to ensure adequate nutrient supply and avoid adverse effects on growth performance, gut health, and immune function. Additionally, improving manure processing technologies is crucial to minimize nutrient losses and emissions while maximizing energy recovery. Socio-economic assessments of sustainable pig farming practices also require greater attention, as consumer perceptions, market incentives, and economic viability play a vital role in the adoption of sustainable practices. Furthermore, ensuring socio-economic sustainability necessitates not only evaluating economic returns but also addressing social equity, equitable access to technology, and inclusive development within rural communities. Therefore, future assessments should integrate these factors to more comprehensively capture the full impact of innovation-driven pig production systems and practices. The inefficiency of using what could be human food to feed pigs requires major change in order that there will not be a substantial decrease in world pig production. The principles of resource efficiency, circular economy, and animal welfare are relevant to global pig production systems, regardless of region or production scale.

Achieving a truly sustainable pig industry requires a multi-stakeholder approach that integrates policy support, scientific innovation, and industry collaboration. Education and stakeholder engagement can be critical aspects for advancing animal welfare, sustainability, and circular bioeconomy principles in pig production systems. Targeted education and training programs equip farmers and farm personnel with the necessary knowledge and skills to implement welfare-enhancing practices, optimize resource use, and adopt environmentally sound technologies, promoting circularity in pig farming, Simultaneously, the active involvement of diverse stakeholders—including producers, researchers, industry actors, policymakers, and consumers—ensures that innovations are co-developed and aligned with both practical feasibility and societal expectations. The multi-actor collaboration is also critical towards a systematic change by integrating ethical, environmental, and economic considerations into decision-making processes, ultimately supporting the transition towards more resilient and sustainable pig production models. As the sector evolves, continued research and policy development will be essential to aligning economic productivity with environmental and ethical responsibility. For pig production, as for all other food production, all components of sustainability should be evaluated and the use of the worst systems discontinued.