Nutritional Strategies for Stress Mitigation and Performance Enhancement in Swine

Stress is an unavoidable component of modern swine production and arises from multiple environmental, physiological, and management-related factors, including weaning, heat exposure, transportation, and social interactions. These stressors disrupt homeostasis and adversely affect growth performance, immune function, reproductive efficiency, and overall animal welfare, leading to significant economic losses. Increasing evidence highlights oxidative stress as a central mechanism underlying stress-induced damage in pigs, emphasizing the need for effective mitigation strategies. This review synthesizes current knowledge on the role of nutrition in modulating stress responses in swine. Key nutritional strategies discussed include the use of functional amino acids, low-protein diets, plant-derived bioactive compounds, organic acids, prebiotics, probiotics, and essential micronutrients such as vitamins and trace minerals. These interventions exert their beneficial effects through multiple mechanisms, including enhancement of antioxidant capacity, modulation of gut microbiota, improvement of intestinal integrity, and regulation of immune and metabolic functions. The review further emphasizes the importance of integrated and precision nutrition approaches tailored to specific stressors and production stages, which can provide synergistic benefits and improve overall resilience. Advances in nutrigenomics and microbiome research offer promising opportunities for developing targeted feeding strategies. In conclusion, nutritional modulation represents a practical, sustainable, and effective approach to mitigating stress in swine production systems, contributing to improved animal welfare, productivity, and system efficiency.

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Introduction

Stress in swine, first conceptualized by Hans Selye as "a non-specific response of the body to any change," represents a complex physiological and behavioural reaction to internal and external challenges. It is an inevitable component of modern swine production systems, arising from a variety of environmental, physiological, and management-related factors such as weaning, transportation, regrouping, thermal extremes, and disease challenges. These stressors disrupt homeostasis and trigger complex neuroendocrine, immune, and metabolic responses mediated primarily through the hypothalamic-pituitary-adrenal (HPA) axis and sympathetic nervous system (Martínez-Miró et al 2016) [1] . As a consequence, stress negatively affects feed intake, growth performance, reproductive efficiency, immune competence, and meat quality, thereby causing substantial economic losses in pig production systems. In practical swine production systems, animals are exposed to multiple stressors that can significantly impair performance and welfare. Common stress factors include longdistance transportation, vaccination and disease challenges, abrupt temperature fluctuations, weaning, pregnancy and parturition, and suboptimal housing management. These stressors negatively influence feed intake, immune competence, growth rate, and reproductive efficiency. Consequently, various forms of stress reduce the animal's resistance to diseases, compromise productivity, and adversely affect reproductive traits. Understanding the nature, classification, and impact of stress is therefore essential for developing effective management and nutritional strategies to enhance resilience and optimize overall swine production efficiency (Martínez-Miró et al., 2016; Renaudeau et al., 2011; Pluske et al., 2018) [1,2,27] . In recent years, there has been increasing recognition of the role of oxidative stress as a central mechanism underlying stress-induced damage in swine. Oxidative stress results from an imbalance between reactive oxygen species (ROS) production and antioxidant defence systems, leading to cellular dysfunction, impaired immunity, and reduced productivity (Bacou et al., 2021) [4] . Nutritional stressors, including poor-quality feed ingredients, mycotoxins, and imbalanced diets, further exacerbate this condition and compromise animal health and performance. [5,6,7] [8,9,10] . In response to growing concerns regarding antimicrobial resistance and food safety, regulatory restrictions on antibiotic use have intensified, thereby necessitating the development and adoption of sustainable and environmentally friendly alternatives in modern swine production systems (Landers et al., 2012) [11] . Nutritional interventions have emerged as a promising and sustainable approach to alleviate stress and improve resilience in swine. These strategies include the use of functional amino acids, antioxidants, plant-derived bioactive compounds, probiotics, prebiotics, organic acids, vitamins, and trace minerals, which act through modulation of gut health, immune function, and antioxidant capacity (Wang et al., 2023) [6] . Therefore, anti-stress nutrition strategies represent an essential tool in modern pig production, aiming not only to enhance animal welfare but also to improve productivity and sustainability. Understanding the mechanisms and applications of these nutritional interventions is critical for developing effective feeding programs to combat stressrelated challenges in swine.

Types of Stress in Swine Production

Types of Stress in Swine Production

Social Stress

Social stress in swine arises primarily from regrouping, high stocking density, social isolation, and the establishment of dominance hierarchies. Such conditions intensify aggressive interactions, including biting and fighting, and increase competition for feed, water, and space, ultimately impairing welfare and performance. Elevated stocking density has been consistently associated with reduced growth rates, poorer feed efficiency, and increased injury incidence (Verdon et al., 2007) [12] . Among all social stressors, weaning is considered one of the most critical events in a piglet's life, as it involves abrupt maternal separation, transition from milk to solid feed, and exposure to unfamiliar pen mates. These combined challenges often result in reduced feed intake, intestinal dysfunction, postweaning diarrhoea, and compromised immunity (Pluske et al., 2018; Campbell et al., 2013) [27,13] . Transportation further exacerbates stress through handling, mixing, feed and water deprivation, and environmental fluctuations. It induces physiological disturbances such as elevated cortisol levels, dehydration, and altered energy metabolism. Interestingly, evidence suggests that pigs subjected to shorter transport durations may exhibit more acute stress responses compared to those transported for longer periods, possibly due to adaptive habituation mechanisms (Sutherland et al., 2012) [14] . Overall, social stressors significantly influence behavioural responses, immune competence, and productivity in swine systems.

Heat Stress

Pigs are highly susceptible to heat stress due to their limited capacity for evaporative cooling, as they lack functional sweat glands and possess a relatively thick subcutaneous fat layer. Their thermoneutral zone is narrow, and exposure to high ambient temperatures results in marked reductions in voluntary feed intake, growth performance, and reproductive efficiency (Renaudeau et al., 2011) [2] . Heat stress also alters endocrine and metabolic functions, characterized by increased secretion of cortisol and adrenocorticotropic hormone (ACTH), reduced thyroid hormone (thyroxine) levels, and impaired insulin sensitivity, all of which contribute to decreased nutrient utilization efficiency (Pearce et al., 2013) [15] . In addition, heat stress negatively impacts intestinal integrity by disrupting tight junction proteins, leading to increased gut permeability ("leaky gut") and reduced nutrient absorption (Johnson et al., 2015; Pearce et al., 2013) [15,16] . This condition is often accompanied by systemic inflammation and oxidative stress, further compromising animal health and performance. Consequently, heat stress represents one of the most economically significant challenges in swine production, particularly in tropical and subtropical regions.

Oxidative Stress

Oxidative stress is a critical physiological condition resulting from an imbalance between the production of reactive oxygen species (ROS) and the antioxidant defense system. Excess ROS can damage lipids, proteins, and nucleic acids, leading to cellular dysfunction and impaired metabolic processes. In swine, oxidative stress is closely associated with reduced growth performance, compromised immune function, reproductive inefficiency, and increased susceptibility to diseases (Bacou et al., 2021; Surai, 2016) [4,17] . Multiple factors contribute to oxidative stress in pigs, including environmental challenges (e.g., heat stress), dietary imbalances, mycotoxin contamination, and high metabolic demands during rapid growth or reproduction. For instance, oxidative stress during gestation can impair placental function and fetal development, while in growing pigs it may reduce feed efficiency and muscle quality (Rooke et al., 2010) [18] . Furthermore, oxidative stress is often interlinked with other stressors, amplifying their negative effects on overall productivity. Therefore, maintaining an optimal antioxidant balance through nutritional strategies is essential for mitigating oxidative damage and enhancing resilience in swine production systems.

Nutritional Strategies to Mitigate Stress 3.1 Functional Amino Acids

Functional amino acids play critical roles beyond protein synthesis, including regulation of metabolism, immune responses, and antioxidant defence mechanisms. Arginine is essential for nitric oxide synthesis, immune modulation, and reproductive function. Supplementation (0.4%) has been shown to alleviate heat stress effects in weaned piglets by improving average daily gain and feed efficiency, while higher levels (0.8-1.0%) enhance antioxidant capacity and semen quality in boars (Yun et al., 2020) [19] . Glutamate and glutamine are vital for maintaining intestinal integrity and serve as precursors for glutathione, a key intracellular antioxidant. Leucine, a branched-chain amino acid, has been reported to reduce post-weaning diarrhoea and improve growth performance under stress conditions by modulating protein synthesis and immune signalling pathways. The application of functional amino acids has emerged as an effective nutritional strategy to enhance stress resistance in swine, with multiple studies demonstrating their beneficial roles across different physiological stages and stress conditions. Arginine, one of the most extensively studied functional amino acids, has been shown to improve antioxidant status and mitigate oxidative stress in sows when supplemented at levels of 33-36 g/day. In weaned piglets, dietary arginine inclusion at approximately 0.4% alleviates heat stress by improving average daily gain and feed efficiency, while higher supplementation levels (0.8-1.0%) in boars enhance reproductive performance and antioxidant capacity under heat stress conditions (Yun et al., 2020) [19] . Furthermore, combined supplementation of arginine and lysine (2%) in finishing pigs has been reported to reduce the adverse effects of transportation stress, highlighting the synergistic role of amino acids in stress mitigation (Li et al., 2018). Glutamate also plays a critical role in stress resistance by supporting intestinal integrity and antioxidant defence (Wu, 2013). Supplementation in piglets, boars, and weaned pigs has been associated with improved resilience against oxidative stress, primarily through its role as a precursor for glutathione synthesis (Rezaei et al., 2013; Wu et al., 2010) [22] . Similarly, monosodium glutamate supplementation at 4% in post-weaning pigs has been shown to alleviate weaning stress by enhancing feed intake and gut health (Rezaei et al., 2013) [22] . Branched-chain amino acids such as leucine have also demonstrated significant benefits; supplementation at 1% in weaned piglets reduces immune stress and improves growth performance, while inclusion at 0.5% in growing pigs enhances tolerance to heat stress (Li et al., 2007; Rhoads et al., 2006) [25] . Additionally, methionine supplementation (0.145%) in nursery pigs contributes to improved antioxidant capacity and reduced oxidative damage, owing to its role in methylation reactions and glutathione metabolism (Martínez et al., 2017) [1] . Collectively, these findings underscore the importance of functional amino acids in modulating physiological responses to various stressors in swine. Their roles in enhancing antioxidant defense, maintaining gut integrity, improving immune function, and supporting growth and reproduction highlight their potential as key components of anti-stress nutritional strategies in modern pig production systems.

Low-Protein Diets

The application of low-protein (LP) diets supplemented with balanced essential amino acids has gained considerable attention as an effective nutritional strategy to enhance stress resilience in swine. High-protein diets often result in excess undigested protein in the hindgut, promoting the growth of pathogenic bacteria and increasing the risk of diarrhoea (Heo et al., 2013) [27] . A reduction of dietary crude protein by 3 percentage units below National Research Council (1998) recommendations did not compromise growth performance or alter blood biochemical profiles in weanling crossbred (Yorkshire × Zovawk) pigs, suggesting the feasibility of protein optimization without negative physiological consequences (Lallawmawmi et al., 2025) [28] Evidence from multiple studies indicates that moderate reductions in dietary crude protein (CP) can alleviate physiological stress, improve gut health, and enhance overall performance without compromising growth. For instance, dietary CP levels of approximately 17.25% in weaned piglets (LWY × Landrace) have been shown to reduce oxidative stress by improving antioxidant status and minimizing the accumulation of undigested protein in the hindgut. More pronounced reductions in CP have demonstrated benefits under specific stress conditions. In growing crossbred pigs exposed to heat stress, lowering dietary CP to 13.5% significantly improves intestinal morphology, enhances nutrient digestibility, and reduces metabolic heat production, thereby improving overall performance (Heo et al., 2009; Yue and Qiao, 2008) [27,31] . Likewise, in weaned pigs, a CP level of around 17% has been associated with reduced incidence of post-weaning diarrhoea and improved gut integrity under weaning stress conditions (Wellock et al., 2008; Nyachoti et al., 2006) [32,33] . In breeding animals, particularly sows of Huzhu Bamei pigs, further reduction of CP to approximately 12% has been reported to alleviate weaning-related stress, likely through improved nitrogen utilization efficiency and reduced metabolic burden (Dourmad et al., 1999; Wu et al., 2014) [34] . Collectively, these findings highlight that strategic reduction of dietary protein, when coupled with adequate amino acid supplementation, can effectively mitigate various forms of stress-including oxidative, heat, and weaning stress-in swine. The underlying mechanisms include reduced substrate availability for pathogenic bacteria in the gut, improved intestinal barrier function, decreased nitrogen excretion, and lower metabolic heat increment. Therefore, low-protein feeding strategies represent a sustainable and environmentally friendly approach to improving animal welfare and production efficiency in modern swine systems.

Plant Extracts

The use of plant-derived bioactive compounds has gained considerable attention as a natural and sustainable approach to enhance stress resistance in swine production systems. These phytogenic additives, including polyphenols, flavonoids, and essential oils, exert their beneficial effects through antioxidant, anti-inflammatory, and antimicrobial mechanisms, thereby improving animal health and performance under various stress conditions (Liu et al., 2018; Windisch et al., 2008) [37] . Silymarin supplementation has been shown to enhance antioxidant enzyme activity and improve reproductive performance in gestating sows, while grape seed proanthocyanidins improve intestinal morphology and reduce oxidative stress. Oregano essential oil, rich in carvacrol and thymol, has demonstrated efficacy in mitigating transport-induced oxidative damage and improving immune responses. These phytogenic additives also enhance feed palatability and digestion. Resveratrol, a polyphenolic compound, has been shown to mitigate oxidative stress in weaned piglets when supplemented at approximately 100 mg/kg. Its protective effects are primarily attributed to the activation of antioxidant enzymes and reduction of lipid peroxidation. Similarly, curcumin, administered at 400 mg/kg in neonatal piglets experiencing growth retardation, enhances antioxidant capacity and reduces oxidative damage by modulating inflammatory pathways and improving cellular defence systems (Niu et al., 2019) [39] . Essential oils such as oregano oil, rich in bioactive components like carvacrol and thymol, have demonstrated efficacy in reducing transportation-induced stress in finishing pigs at inclusion levels of around 25 mg/kg. These compounds improve immune responses and reduce oxidative damage during handling and transport (Zou et al., 2016) [40] . Quercetin, another potent flavonoid, is also recognized for its antioxidant and anti-inflammatory properties, although its application in swine requires further dose optimization studies. Betaine, a methyl donor and osmolyte, plays a crucial role in maintaining cellular water balance and metabolic stability. Supplementation at 1 g/kg in growing pigs has been shown to alleviate heat stress by improving thermoregulation, enhancing feed intake, and reducing oxidative stress. Grape seed proanthocyanidins, administered at approximately 250 mg/kg, have been reported to improve intestinal morphology, enhance antioxidant enzyme activity, and reduce weaning stress in gilts and barrows. Furthermore, Eucommia ulmoides, a traditional medicinal plant, has demonstrated promising results in alleviating weaning stress in piglets when included at 0.5% of the diet. Its bioactive constituents contribute to improved gut health, immune function, and antioxidant status (Peng et al., 2019) [41] . Collectively, these studies highlight the potential of plant extracts as effective alternatives to antibiotics, offering multifaceted benefits in mitigating oxidative, heat, transportation, and weaning stress in swine production systems.

Organic Acids

Organic acids, including short-chain fatty acids (SCFA), medium-chain fatty acids (MCFA), and organic acids such as lactic, formic, and propionic acid, play a pivotal role in maintaining gastrointestinal health and enhancing feed efficiency in swine. Their primary mode of action involves reducing the pH of the gastrointestinal tract, thereby creating an unfavorable environment for pathogenic microorganisms such as Escherichia coli and Salmonella spp., while simultaneously promoting the proliferation of beneficial microbiota (Partanen & Mroz, 1999; Dibner & Buttin, 2002) [42,43] . This acidification also improves the activity of digestive enzymes, particularly pepsin, leading to enhanced protein digestion and nutrient utilization. In addition to their antimicrobial effects, SCFAsespecially butyrate-serve as an important energy source for intestinal epithelial cells, promoting villus growth, improving mucosal integrity, and strengthening the intestinal barrier function (Guilloteau et al., 2010) [44] . Butyrate also exhibits anti-inflammatory properties by modulating cytokine production and reducing oxidative stress. MCFA, such as caprylic and capric acids, possess strong antimicrobial activity due to their ability to disrupt bacterial cell membranes, thereby reducing pathogen load and improving gut health (Hanczakowska et al., 2013) [45] . Lactic acid further contributes by lowering gastric pH and enhancing mineral solubility and absorption. Sodium butyrate, a key SCFA, has been shown to reduce inflammatory cytokine expression, improve intestinal barrier integrity, and promote epithelial cell proliferation. MCFA improves energy utilization and immune response, while lactic acid enhances gastric digestion and microbial balance. Organic acids are widely recognized as effective alternatives to antibiotic growth promoters in swine diets (Ren et al., 2014) [29] . Moreover, organic acids have been shown to improve growth performance, feed conversion ratio, and overall health status in pigs, particularly during critical periods such as weaning when animals are highly susceptible to digestive disturbances. Their inclusion in diets is widely recognized as an effective alternative to antibiotic growth promoters, contributing to sustainable and safe swine production systems (Partanen & Mroz, 1999; Upadhaya et al., 2014) [1,42] .

Prebiotics

Prebiotics are non-digestible dietary components that selectively stimulate the growth and activity of beneficial gut microbiota. Common prebiotics include fructooligosaccharides (FOS), mannan-oligosaccharides (MOS), and chito-oligosaccharides (COS) (Gibson & Roberfroid, 1995) [47] . Research into the application of prebiotics in swine nutrition has demonstrated their potential to mitigate various forms of physiological stress, including immune, oxidative, weaning, and transportation stress. Several classes of prebiotics-such as fructooligosaccharides (FOS), chitooligosaccharides (COS), γ-aminobutyric acid (GABA), inulin, microalgae-derived compounds, and mannan-oligosaccharides (MOS)-have been evaluated across different stages of pig development. (Czech et al., 2009) [49] . For instance, supplementation with FOS in weaned piglets was shown to enhance immune resilience under stress conditions (Liu et al., 2020) [50] . Similarly, COS has been investigated in both weaning piglets and sows. At a dosage of 158.8 mg/kg, COS alleviated weaning stress in piglets (Liu et al., 2018) [36] , while a lower dose of 30 mg/kg improved antioxidant capacity in sows experiencing oxidative stress (Xie et al., 2016) [51] . GABA has also demonstrated promising antistress effects. In weaned piglets, supplementation at 20 mg/kg improved immune responses. In growing-finishing pigs, a higher dose of 30 mg/kg helped mitigate transportation-induced stress (Bi et al., 2020). Inulin, administered at 4% of the diet, was found to enhance immune function in piglets (Tako et al., 2008) [53] . Additionally, microalgae supplementation at 3.12% in pregnant sows showed protective effects against oxidative stress (Lee et al., 2019) [54] . MOS, provided at 8 g/day, also improved oxidative stress parameters in sows (Czech et al., 2009) [49] . Overall, these findings indicate that prebiotic supplementation can play a significant role in improving stress resistance in swine, with effects varying depending on the type of prebiotic, dosage, and physiological stage of the animal.

Probiotics

Probiotics, including Lactobacillus, Bacillus, and Bifidobacterium species, improve gut health and stress resilience by modulating microbial balance and enhancing host immunity. They function through competitive exclusion of pathogens, production of antimicrobial metabolites, and stimulation of immune signaling pathways. Probiotics also activate antioxidant pathways such as Nrf2-Keap1 and MAPK, thereby reducing oxidative stress and improving intestinal integrity. Their supplementation has been associated with improved growth performance, reduced diarrhea incidence, and enhanced immune responses in pigs. Probiotic supplementation has been increasingly explored as a nutritional strategy to enhance stress resistance in swine, particularly during critical production stages such as weaning, growth, and transportation (Wang et al., 2018) [55] . Various microbial strains, including Lactobacillus spp. and Saccharomyces boulardii, have demonstrated beneficial effects in alleviating immune and oxidative stress. For example, administration of Lactobacillus plantarum to female crossbred piglets at a concentration of 2.5 × 10⁷ CFU/mL significantly improved immune responses under stress conditions, suggesting its role in modulating host immunity (Shin et al., 2019) [48] . Similarly, supplementation with Lactobacillus fermentum at 1 × 10⁹ CFU/mL in growingfinishing pigs was shown to reduce oxidative stress, likely through enhancement of antioxidant defence systems (Wang et al., 2013) [22] . The probiotic yeast Saccharomyces boulardii has also been evaluated in weaned piglets. At a dosage of 1 × 10⁸ CFU/kg, it effectively alleviated oxidative stress, potentially by stabilizing gut microbiota and improving intestinal health (Zhang et al., 2020) [57] . In addition, Lactobacillus reuteri, administered at 3.0 × 10⁶ CFU/g in weaning piglets, demonstrated efficacy in mitigating weaning stress, a critical period associated with immune suppression and intestinal dysfunction. Collectively, these findings highlight the strain-specific and dose-dependent effects of probiotics in improving stress resilience in swine. Their mechanisms of action are often linked to modulation of gut microbiota, enhancement of immune function, and reduction of oxidative damage.

Minerals and Vitamins

Micronutrients play a crucial role in mitigating stress by supporting immune function and antioxidant defence systems. Selenium and vitamin E act synergistically to protect cellular components from oxidative damage by enhancing glutathione peroxidase activity (Surai, 2016) [17] . Dietary supplementation of minerals and vitamins plays a crucial role in enhancing stress tolerance in swine by improving antioxidant capacity, immune function, and metabolic stability under challenging environmental and physiological conditions. Chromium supplementation has been widely studied for its role in alleviating heat stress in finishing pigs. At a dietary inclusion level of 200 µg/kg, chromium has been shown to improve growth performance and reduce physiological stress markers, likely through its involvement in glucose metabolism and insulin sensitivity (Mayorga et al., 2019) [58] . Iron is particularly important in neonatal piglets, where rapid growth and limited iron reserves predispose them to oxidative stress. Supplementation at 60 mg/kg has been reported to enhance antioxidant status and reduce oxidative damage, thereby supporting early-life health and development. Selenium, a key component of antioxidant enzymes such as glutathione peroxidase, has demonstrated broad protective effects across different categories of pigs, including boars, gilts, and sows. At a supplementation level of 1 mg/kg, selenium has been shown to mitigate both oxidative and heat stress by enhancing cellular antioxidant defenses and maintaining redox balance (Wang et al., 2023) [6] . Vitamin A plays a significant role in maintaining epithelial integrity and immune competence. Supplementation at 13,500 IU/kg in weaned piglets has been associated with improved resilience to weaning stress, a critical period characterized by intestinal and immunological challenges (Hu et al., 2020) [59] . Vitamin C, a potent watersoluble antioxidant, has been shown to reduce oxidative stress in weaning piglets when administered at 150 mg/kg. Its ability to scavenge free radicals and regenerate other antioxidants contributes to improved stress resistance (Shi et al., 2017) [61] . Vitamin E, a lipid-soluble antioxidant, is particularly effective in protecting cell membranes from oxidative damage. In growing pigs, supplementation at varying levels (50, 100, and 200 IU/kg) has been demonstrated to alleviate heat stress by improving antioxidant status and reducing lipid peroxidation. Similarly, in multiparous sows, dietary inclusion of 200 IU/kg vitamin E has been shown to mitigate oxidative stress and improve reproductive and physiological performance (Wang et al., 2023) [6] . Overall, these findings highlight the importance of targeted mineral and vitamin supplementation strategies in improving stress resilience in swine, with effects depending on nutrient type, dosage, and physiological stage.

Integrated Nutritional Approaches

While individual nutritional interventions provide measurable benefits, integrated strategies that combine functional amino acids, prebiotics, probiotics, organic acids, and micronutrients are more effective due to synergistic interactions. Such approaches simultaneously target multiple physiological pathways, including oxidative stress, immune function, and gut health. Precision nutrition, tailored to specific stressors (e.g., heat stress, weaning stress), production stages, and environmental conditions, is increasingly recognized as a key strategy for optimizing swine performance. Advances in nutrigenomics and microbiome research are further enhancing the ability to design targeted dietary interventions that improve stress resilience, animal welfare, and production efficiency (Dawson et al., 2021; Wang et al., 2023) [62,6] .

Conclusion

Nutrition plays a critical role in modulating stress responses in swine by supporting gut health, immune function, and antioxidant capacity. Dietary interventions-including prebiotics, probiotics, and key micronutrients such as selenium, chromium, and vitamins A, C, and E-have been shown to effectively mitigate physiological and environmental stressors, particularly during weaning, heat exposure, and transportation. The efficacy of these strategies depends on appropriate formulation, dosage, and the physiological stage of the animal, highlighting the importance of targeted and precision nutrition. Overall, optimized nutritional management represents a practical and sustainable approach to improving stress resilience, animal welfare, and productivity in swine production systems.

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