Phytogenic feed additives in animal nutrition: Multifunctional roles in productivity, gut health and emission mitigation

Introduction

Livestock production is fundamental to global food security, providing high-quality animalderived proteins such as meat and milk, which are rich in essential amino acids and critical micronutrients including heme iron, vitamin B12, vitamin D3, zinc, and calcium (Pulina et al., 2017; Caprarulo et al., 2022) [10,48] . With the continuous growth of the global population, there is increasing pressure on animal production systems to enhance productivity, efficiency, and sustainability while ensuring food safety and minimizing environmental impact. A major objective of modern livestock farming is to reduce dependence on synthetic inputs such as antibiotics, hormones, and other xenobiotics. Feed additives have therefore become an essential component of animal nutrition strategies, as they improve feed quality, enhance nutrient utilisation, support digestion, mitigate anti-nutritional factors, and promote growth performance, while also strengthening disease resistance and metabolic efficiency (Singh, 2015; Silveira et al., 2021) [55,57] . These additives are broadly classified into technological, sensory, nutritional, and zootechnical categories, as well as coccidiostats and histomonostats (Karásková et al., 2015; Pandey et al., 2019; Purba et al., 2020) [24,46,49] , and include compounds such as probiotics, prebiotics, enzymes, organic acids, immunomodulators, and phytogenic substances. Historically, antibiotic growth promoters (AGP) have been widely used to enhance livestock productivity. However, growing concerns regarding antimicrobial resistance and public health risks led to regulatory [15,46,58,60] . Despite this, antibiotics continue to be used in some regions due to limited regulatory enforcement and insufficient monitoring data (Sampath et al., 2023) . The withdrawal of AGP has been associated with challenges such as reduced growth performance, increased incidence of enteric diseases, impaired nutrient absorption, and higher microbial load in the gastrointestinal tract, particularly in swine production systems (Kikusato, 2021; Muniyappan et al., 2021; Sampath et al., 2023) [26,45] . Consequently, the development of effective and safe alternatives to AGP has become a critical research priority (Jha and Berrocoso, 2015) [22] . Among the proposed alternatives, phytogenic feed additives (PFA), also known as phytogenics, phytobiotics, or plantderived bioactive compounds, have gained considerable attention. These additives are derived from herbs, spices, fruits, and other plant materials and contain a wide range of bioactive constituents, including polyphenols, flavonoids, alkaloids, quinones, terpenoids, and essential oils (Yang et al., 2015) [74]. The use of plant-based compounds in animal nutrition is not new; historical evidence indicates their application in livestock care dating back to ancient Mesopotamia around 2600 BC (Franz et al., 2020) [15]. Phytogenic compounds exhibit diverse biological activities, including antimicrobial, antioxidant, anti-inflammatory, antifungal, antiparasitic, and immunomodulatory effects (Zhai et al., 2018; Kuralkar and Kuralkar, 2021) [29,77] . These properties contribute to improved gut health, enhanced immune function, and better nutrient utilisation. In ruminants, phytogenics modulate rumen fermentation, alter microbial populations, reduce methane emissions, and improve feed intake, palatability, and productivity parameters such as milk yield and growth rate (Artuso-Ponte et al., 2020; Purba et al., 2020) [4,49] . In swine, phytogenic supplementation has been shown to improve gastrointestinal function, enhance digestive enzyme activity, modulate gut microbiota, and increase nutrient digestibility and growth performance (Peng et al., 2021; Szabó et al., 2023) [47,63] . Several plant-derived compounds have demonstrated specific functional benefits. For instance, quillaja saponin has been reported to enhance growth performance, nutrient utilisation, and gut microbiota in pigs (Muniyappan et al., 2022a) [44] , while chestnut wood extract improves antioxidant status and intestinal health (Biagi et al., 2010) [6] . Phytogenic supplementation has also been shown to reduce diarrhoea in weaned pigs challenged with E. coli K88 and improve immune and metabolic indicators (Li et al., 2015; Cairo et al., 2018) [8,31] . Similarly, compounds such as limonene and carvacrol improve energy utilisation and productivity in ruminants (Varga-Visi et al., 2023) [67] , while plant-based additives including Coleus amboinicus and composite plant extracts enhance rumen fermentation, nutrient utilisation, and meat quality while reducing methane emissions (Yanza et al., 2022) [75] . The mechanisms of action of phytogenic feed additives are complex and involve multiple pathways, including modulation of gastrointestinal and rumen microbiota, stimulation of digestive enzyme activity, enhancement of gut integrity, and reduction of oxidative stress (Singh and Gaikwad, 2020). Additionally, phytogenics contribute to environmental sustainability by reducing ammonia, hydrogen sulfide, and other harmful gas emissions associated with livestock production. From an economic perspective, the global phytogenics market reflects increasing adoption, with estimates indicating growth from 500.46 million USD in 2024 to 613.94 million USD by 2029 (Kazempoor et al., 2022) [25] . This trend underscores the growing demand for natural, antibiotic-free production systems and highlights the commercial relevance of phytogenic additives. Despite these promising attributes, variability in efficacy across studies remains a challenge, largely due to differences in plant composition, dosage, and experimental conditions. The concentration of bioactive compounds can vary significantly depending on plant source, processing, and storage conditions, which complicates standardisation. Advanced approaches such as metagenomics, transcriptomics, proteomics, and network pharmacology offer new opportunities to elucidate the mechanisms and optimise the application of phytogenic feed additives in livestock systems. Therefore, this review aims to provide a comprehensive synthesis of the role of phytogenic feed additives as natural alternatives to antibiotic growth promoters in livestock production. It focuses on their effects on growth performance, nutrient utilisation, gut health and microbiota, antioxidant and antimicrobial activity, immune function, product quality, and environmental emissions. By integrating evidence across ruminant and swine systems, this review seeks to support the development of sustainable and efficient feeding strategies for modern livestock production.

Literature Search Strategy

To obtain a comprehensive understanding of the effects of dietary supplementation with phytogenic feed additives (PFA) on livestock health and performance, an extensive literature survey was conducted. A total of 180 studies were systematically selected from multiple scientific databases, including ScienceDirect, PubMed, Scopus, Google Scholar, and Web of Science. The search strategy was designed to ensure broad coverage of relevant research across ruminant and monogastric systems. Specific keywords were employed, including "phytogenic feed additives," "ruminants," "cow," "sheep," "goat," "buffalo," "growth performance," "carcass traits," "ruminal fermentation," "antioxidant," "immunity," "antimicrobial," "methane and ammonia emission," "milk composition," "blood metabolites," and "meat quality." This structured approach enabled the compilation of a robust dataset encompassing multiple dimensions of animal production. The selected studies provided insights into the influence of PFA on rumen fermentation dynamics, greenhouse gas emissions (methane and ammonia), milk production and composition, reproductive performance, blood biochemical profiles, antioxidant and antimicrobial activities, immune responses, and meat quality attributes. Collectively, this methodology ensured a comprehensive and multidimensional evaluation of phytogenic feed additives in livestock systems.

Phytogenic Feed Additives: Concept and Functional Basis

Phytogenic feed additives (PFA), also referred to as phytobiotics or phytochemicals, encompass plant-derived natural compounds, herbal formulations, and bioactive extracts incorporated into animal diets to enhance feed quality, animal performance, and product quality (Wang et al., 2024) [68] . These additives are commonly derived from herbs, spices, and plant extracts, and may also be classified based on their origin and processing methods, such as Chinese herbal additives or plant extract-based formulations. The use of herbal remedies in human health dates back over 3,000 years (van Wyk and Prinsloo, 2020) [66] , while their application in livestock production extends over two millennia (Gong et al., 2014; Bhosale et al., 2025) [5,16] . Over time, phytogenic compounds have gained recognition for their beneficial effects on animal performance, including improvements in body weight, feed conversion ratio, and meat quality. Subsequent research has demonstrated their antimicrobial, antiviral, antifungal, antioxidant, and antiinflammatory properties, which underpin their functional role in animal nutrition. Phytogenics act through multiple biological pathways, including modulation of nutrient metabolism, enhancement of immune responses, and improvement of gut health. Their mode of action differs fundamentally from antibiotic growth promoters (AGP), as they do not suppress microbial populations indiscriminately but rather promote a balanced intestinal ecosystem. Advances in extraction technologies and identification of active constituents have further strengthened their potential as natural alternatives to antibiotics. Additionally, phytogenic additives are generally considered safe, cost-effective, environmentally friendly, and widely applicable in livestock production systems (Gong et al., 2014) [16] .

Effects of Phytogenic Feed Additives on Growth Performance

The reduction and eventual removal of AGP from animal diets have intensified research into phytogenics as alternative growth promoters, particularly in swine production systems. Unlike AGP, which primarily reduce microbial load and intestinal inflammation (Windisch et al., 2008) [69] , phytogenics enhance growth performance by modulating the intestinal environment, strengthening gut barrier integrity, promoting beneficial microbiota, and stimulating digestive enzyme secretion (Ali et al., 2021) [3] . Phytogenic supplementation has been associated with improvements in key growth performance indicators, including average daily gain (ADG), average daily feed intake (ADFI), gain-to-feed ratio (G:F), and body weight gain (BWG). For instance, supplementation with green tea leaves and pomegranate extracts significantly improved ADG and G:F compared to control diets (Bontempo et al., 2014) [7] . Similar improvements in growth performance have been reported with various herbal extracts (Lan et al., 2017) [30] . Specific phytogenic compounds and plant sources have demonstrated notable effects (table 1). Extracts of Scutellaria baicalensis and Lonicera japonica increased feed intake and body weight gain (Liu et al., 2016) [35] . A mixture of essential oils containing eucalyptol, p-cymene, linalool, anethole, and thymol improved reproductive and growth parameters in piglets (Reyes-Camacho et al., 2020) [50] . Herbal mixtures comprising buckwheat, thyme, curcuma, black pepper, and ginger enhanced BW, ADG, and G:F in weaning piglets (Yan and Kim, 2012) [72] . Similarly, combinations of herbal extracts such as Astragalus membranaceus, Codonopsis pilosula, and Salvia miltiorrhiza improved growth performance indices (Kong et al., 2007) [27] . [11,21,51] . For example, supplementation with Yucca schidigera and Quillaja saponaria increased feed intake and ADG by up to 8.6% compared to control diets, likely due to improved nutrient utilisation and energy efficiency (Ma et al., 2022b) [10,33] . However, the effects of phytogenics on feed intake remain inconsistent. Reported variations range from -9% to +12% (Wang et al., 2024) [68] and -3% to +19% (Abdelli et al., 2021) [1] , suggesting that increased intake may not necessarily reflect feed preference but could be a consequence of improved growth rates (Windisch et al., 2008) [69] . Several studies have reported no significant effects on growth performance parameters, including BW, ADFI, and feed conversion ratio (Li et al., 2012a; Frankič et al., 2010) [14,32] . These inconsistencies may be attributed to differences in experimental conditions, phytogenic composition, dosage, animal age, and health status. [38] Effects of Phytogenic Feed Additives on Nutrient Digestibility Phytogenic feed additives have been widely studied for their effects on nutrient digestibility, which is a critical determinant of animal performance. [5,72] . Similarly, supplementation with Achyranthes japonica extract improved DM, CP, and GE digestibility in growing and finishing pigs (Kumar et al., 2021) [28] .

Other phytogenic compounds, such as Silybum marianum (Hossain et al., 2023) [18] , Forsythia suspensa (Long et al., 2021b) [36,38] , and black pepper (Sampath et al., 2020) [51] , have also demonstrated positive effects on nutrient digestibility. Improvements in intestinal morphology, including increased villus height and villus height-to-crypt depth ratio, have been associated with enhanced nutrient absorption (Long et al., 2021b). Essential oil-based phytogenics, such as cinnamaldehyde, thymol, and menthol-containing blends, have been shown to improve ileal digestibility of crude protein and amino acids (Yan et al., 2010b) [73] . Similarly, supplementation with medicinal plants (Zhao et al., 2016) [78] and compounds such as quillaja saponin (Muniyappan et al., 2022a) [44] has been linked to enhanced nutrient utilisation. Despite these positive findings, inconsistent results have also been reported. Some studies have shown no significant effects or even negative impacts of phytogenic supplementation on nutrient digestibility (Hossain et al., 2024) [19] . These discrepancies are likely due to variations in phytogenic composition, dosage, animal physiological status, and experimental conditions. [12] . Cytokines, which are lowmolecular-weight proteins involved in immune signaling, play a central role in regulating inflammatory responses (Taranu et al., 2010) [64] . Phytogenics also strengthen intestinal barrier function by promoting the expression of tight junction proteins, reducing intestinal permeability, and preventing bacterial translocation. Additionally, they enhance humoral and cellular immunity by stimulating immunoglobulin production (e.g., IgA) and activating immune cells such as macrophages, dendritic cells, and T lymphocytes.

Role of Phytogenic

The antimicrobial properties of phytogenics further contribute to diarrhea control. Compounds such as thymol, carvacrol, tannins, and saponins inhibit the growth of pathogenic bacteria including E. coli and Clostridium perfringens. For example, dietary supplementation with quillaja saponin significantly reduced diarrhea scores in pigs (Muniyappan et al., 2022a) [44] , while essential oil supplementation decreased diarrhea incidence in weaned piglets (Ma et al., 2022a) [10,33] .

Several studies have demonstrated that phytogenics can alleviate E. coli-induced diarrhea (Deng et al., 2007) [27] . Further evidence highlights the role of phytogenics in oxidative stress mitigation and intestinal health. Stevia residue extract increased antioxidant enzyme activities, including superoxide dismutase (SOD), glutathione peroxidase (GSH-PX), and total antioxidant capacity (T-AOC), while reducing malondialdehyde (MDA) levels and intestinal permeability in piglets (Liu et al., 2022b) [33] . Similarly, Forsythia suspensa extract reduced diarrhea incidence and duration, enhanced serum immunoglobulin levels, and inhibited E. coli colonization (Long et al., 2019) [37] . Herbal supplementation has also been associated with increased beneficial bacteria and reduced pathogenic populations in fecal microbiota (Zhao et al., 2016) [78] .

Additional studies have demonstrated that phytogenics modulate gene expression related to intestinal barrier integrity. For instance, mulberry leaf extract improved the expression of mucin-2, occludin, and claudin-1, while enhancing digestive enzyme activities and reducing pathogenic bacteria in piglets (Ma et al., 2023) [41] . Combined supplementation of essential oils and organic acids reduced diarrhea scores, downregulated pro-inflammatory cytokines, and improved intestinal barrier function in piglets challenged with enterotoxigenic E. coli (Xu et al., 2020) [70] . Furthermore, lactoferricin B has been shown to modulate mucosal transcriptomics and reduce pathogen-induced diarrhea (Hou et al., 2011) [20] . Overall, phytogenic feed additives exert multifaceted protective effects against diarrhea by integrating antimicrobial, immunomodulatory, antioxidant, and barrierenhancing mechanisms.

Modulation of Gut Microbiota by Phytogenic Feed Additives

The gut microbiota plays a fundamental role in host nutrition, metabolism, immune function, and overall health. However, its complex interactions with dietary components and host physiology are not yet fully understood. Phytogenic feed additives have been widely investigated for their capacity to modulate gut microbial communities and improve intestinal health. Phytogenics exhibit well-documented antimicrobial properties, with generally stronger activity against Grampositive bacteria compared to Gram-negative species (Windisch et al., 2008) [69] . Their antimicrobial action is dosedependent and may involve disruption of microbial cell membranes, inhibition of enzyme systems, and interference with microbial metabolism (Thapa et al., 2012) [65] . Importantly, phytogenics do not indiscriminately suppress microbial populations but rather promote a balanced microbiota by inhibiting pathogenic bacteria and supporting beneficial microbial groups. Key phytogenic compounds, including tannins, flavonoids, saponins, and essential oils, contribute to gut health by competitively excluding harmful bacteria and modulating immune responses within the gastrointestinal tract (Zeng et al., 2015) [76] . Numerous studies have demonstrated their effects on microbial fermentation, enzyme activity, and microbial composition (Cairo et [8,36,38] . For example, supplementation with thymol and cinnamaldehyde reduced E. coli populations in the cecum, colon, and rectum, while improving the Lactobacillus-to-E. coli ratio (Li et al., 2012a) [32] . Red pepper essential oil increased Lactobacillus counts and reduced Enterobacteria and diarrhea incidence in piglets (Cairo et al., 2018) [8] . Similarly, essential oils containing eugenol, thymol, and piperine reduced Escherichia-Shigella and Campylobacter populations (Silva Júnior et al., 2020) [54] . Herbal extracts have also been shown to enhance beneficial microbial populations. Increased levels of lactic acid bacteria and Bifidobacterium, along with reduced Enterobacteriaceae and Campylobacter jejuni, were observed following herbal supplementation (Kafantaris et al., 2018) [23] . Likewise, supplementation with quillaja saponin, Achyranthes japonica, and Silybum marianum increased fecal Lactobacillus counts and reduced E. coli populations (Muniyappan et al., 2022a; Hossain et al., 2023) [18,44] . Phytogenics also influence microbial metabolites. For instance, supplementation with Alliaceae extract increased beneficial bacterial populations and enhanced the production of short-chain fatty acids such as propionic and branchedchain fatty acids (Sánchez et al., 2020) [53] . Stevia residue extract increased the abundance of microbial families such as Lachnospiraceae and Prevotellaceae and enhanced fermentation metabolites in the colon (Liu et al., 2022b) [33] . Additionally, increased populations of Bifidobacterium and Lactobacillus, along with reduced E. coli, were associated with improved propionic acid production and reduced serum cholesterol levels (Su et al., 2018) [61] . Overall, phytogenic feed additives contribute to gut microbial homeostasis by selectively modulating microbial populations, enhancing beneficial bacteria, suppressing pathogens, and improving fermentation profiles. These effects collectively support improved nutrient utilisation, immune function, and overall animal health.

Immunomodulatory Effects of Phytogenic Feed Additives

The intestinal immune system represents a critical interface between the host and its environment, where physical, microbial, and immunological barriers collectively determine disease resistance. Phytogenic feed additives influence immune responses through complex molecular and cellular mechanisms involving both innate and adaptive immunity. Phytogenic compounds interact with immune signaling pathways by binding to Toll-like receptors (TLR) on intestinal epithelial and immune cells, leading to activation of key intracellular pathways such as nuclear factor kappa B (NF-κB) and mitogen-activated protein kinase (MAPK) (Zhou et al., 2014). These pathways regulate the expression of proinflammatory cytokines, including tumor necrosis factor-α (TNF-α), interleukins (IL-1β, IL-6), monocyte chemoattractant protein-1 (MCP-1), and interferon-γ (IFN-γ). Phytogenics modulate these inflammatory responses by either stimulating or suppressing cytokine production, thereby maintaining immune homeostasis. They enhance lymphocyte proliferation, macrophage phagocytic activity, and natural killer cell function, contributing to improved host defense (Abdelli et al., 2021) [1] . Additionally, they influence immunoglobulin production, including Ig A, Ig M, and secretory Ig A, which play essential roles in mucosal immunity (Zhao et al., 2022) [62] . Several studies highlight the anti-inflammatory effects of phytogenic compounds. Essential oils such as thymol and cinnamaldehyde reduced serum IL-6 levels while increasing lymphocyte counts and growth-related factors (Li et al., 2012a) [32] . Capsicum extract decreased pro-inflammatory cytokines (TNF-α, IL-1β, IL-6) and modulated immune responses through inhibition of the NFκB pathway (Mendivil et al., 2019) [42] . Similarly, Forsythia suspensa reduced inflammatory cytokines while enhancing antioxidant enzyme activity and immunoglobulin levels (Long et al., 2021b) [36,38] . Oxidative stress is closely linked to inflammation, and phytogenics play a crucial role in regulating antioxidant defense systems. The nuclear factor erythroid 2-related factor 2 (Nrf2) pathway is a central regulator of antioxidant responses, controlling the expression of detoxifying enzymes and protecting against oxidative damage (Yamamoto et al., 2018) [71] . Phytogenic supplementation has been shown to activate Nrf2 signaling while inhibiting NF-κB-mediated inflammation, thereby reducing intestinal disease risk (Su et al., 2018) [61] . Overall, phytogenic feed additives exhibit strong immunomodulatory properties through coordinated regulation of inflammatory pathways, antioxidant systems, gut barrier function, and microbial interactions. These effects collectively contribute to improved disease resistance, intestinal health, and overall animal performance.

Biochemical Indicators and Oxidative Status

Biochemical parameters serve as critical indicators of metabolic status, immune function, and physiological wellbeing in livestock. Numerous studies have evaluated the influence of phytogenic feed additives (PFA) on these indicators, demonstrating their role in modulating metabolic and antioxidant pathways (Wang et al., 2024) [68] . Phytogenic additives such as Forsythia suspensa have also demonstrated improvements in hematological parameters, including increased red blood cell counts and serum IgA levels in weaning pigs (Long and Piao, 2021) [36,38] . In addition, grape seed proanthocyanidin extract significantly enhanced antioxidant capacity by increasing total antioxidant capacity (T-AOC), glutathione (GSH), and glutathione peroxidase (GSH-Px), while reducing malondialdehyde (MDA) levels and improving lipid profiles (Feng et al., 2023) [13] . These hypocholesterolemic effects are attributed to reduced activity of key lipid metabolism enzymes such as HMG-CoA reductase, cholesterol-7 hydroxylase, and fatty acid synthase (Abdelli et al., 2021) [1] . Furthermore, phytogenic compounds such as quillaja saponin, micelle silymarin, and herbal extracts have been associated with reduced serum cholesterol and triglyceride levels (Muniyappan et al., 2022a) [44] . Enhancement of antioxidant enzyme activity (SOD, CAT, GSH-Px) and reduction of oxidative stress markers (ROS, TBARS, MDA) have also been consistently reported following supplementation with essential oils, organic acids, and plant extracts (Li et al., 2012a; Zou et al., 2016b; Wang et al., 2020) [32] . Overall, phytogenic feed additives improve biochemical and oxidative status by enhancing immune responses, regulating lipid metabolism, and strengthening antioxidant defense systems. Effects of Phytogenic Feed Additives on Fecal Gas Emissions Noxious gas emissions from livestock production systemsparticularly ammonia, methyl mercaptans, carbon dioxide, and hydrogen sulfide-represent a significant environmental and occupational health concern (Muniyappan et al., 2021) [45] . Among these, ammonia emissions are closely associated with environmental issues such as soil and water acidification, as well as nitrogen deposition in ecosystems. The generation of fecal gases is closely linked to nutrient metabolism and the activity of gut microbiota (table 1). Microbial degradation of undigested nutrients in feces leads to the production of ammonia and other harmful gases (Li et al., 2012b) [32] . Therefore, strategies that improve nutrient utilisation and modulate gut microbial populations can effectively reduce fecal gas emissions. Phytogenic feed additives (PFA) have demonstrated considerable potential in mitigating fecal gas emissions through their effects on gut microbiota and nutrient digestibility. For instance, supplementation with black pepper significantly reduced emissions of ammonia, methyl mercaptans, and acetic acid (Sampath et al., 2020) [51] . Similarly, inclusion of medicinal plant extracts reduced coliform bacterial populations, increased Lactobacillus counts, and improved nutrient digestibility, thereby lowering fecal ammonia and methyl mercaptan emissions (Yan and Kim, 2012) [72] . Quillaja saponin supplementation has been shown to reduce fecal ammonia and carbon dioxide emissions (Muniyappan et al., 2022a) [44] , likely due to enhanced populations of beneficial bacteria such as Lactobacillus. Likewise, Achyranthes japonica extract modulated gut microbiota by increasing beneficial bacteria and reducing pathogenic populations, leading to decreased emissions of ammonia and hydrogen sulfide (Liu et al., 2021) [36] . Essential oils have also demonstrated consistent reductions in fecal gas emissions. Dietary supplementation with essential oils reduced hydrogen sulfide and ammonia emissions in growingfinishing pigs (Yan et al., 2010a) [73] , while similar effects were observed with herbal extracts and essential oil supplementation (Yan et al., 2011b). Overall, phytogenic feed additives reduce fecal gas emissions primarily by enhancing nutrient digestibility, shifting gut microbial populations toward beneficial species, and limiting the proliferation of gas-producing pathogenic bacteria. These effects contribute not only to improved animal performance but also to reduced environmental pollution and enhanced sustainability of livestock production systems.

Conclusion

Phytogenic feed additives (PFA) represent a credible and biologically effective alternative to antibiotic growth promoters in livestock production. The evidence synthesized in this review indicates that PFA influence multiple physiological and production parameters in both ruminants and swine, including growth performance, nutrient utilisation, gut microbiota composition, immune responses, antioxidant status, product quality, and environmental emissions. In ruminants, PFA improve rumen fermentation efficiency, modulate microbial populations, and contribute to reductions in methane and ammonia emissions, thereby enhancing both productivity and environmental sustainability. In swine, phytogenics support gut health, reduce post-weaning diarrhea, and improve nutrient digestibility and growth performance, largely through modulation of intestinal microbiota and immune pathways. Across species, PFA consistently exhibit antimicrobial, antioxidant, and immunomodulatory effects, including regulation of inflammatory signaling pathways and enhancement of endogenous antioxidant systems. These mechanisms contribute to improved physiological resilience and product quality, including meat characteristics and, to a lesser extent, milk production traits. However, responses remain variable due to differences in phytogenic composition, dosage, animal factors, and experimental conditions. Future research should prioritise standardisation of phytogenic formulations, identification of active compounds and their interactions, and validation under practical production systems using advanced molecular approaches. Overall, PFA offer a sustainable and multifunctional approach to improving animal health, productivity, and environmental outcomes, supporting their integration into antibiotic-free livestock production systems.

References

1. Abdelli N, Sola-Oriol D, Perez JF. Phytogenic feed additives in poultry: achievements, prospective and challenges. Animals. 2021;11(12):3471.
2. Aguerre MJ, Capozzolo MC, Lencioni P, Cabral C, Wattiaux MA. Effect of quebracho-chestnut tannin extracts at 2 dietary crude protein levels on performance, rumen fermentation, and nitrogen partitioning in dairy cows. J Dairy Sci. 2016;99:4476-4486.
3. Ali A, Ponnampalam EN, Pushpakumara G, Cottrell JJ, Suleria HA, Dunshea FR. Cinnamon: a natural feed additive for poultry health and production-A review. Animals. 2021;11(7):2026.
4. Artuso-Ponte V, Pastor A, Andratsch M. The effects of plant extracts on the immune system of livestock: The isoquinoline alkaloids model. In: Florou-Paneri P, Christaki E, Giannenas I. Feed Additives. Aromatic Plants and Herbs in Animal Nutrition and Health. Elsevier Inc. Amsterdam (Nederland); 2020:295-310.
5. Bhosale TR, Kamble DK, Adangale SB, Gaikwad US, Birari DR. Antinutritional factors in animal feed and it's prospects in greenhouse gas mitigation: A review. Int J Vet Sci Anim Husb. 2025;10(6):196-202.
6. Biagi G, Cipollini I, Paulicks BR, Roth FX. Effect of tannins on growth performance and intestinal ecosystem in weaned piglets. Arch Anim Nutr. 2010;64(2):121-135.
7. Bontempo V, Jiang X, Cheli F, Verso LL, Mantovani G, Vitari F, Domeneghini C, Agazzi A. Administration of a novel plant extract product via drinking water to postweaning piglets: effects on performance and gut health. Animal. 2014;8(5):721-730.
8. Cairo PLG, Gois FD, Sbardella M, Silveira H, de Oliveira RM, Allaman IB, et al. Effects of dietary supplementation of red pepper (Schinus terebinthifolius Raddi) essential oil on performance, small intestinal morphology and microbial counts of weanling pigs. J Sci Food Agric. 2018;98(2):541-548.
9. Canaes TS, Zanferari F, Maganhe BL, Takiya CS, Silva TH, Del Valle TA, et al. Increasing dietary levels of citral oil on nutrient total tract digestibility, ruminal fermentation, and milk composition in Saanen goats. Anim Feed Sci Tech. 2017;229:47-56.
10. Caprarulo V, Ventura V, Amatucci A, Ferronato G, Gilioli G. Innovations for reducing methane emissions in livestock toward a sustainable system: analysis of feed additive patents in ruminants. Animals. 2022;12:2760.
11. Dang DX, Cho S, Kim IH. Silybum marianum seed extract supplementation positively affects the body weight of weaned piglets by improving voluntary feed intake. J Anim Sci Technol. 2022;64(4):696.
12. Ding S, Yan W, Ma Y, Fang J. The impact of probiotics on gut health via alternation of immune status of monogastric animals. Anim Nutr. 2021;7(1):24-30.
13. Feng Y, Chen X, Chen D, He J, Zheng P, Luo Y, et al. Dietary grape seed proanthocyanidin extract supplementation improves antioxidant capacity and lipid metabolism in finishing pigs. Anim Biotechnol. 2023;34(8):4021-4031.
14. Frankič T, Levart A, Salobir J. The effect of vitamin E and plant extract mixture composed of carvacrol, cinnamaldehyde and capsaicin on oxidative stress induced by high PUFA load in young pigs. Animal. 2010;4(4):572-578.
15. Franz CM, Baser K, Hahn-Ramssl I. Herbs and aromatic plants as feed additives: aspects of composition, safety, and registration rules. In: Florou-Paneri P, Christaki E, Giannenas I, editors. Feed Additives. Academic Press. Cambridge, MA (USA); 2020:35-56.
16. Gong J, Yin F, Hou Y, Yin Y. Chinese herbs as alternatives to antibiotics in feed for swine and poultry production: potential and challenges in application. Can J Anim Sci. 2014;94(2):223-241.
17. Hashemzadeh F, Rafeie F, Hadipour A, Rezadoust MH. Supplementing a phytogenic-rich herbal mixture to heatstressed lambs: Growth performance, carcass yield, and muscle and liver antioxidant status. Small Rumin Res. 2022;206:106596.
18. Hossain MM, Bo Cho S, Kim IH. Silybum marianum seed extract as a potential phytogenic feed additive for improving growth performance and nutrient digestibility in growing pigs. Can J Anim Sci. 2023;104(1):80-85.
19. Hossain MM, Hwang HS, Pang M, Choi MK, Kim IH. Effect of dietary Achyranthes japonica extract on growth performance of growing pigs and absorption rate of quercetin in blood. J Anim Sci Technol. 2024;66(1):103114.
20. Hou ZP, Wang WJ, Liu JX, Liu G, Liu ZQ, Souffrant WB, et al. Effects of lactoferricin B and cecropin P1 on growth performance, faecal score and dry matter in weaned piglets orally challenged with enterotoxigenic Escherichia coli F4. J Food Agric Environ. 2011;9(2):275-80.
21. Hsu JE, Lo SH, Lin YY, Wang HT, Chen CY. Effects of essential oil mixtures on nitrogen metabolism and odor emission via in vitro simulated digestion and in vivo growing pig experiments. J Sci Food Agric. 2022;102(5):1939-1947.
22. Jha R, Berrocoso J. Dietary fiber utilization and its effects on physiological functions and gut health of swine. Animal. 2015;9(9):1441-1452.
23. Kafantaris I, Stagos D, Kotsampasi B, Hatzis A, Kypriotakis A, Gerasopoulos K, et al. Grape pomace improves performance, antioxidant status, fecal microbiota and meat quality of piglets. Animal. 2018;12(2):246-255.
24. Karásková K, Suchý P, Straková E. Current use of phytogenic feed additives in animal nutrition: a review. Czech J Anim Sci. 2015;60:521-530.
25. Kazempoor R, Alavinezhad S, Pargari M, Shakeri Y, Haghighi M. A review on the application of phytogenics as feed additives for aquatic animals. Int J Aquat Res Environ Stud. 2022;2(2):46-78.
26. Kikusato M. Phytobiotics to improve health and production of broiler chickens: functions beyond the antioxidant activity. Anim Biosci. 2021;34(3):345-353.
27. Kong XF, Wu GY, Liao YP, Hou ZP, Liu HJ, Yin FG, et al. Effects of Chinese herbal ultra-fine powder as a dietary additive on growth performance, serum metabolites and intestinal health in early-weaned piglets. Livest Sci. 2007;108(1):272-275.
28. Kumar S, Kim Y, Sampath V, Kim IH. Effects of Achyranthes japonica extract on the performance of finishing pigs fed diets containing palm kernel meal and rapeseed meal as a partial alternative to soybean meal. J Anim Physiol Anim Nutr. 2021;106(1):88-97.
29. Kuralkar P, Kuralkar S. Role of herbal products in animal production-An updated review. J Ethnopharmacol. 2021;278:114246.
30. Lan R, Park J, Lee D, Kim I. Effects of Astragalus membranaceus, Codonopsis pilosula and allicin mixture on growth performance, nutrient digestibility, faecal microbial shedding, immune response and meat quality in finishing pigs. J Anim Physiol Anim Nutr. 2017;101(6):1122-1129.
31. Li H, Zhao P, Lei Y, Li T, Kim I. Response to an Escherichia coli K88 oral challenge and productivity of weanling pigs receiving a dietary nucleotides supplement. J Anim Sci Biotechnol. 2015;6:49.
32. Li P, Piao X, Ru Y, Han X, Xue L, Zhang H. Effects of adding essential oil to the diet of weaned pigs on performance, nutrient utilization, immune response and intestinal health. Asian Australas J Anim Sci. 2012;25(11):1617-1626.
33. Liu S, Xiong Y, Cao S, Wen X, Xiao H, Li Y, et al. Dietary stevia residue extract supplementation improves antioxidant capacity and intestinal microbial composition of weaned piglets. Antioxidants. 2022;11(10):2016.
34. Liu T, Chen H, Bai Y, Wu J, Cheng S, He B, et al. Calf starter containing a blend of essential oils and prebiotics affects the growth performance of Holstein calves. J Dairy Sci. 2020;103:2315-2323.
35. Liu WC, Pi SH, Kim IH. Effects of Scutellaria baicalensis and Lonicera japonica extract mixture supplementation on growth performance, nutrient digestibility, blood profiles and meat quality in finishing pigs. Ital J Anim Sci. 2016;15(3):446-452.
36. Liu X, Kim IH. Effects of long-term feeding of Achyranthes japonica Nakai extract as a supplement to diets with different protein levels diets on the growth performance and meat quality characteristics of growing fattening pigs. Anim Feed Sci Technol. 2021;279:115030.
37. Long S, Liu L, Liu S, Mahfuz S, Piao X. Effects of Forsythia suspense extract as an antibiotics substitute on growth performance, nutrient digestibility, serum antioxidant capacity, fecal Escherichia coli concentration and intestinal morphology of weaned piglets. Animals. 2019;9(10):729.