Brown seaweed supplementation in dairy and beef cattle: effects on methane mitigation, productivity, reproductive responses, and safety

Introduction

Enteric methane (CH₄) emissions from ruminant livestock contribute substantially to global anthropogenic CH₄ emissions (approximately 30%) and represent a loss of approximately 2–12% of gross dietary energy intake that could otherwise be utilized for animal productivity (Johnson and Johnson, 1995; Vargas et al., 2022).

Consequently, reducing CH₄ emissions has become an important objective for improving both the environmental sustainability and production efficiency of cattle systems (Hook et al., 2010; Hristov et al., 2013). Among the available mitigation strategies, nutritional interventions have received considerable attention because they can often be implemented within existing management systems and may directly influence rumen fermentation processes (Beauchemin et al., 2022).

Marine macroalgae have emerged as promising dietary supplements for reducing enteric CH₄ production in ruminants. Interest in seaweed-based CH₄ mitigation strategies has increased substantially over the past decade because many seaweed species contain bioactive compounds capable of modifying rumen microbial activity and fermentation pathways associated with methanogenesis (Min et al., 2021; Lean et al., 2021). Much of the recent research has focused on red seaweed, particularly Asparagopsis spp., whose potent antimethanogenic effects are primarily attributed to bromoform and other halogenated compounds that disrupt methanogenesis (Vijn et al., 2020; Liu et al., 2025). In contrast, brown seaweed contains negligible concentrations of bromoform, and its biological effects are primarily attributed to bioactive compounds such as phlorotannins, laminarin, fucoidan, and alginates, which are thought to modulate rumen microbial populations and fermentation pathways through mechanisms distinct from those of Asparagopsis spp. However, concerns regarding the presence of bromoform, regulatory approval, large-scale production, environmental sustainability, and long-term implementation have raised questions about the practical applicability of Asparagopsis spp. in commercial livestock systems (Vijn et al., 2020; Eason and Fennessy, 2025; Liu et al., 2025).

Compared with red seaweed, brown seaweed is a biologically distinct group whose effects are generally attributed to polyphenols, polysaccharides, and mineral constituents rather than halogenated compounds. Representative temperate brown seaweeds include Ascophyllum nodosum, Saccharina latissima, Laminaria spp., and Fucus spp., whereas Sargassum spp. are predominantly distributed in tropical and subtropical marine environments. These species have been evaluated as feed ingredients or feed additives in ruminant diets (Craigie, 2011; Min et al., 2021). These species differ considerably in chemical composition, and their nutritional value and biological activity are influenced by harvest season, geographic location, processing method, preservation strategy, and inclusion level (Dhakal et al., 2024). Such variation may contribute to the inconsistent responses reported across studies.

Research evaluating brown seaweed supplementation has expanded considerably in recent years; however, the available evidence remains fragmented. CH₄ mitigation has been the primary focus of most studies, yet reported responses vary substantially among seaweed species, experimental systems, and production conditions. Furthermore, relatively few investigations have simultaneously evaluated reproductive and endocrine outcomes, productivity responses, safety considerations, and implementation challenges within the same experimental framework. Reproductive responses are an important component of this review because dietary interventions that modify rumen fermentation, nutrient utilization, endocrine function, or iodine metabolism may influence reproductive physiology and, ultimately, the overall suitability of brown seaweed supplementation in cattle production systems. Consequently, a comprehensive evaluation that integrates CH₄ mitigation with productivity, reproductive responses, and safety considerations is needed to better assess the practical application of brown seaweed in dairy and beef production. However, the practical significance of many reported findings remains uncertain, particularly because favorable responses observed under in vitro conditions are often not replicated in commercial cattle production systems (Beauchemin et al., 2022; Hodge et al., 2024).

Several reviews have examined seaweed supplementation in ruminant diets or assessed CH₄-mitigation strategies across a range of feed additives (Min et al., 2021; Lean et al., 2021; Costa et al., 2021; Hodge et al., 2024). However, relatively few reviews have focused specifically on brown seaweed supplementation in cattle while integrating CH₄ emissions, reproductive-related responses, productivity outcomes, and safety considerations within a single framework. Such a synthesis is needed because the growing body of literature encompasses studies that differ substantially in seaweed species, supplementation strategies, processing methods, study duration, and outcome assessments.

Therefore, the objective of this review was to critically synthesize current evidence on brown seaweed supplementation in dairy and beef cattle, with particular emphasis on enteric CH₄ emissions, reproductive-related responses, productivity outcomes, and safety considerations. Special attention was given to species-specific responses, differences between in vitro and in vivo findings, practical implementation challenges, and key research gaps that must be addressed before broad recommendations can be made for commercial cattle production systems.

Materials and methods

Review design and rationale

This study was conducted as a structured narrative review to synthesize current evidence regarding brown seaweed supplementation in cattle. This approach was selected because the available literature remains limited and highly heterogeneous with respect to study design, encompassing in vitro fermentation systems, in vivo animal trials, field-based investigations, and observational studies, as well as substantial variation in cattle type, seaweed species, processing methods, supplementation strategies, and measured outcomes. Furthermore, relatively few directly comparable controlled studies were available, limiting the suitability of a formal meta-analysis (Grant and Booth, 2009; Ferrari, 2015; Hodge et al., 2024). Recent investigations have introduced additional variability through the evaluation of alternative processing methods, extract preparations, and delivery systems (De Bhowmick et al., 2025; Casey et al., 2026a, b, c). Therefore, a structured narrative approach was considered the most appropriate method for integrating available evidence while identifying major knowledge gaps, methodological limitations, and future research priorities. The literature search, study selection, and reporting were conducted using a structured and reproducible literature-search strategy based on predefined search terms, eligibility criteria, and screening procedures, and the study-selection process is summarized using a PRISMA-style flow diagram (Figure 1). Because of the substantial methodological heterogeneity among the included studies, the evidence was synthesized narratively rather than through quantitative meta-analysis.

The review focused on brown seaweed species evaluated in bovine nutrition, including Ascophyllum nodosum, Saccharina latissima, Laminaria spp., Fucus spp., and Sargassum spp. Evidence involving red or green algae was excluded from the primary synthesis but was occasionally cited to provide limited comparative context regarding CH₄-mitigation mechanisms, implementation challenges, and delivery strategies.

The review addressed four principal questions: (1) What evidence is available regarding the CH₄-mitigation potential of brown seaweed supplementation in cattle? (2) What reproductive-related responses, including endocrine- and fertility-related outcomes, have been reported? (3) What effects have been observed on productivity traits, including feed intake, digestibility, milk production, milk composition, and growth performance? and (4) How do in vitro findings compare with in vivo evidence, and what implications do these differences have for practical application in cattle production systems?

Literature search strategy

A structured literature search was conducted using Web of Science Core Collection, Scopus, PubMed/MEDLINE, AGRICOLA, and CAB Abstracts. These databases were selected because of their broad coverage of animal science, veterinary, agricultural, and biological research (Falagas et al., 2008; Bramer et al., 2017). Additional records were identified through ProQuest Dissertations and Theses Global, conference proceedings, institutional repositories, citation tracking, and manual screening of reference lists from relevant reviews and recent publications.

Search terms were developed to balance sensitivity and specificity (Methley et al., 2014) and were organized around four components: (1) cattle-related terms (e.g., dairy cattle, beef cattle, bovine); (2) brown seaweed-related terms, including brown seaweed, macroalgae, kelp, Ascophyllum nodosum, Saccharina latissima, Laminaria digitata, Laminaria spp., Fucus vesiculosus, Fucus serratus, Fucus spp., and Sargassum spp.; (3) outcome-related terms, including CH₄ emissions, rumen fermentation, milk production, feed intake, digestibility, reproductive performance, fertility, animal health, and productivity; and (4) study-design terms. The search included studies published between January 2008 and April 2026 to capture contemporary evidence related to CH₄ mitigation, productivity, safety, implementation, processing methods, and delivery strategies.

To ensure inclusion of the most recent evidence, the literature search was updated through April 2026. Retrieved records included both primary research studies and additional sources used to provide methodological, regulatory, mechanistic, and implementation-related context. Primary synthesis studies were used to evaluate CH₄ emissions, reproductive-related responses, productivity outcomes, and safety considerations associated with brown seaweed supplementation in cattle. In contrast, review articles, methodological papers, regulatory documents, and other contextual references were used to support interpretation of biological mechanisms, regulatory considerations, implementation challenges, methodological context, and broader discussion of the findings. Only studies meeting the predefined eligibility criteria were retained as primary synthesis studies.

Data from eligible studies were extracted using a standardized electronic data-extraction form developed specifically for this review. Information collected included publication details, study design, animal characteristics, seaweed species, inclusion level, supplementation method, measured outcomes, and principal findings. Data extraction followed established reporting principles for animal research (Kilkenny et al., 2010).

Following duplicate removal, screening, and full-text eligibility assessment, 37 studies met the eligibility criteria. During final eligibility reassessment and standardization of study classification, three studies were reclassified as contextual references because they did not meet the final criteria for inclusion as primary synthesis studies. Consequently, 34 studies were retained for the narrative synthesis. The literature search and study-selection process are summarized in the PRISMA-style flow diagram presented in Figure 1. Additional details regarding the search strategy, eligibility criteria, retained studies, study classification, and identified research gaps are provided in Supplementary Tables S1–S6.

Inclusion and exclusion criteria

The target population included dairy and beef cattle, as well as cattle-derived rumen systems used in in vitro fermentation studies. Eligible studies evaluated brown seaweed supplementation or brown seaweed-derived preparations and reported at least one relevant outcome, including CH₄ emissions, rumen fermentation, productivity, mineral transfer, endocrine-related responses, reproductive-related traits, or implementation-related outcomes. Acceptable study designs included in vitro experiments, in vivo animal trials, mechanistic fermentation studies, field-based investigations, and observational studies.

Peer-reviewed publications and selected grey-literature sources, including theses, dissertations, and institutional reports available in full text, were considered if they were published in English and provided sufficient methodological detail for interpretation. Grey-literature sources were limited in the final dataset and were interpreted more cautiously than peer-reviewed studies because of potential differences in methodological rigor, reporting consistency, and accessibility of supporting information.

Studies were excluded if they focused exclusively on red or green algae, involved only non-ruminant species, lacked sufficient methodological detail, or were available only as abstracts without complete data. Review articles, methodological papers, comparative red-seaweed studies, and implementation- or mechanism-focused references were used to provide contextual information but were not included as retained primary synthesis studies unless they satisfied the predefined eligibility criteria for brown seaweed supplementation in cattle-related systems.

Data extraction and quality assessment

Data were extracted from each eligible study using a standardized electronic extraction form. Information collected included publication details, animal system, seaweed species, seaweed form, inclusion level, supplementation method, study design, measured outcomes, and principal findings. Studies were subsequently grouped according to seaweed species, study type, supplementation approach, and outcome category to facilitate thematic comparison and synthesis, following the narrative synthesis framework described by Popay et al. (2006).

Because this review was conducted as a structured narrative review, a formal quantitative risk-of-bias assessment was not performed. Given the substantial methodological heterogeneity among the included studies, including differences in experimental systems, study designs, seaweed species, supplementation strategies, and outcome measures, formal risk-of-bias tools developed primarily for homogeneous intervention studies were not considered appropriate. Instead, evidence was evaluated qualitatively based on study design, methodological transparency, sample size, study duration, relevance to cattle systems, and consistency with findings from other studies. Greater caution was applied when interpreting findings derived from single studies, combined feed-additive interventions, alternative delivery systems, in vitro studies lacking in vivo validation, or grey-literature sources with limited methodological detail (Greenhalgh et al., 2018).

Data synthesis

Data were synthesized narratively according to the framework described by Popay et al. (2006). Findings were organized into four major themes: (1) methane mitigation, (2) productivity, (3) reproductive-related responses, and (4) safety considerations. Within each theme, both quantitative outcomes (e.g., CH₄ production, milk yield, average daily gain, feed intake, and digestibility) and qualitative observations (e.g., endocrine, health, physiological, and implementation-related responses) were synthesized when available (Stern et al., 2020).

A formal meta-analysis was not conducted because of substantial heterogeneity among studies with respect to seaweed species, inclusion levels, processing methods, supplementation strategies, study duration, experimental design, and outcome definitions. Consequently, findings were interpreted qualitatively, with emphasis placed on identifying consistent patterns, sources of variation, practical implications, and key knowledge gaps across the available literature.

Results and discussion

The available literature was dominated by in vitro fermentation studies, whereas comparatively fewer investigations evaluated responses under in vivo cattle conditions. Consequently, the strength and applicability of the evidence varied among research areas. CH₄-related outcomes were the most extensively studied, whereas reproductive-related and productivity outcomes were evaluated less frequently and were generally supported by a smaller and more heterogeneous body of evidence.

Across studies, responses to brown seaweed supplementation varied according to seaweed species, inclusion level, processing method, supplementation strategy, and experimental system. This heterogeneity limited direct comparisons among studies and contributed to inconsistencies in reported outcomes. Overall, current evidence suggests that brown seaweed supplementation has the potential to influence CH₄ production and selected physiological and productive responses in cattle. However, available data remain insufficient to support broad conclusions regarding its consistent effectiveness under practical cattle-production conditions. The following sections summarize the available evidence related to CH₄ emissions, reproductive-related responses, and productivity outcomes.

Effects of brown seaweed on enteric methane emissions

Methane (CH₄) is a natural by-product of ruminal fermentation. During microbial degradation of dietary carbohydrates, rumen microorganisms produce volatile fatty acids (VFAs), which provide a major energy source for the host animal, together with carbon dioxide (CO₂) and hydrogen (H₂) as fermentation end products. Methanogenic archaea utilize H₂ to reduce CO₂, producing CH₄, which is subsequently released primarily through eructation (Hook et al., 2010; Hristov et al., 2013). Although this process helps maintain a low partial pressure of H₂ within the rumen, thereby supporting efficient microbial fermentation, it also represents an energy loss of approximately 2–12% of gross dietary energy intake and contributes substantially to agricultural greenhouse-gas emissions (Johnson and Johnson, 1995; Hristov et al., 2013). Consequently, nutritional strategies that modify rumen fermentation or redirect H₂ away from methanogenesis have received considerable attention as approaches to mitigate enteric CH₄ emissions (Beauchemin et al., 2022; Hodge et al., 2024).

Mitigation potential and species-specific responses

Table 1 summarizes CH₄-related responses reported for brown seaweed supplementation. Across the available literature, reductions in CH₄ production were generally greater and more consistent in vitro than in vivo. Overall, evidence suggests that some brown seaweed species can influence rumen fermentation and reduce CH₄ production under specific conditions; however, responses vary substantially according to seaweed species, inclusion level, processing method, basal diet, supplementation strategy, and experimental system (Lean et al., 2021; Orzuna-Orzuna et al., 2024; Hodge et al., 2024).

Table 1. Summary of reported methane (CH₄)-related responses to brown seaweed supplementation in ruminant studies.

CH₄, methane; DM, dry matter; DMI, dry matter intake; RUSITEC, rumen simulation technique.

Review articles were excluded from this table. Studies evaluating combinations of brown seaweed with red seaweeds, microalgae, or other feed additives were omitted to maintain emphasis on brown seaweed-specific responses. In vitro fermentation, RUSITEC, and in vivo cattle studies represent different levels of biological evidence and should not be interpreted as equivalent when evaluating CH₄-mitigation efficacy. Reported responses are summarized qualitatively because study designs, seaweed species, inclusion levels, processing methods, harvest locations, supplementation strategies, and response variables differed substantially among studies. Consequently, reported CH₄ responses should be interpreted within the context of individual experimental systems.

¹ Approximate dietary DM equivalents were calculated from the reported mean dry matter intake in the original studies and are provided solely to facilitate comparison among studies. Original supplementation units reported by the source publications were retained.

A meta-analysis evaluating seaweed supplementation in cattle reported an average reduction of 5.28 ± 3.5 g CH₄ kg^-1 DMI, although substantial heterogeneity among studies was observed (Lean et al., 2021). Because this meta-analysis included multiple seaweed species, including both red and brown seaweeds, the reported reduction should be interpreted as broad contextual evidence rather than a direct estimate of the efficacy of brown seaweed supplementation alone. The overall reduction likely reflects, in part, the inclusion of studies evaluating Asparagopsis spp., whereas studies investigating brown seaweed have generally reported more modest and variable CH₄ responses under in vivo cattle conditions (Muizelaar et al., 2023; Thorsteinsson et al., 2023). Furthermore, the marked heterogeneity reported in the meta-analysis highlights the considerable variability in CH₄ responses among seaweed species, experimental conditions, and supplementation strategies.

Among brown seaweed, Ascophyllum nodosum has received the greatest research attention. In vitro studies generally reported low to moderate reductions in CH₄ production, whereas cattle studies reported variable responses ranging from negligible effects to modest reductions (Durmic et al., 2014; Belanche et al., 2016; Antaya et al., 2019; Silva et al., 2022; Bošnjaković et al., 2024). Recent investigations evaluating extract-based products and alternative delivery approaches have similarly reported inconsistent CH₄ responses despite observed effects on rumen fermentation, nitrogen utilization, or animal performance (Casey et al., 2026a, c).

Evidence for Saccharina latissima, Laminaria spp., and Fucus spp. demonstrates a similar pattern. Several laboratory studies reported reductions in CH₄ production, whereas cattle studies generally reported limited or inconsistent responses depending on dietary and management conditions (Kinley and Fredeen, 2015; Muizelaar et al., 2023; Thorsteinsson et al., 2023, 2024). For example, CH₄ reductions observed in vitro for North Atlantic seaweed mixtures containing Laminaria longicruris and Fucus vesiculosus have not yet been consistently validated under cattle-production conditions (Kinley and Fredeen, 2015). Likewise, favorable in vitro responses reported for Fucus species, including reductions in CH₄ concentration under selected experimental conditions, have not been consistently reproduced in vivo (Yergaliyev et al., 2024; Thorsteinsson et al., 2024).

Several studies involving Sargassum spp. also reported reductions in CH₄ production under in vitro conditions, suggesting potential antimethanogenic activity; however, evidence from cattle studies remains limited (Choi et al., 2020, 2021). Screening studies evaluating multiple brown seaweed species have likewise demonstrated considerable variability among taxa, with CH₄ reductions observed for some seaweed but often accompanied by changes in fermentation characteristics and digestibility (Ahmed et al., 2022). Importantly, not all brown seaweed species produce CH₄-reducing effects. For example, supplementation with Ecklonia stolonifera extract increased CH₄ and H₂ production under in vitro conditions despite altering rumen microbial populations, highlighting the species-specific nature of brown-seaweed responses (Lee et al., 2019). The authors suggested that this response was associated with increased H₂ availability resulting from shifts in fibrolytic bacterial populations, particularly an increase in Ruminococcus flavefaciens, which may have provided additional substrate for hydrogenotrophic methanogens despite reductions in methanogen abundance.

A consistent pattern across the CH₄-mitigation literature is the contrast between promising laboratory findings and the more variable responses observed in cattle. The largest reductions in CH₄ production have generally originated from in vitro screening systems and controlled rumen-simulation studies, whereas cattle trials more commonly reported modest reductions or no significant effect. This discrepancy likely reflects differences in rumen adaptation, animal physiology, feed intake, diet composition, supplementation strategy, and the complex biological interactions that occur under commercial production conditions. Recent studies evaluating seaweed mixtures and alternative supplementation approaches have similarly reported inconsistent responses across feeding systems and production environments (Reynolds et al., 2025; Ramos et al., 2025). Controlled rumen-simulation studies using Ascophyllum nodosum extracts have further demonstrated that CH₄ and fermentation responses may vary according to experimental conditions and supplementation strategy (Casey et al., 2026b).

Overall, current evidence suggests that brown seaweed supplementation possesses CH₄-mitigation potential under selected conditions. However, the magnitude and consistency of responses remain highly variable, and robust evidence supporting consistent CH₄ reduction across commercial cattle-production systems is still lacking.

Comparisons with red seaweed should be interpreted cautiously. Red seaweed such as Asparagopsis spp. are often associated with substantially greater CH₄ reductions; however, their antimethanogenic mechanisms, bioactive compounds, delivery strategies, and safety considerations differ fundamentally from those proposed for brown seaweed. Consequently, evidence from red seaweed studies provides useful context but should not be considered direct evidence of brown seaweed efficacy.

Underlying mechanisms of methane response

Several mechanisms have been proposed to explain CH₄-related responses to brown seaweed supplementation, including alterations in rumen microbial ecology, direct or indirect effects on methanogenic archaea, and shifts in H₂-utilization pathways within the rumen. Among the bioactive compounds present in brown seaweed, phlorotannins have received considerable attention because of their potential to influence rumen microbial activity, nutrient degradation, and fermentation pathways associated with methanogenesis (Wang et al., 2008; Min et al., 2021; Choi et al., 2021). Phlorotannins are polyphenolic compounds composed of phloroglucinol units that can form complexes with dietary proteins, cell-wall polysaccharides, microbial enzymes, and cell membranes, thereby modifying microbial activity, nutrient degradation, and rumen fermentation (Wang et al., 2008; Min et al., 2021). These interactions may suppress methanogenesis by reducing H₂ availability for methanogenic archaea, altering fermentation pathways toward less methanogenic end products, and limiting microbial activities that support CH₄ production. However, the magnitude and consistency of these effects vary among brown seaweed species, phlorotannin concentrations, and experimental conditions. Other constituents, including laminarin, fucoidan, and mineral fractions, may also contribute to CH₄-related responses through their effects on rumen microbial activity, substrate utilization, and fermentation pathways.

Current evidence indicates that CH₄ responses are influenced not only by seaweed species but also by processing method, inclusion level, fermentation conditions, extract preparation, and interactions with the rumen microbiome (Andreen et al., 2025; Malik et al., 2025). For example, studies evaluating brown-seaweed hydrolysates, fermentates, and extract-based products have demonstrated that processing can modify the availability and biological activity of seaweed-derived compounds, potentially altering their effects on rumen fermentation and CH₄ production (De Bhowmick et al., 2025). Similarly, a RUSITEC study using Ascophyllum nodosum extracts reported reduced CH₄ production under some dietary conditions, although these responses were occasionally accompanied by changes in nutrient degradation and fermentation characteristics (Casey et al., 2026b).

The complexity of these interactions may partially explain why CH₄ reductions observed under controlled laboratory conditions are often less consistent in cattle studies. Differences in rumen adaptation, feed intake, diet composition, microbial community structure, and host-animal responses may influence the effectiveness of seaweed-derived compounds under practical production conditions. Consequently, although several plausible mechanisms have been identified, the relative contribution of individual bioactive compounds and microbial pathways remains incompletely understood. Further research integrating microbial, biochemical, and animal-performance measurements is needed to clarify the mechanisms responsible for CH₄ mitigation in cattle systems.

Bioactive compounds contributing to brown seaweed responses

Several bioactive compounds have been proposed to contribute to the biological effects of brown seaweed supplementation in ruminant systems, although their relative importance likely varies among seaweed species, processing methods, and supplementation strategies. Phlorotannins are among the most frequently discussed compounds because they are characteristic polyphenols of brown seaweed and have been associated with changes in rumen microbial activity, nutrient degradation, and fermentation characteristics under laboratory conditions (Wang et al., 2008; Min et al., 2021; Choi et al., 2021). These compounds may influence microbial activity directly or indirectly through effects on H₂ utilization and fermentation pathways associated with methanogenesis. However, the magnitude and consistency of these responses under cattle conditions remain unclear.

Other bioactive constituents, including laminarin, fucoidan, alginates, and mineral fractions, have also been suggested to contribute to observed responses through their effects on rumen microbial activity, substrate utilization, and fermentation processes. Nevertheless, the relative contribution of individual compounds remains difficult to determine because brown seaweed contains complex mixtures of bioactive substances that may act synergistically.

Laminarin and fucoidan are important brown-seaweed polysaccharides that may influence rumen microbial ecology and fermentation processes (Choi et al., 2021). However, compared with phlorotannins, evidence supporting their specific contribution to CH₄ mitigation remains limited. Their effects appear to be more closely associated with broader modulation of microbial populations and fermentation pathways than with direct inhibition of methanogenesis.

The mineral composition of brown seaweed may also contribute to their biological effects. Iodine is of particular interest because many brown seaweed species, especially Ascophyllum nodosum, contain substantially higher iodine concentrations than conventional feed ingredients. Although iodine is not considered an antimethanogenic compound, it remains highly relevant because of its physiological importance and because increased iodine transfer into milk has been consistently reported following brown seaweed supplementation, raising important food-safety considerations in dairy systems (Sorge et al., 2016; Silva et al., 2022; Newton et al., 2023).

In contrast, bromoform is frequently discussed in CH₄-mitigation research because of the substantial CH₄ reductions reported for some red seaweed species, particularly Asparagopsis spp. However, bromoform is not considered a major bioactive compound in brown seaweed and is therefore unlikely to explain the responses observed in most brown-seaweed studies. Consequently, CH₄-related responses associated with brown seaweed supplementation are more commonly attributed to phlorotannins, sulfated polysaccharides (including laminarin and fucoidan), alginates, mineral fractions, and the resulting alterations in rumen microbial activity and fermentation pathways rather than to halogenated compounds (Min et al., 2021; Hodge et al., 2024). Brown seaweed also contains a variety of other secondary metabolites that may influence rumen microbial ecology; however, the specific contribution of individual compounds to CH₄ mitigation remains incompletely understood. Consequently, the mechanisms underlying CH₄-related responses are likely multifactorial and may vary according to seaweed species, processing method, inclusion level, and experimental conditions.

Several studies have reported alterations in rumen microbial communities and fermentation-related responses following brown seaweed supplementation (Zhou et al., 2018; Künzel et al., 2022; Yen et al., 2024). In addition, the stability and biological activity of seaweed-derived compounds may be influenced by post-harvest handling, preservation, and processing methods. Ensiling, hydrolysis, fermentation, and extract preparation can alter nutrient composition and the availability of bioactive compounds, potentially affecting rumen fermentation and CH₄-related responses (Campbell et al., 2020; Navarro Marcos et al., 2023; De Bhowmick et al., 2025). However, these effects likely reflect interactions between seaweed species and processing conditions rather than the effects of processing alone.

Overall, the available evidence suggests that the effects of brown seaweed supplementation are unlikely to be driven by a single bioactive compound. Instead, CH₄-related responses appear to result from interactions among multiple compounds and their effects on rumen microbial populations and fermentation pathways. The magnitude of these responses is further influenced by seaweed species, inclusion level, diet composition, processing method, animal characteristics, and microbial adaptation. As a result, reported responses vary considerably among studies, making it difficult to establish universal recommendations for brown seaweed supplementation. Therefore, CH₄-mitigation outcomes should be interpreted within the context of individual studies rather than extrapolated broadly across all brown seaweed species, supplementation strategies, and cattle-production systems (Silva et al., 2022; Hristov, 2024).

In vitro versus In vivo discrepancies

One of the most consistent patterns across the reviewed literature is that CH₄ reductions observed under in vitro conditions are often greater than those reported in vivo. Laboratory systems are valuable for screening candidate seaweed and investigating potential mechanisms of action; however, they do not fully replicate feed-intake behavior, rumen passage dynamics, host metabolism, dietary interactions, dosage variability, or long-term microbial adaptation. Consequently, in vitro findings are useful for identifying biologically active seaweed species but should not be interpreted as direct evidence of efficacy under commercial cattle-production systems (Beauchemin et al., 2022; Hodge et al., 2024). This distinction is particularly important because several brown seaweed species that demonstrated promising CH₄ reductions in laboratory studies produced limited or inconsistent responses in cattle trials (Thorsteinsson et al., 2023, 2024; Muizelaar et al., 2023).

Additional evidence from continuous-culture and RUSITEC systems further highlights the differences between controlled fermentation studies and animal responses. These experimental systems have reported measurable effects on CH₄ production and rumen fermentation characteristics, whereas cattle studies evaluating similar seaweed species or extract-based products generally reported more modest and variable responses (Andreen et al., 2025; Casey et al., 2026c). Likewise, studies evaluating alternative delivery systems in dairy cows and beef steers demonstrated that responses observed under controlled experimental conditions do not always translate into consistent CH₄ mitigation under practical production settings (Casey et al., 2026a, c). Similar conclusions have been reported from studies investigating controlled dosing and direct ruminal delivery approaches, which emphasized the importance of supplementation strategy, dosage consistency, and animal adaptation in determining CH₄-related responses (Muizelaar et al., 2026).

The reduced efficacy frequently observed under in vivo conditions likely reflects the combined influence of several animal- and diet-related factors. In contrast to laboratory systems, where bioactive compounds are maintained at fixed concentrations, the effective dose in vivo is influenced by voluntary feed intake, dilution within the rumen contents, and ruminal passage rate, which may reduce the duration of microbial exposure to seaweed-derived compounds. In addition, prolonged supplementation may result in microbial adaptation, potentially reducing the persistence of antimethanogenic responses over time.

These findings indicate that CH₄ responses are influenced not only by seaweed chemistry but also by animal-level factors, including feed intake, rumen passage rate, microbial adaptation, diet composition, and host physiology. Therefore, although in vitro studies clearly demonstrate the biological potential of several brown seaweed species, current in vivo evidence remains less consistent and suggests that CH₄-mitigation responses are highly dependent on production conditions and supplementation strategy.

Effects of brown seaweed on productivity

Feed intake and digestibility

A summary of findings related to feed intake, digestibility, and nutrient-utilization responses associated with brown seaweed supplementation is presented in Table 2.

Table 2. Summary of reported digestibility-, nutrient-utilization-, and fermentation-related responses to brown seaweed supplementation in cattle studies.

ADF, acid detergent fiber; CP, crude protein; DM, dry matter; NDF, neutral detergent fiber; OM, organic matter.

This table summarizes digestibility-, fermentation-, and nutrient-utilization-related findings associated with brown seaweed supplementation or preservation. Production variables such as milk-protein yield and milk urea concentration were excluded because they do not directly represent digestibility traits. Findings from Campbell et al. (2020) should be interpreted cautiously because the reported nutrient changes may reflect species-specific responses to preservation and ensiling conditions, rather than the inherent nutritional properties of the seaweed itself. Responses may also vary according to seaweed species, inclusion level, processing method, preservation strategy, and experimental conditions. In vitro findings should not be interpreted as direct evidence of efficacy under in vivo cattle-production conditions.

Across the available literature, responses varied considerably among studies and appeared to depend on seaweed species, inclusion level, processing method, preservation approach, supplementation strategy, and production system.

Feed-intake responses were generally inconsistent across studies. In pasture-based dairy systems, supplementation with 113 g/day of kelp meal (Ascophyllum nodosum) increased total dry matter intake (DMI) in Jersey cows (Antaya et al., 2019). In contrast, supplementation with up to 4% dietary DM of Saccharina latissima, Ascophyllum nodosum, or Sargassum muticum in total mixed rations for lactating Holstein cows did not significantly affect DMI (Thorsteinsson et al., 2023). Similar variability has been reported in more recent studies evaluating seaweed mixtures, extracts, and alternative delivery systems, in which changes in rumen fermentation or CH₄-related responses were not always accompanied by corresponding changes in feed intake (Reynolds et al., 2025; Casey et al., 2026a). Differences in feed-intake responses among studies may partly reflect variation in the chemical composition of brown seaweeds, particularly their relatively high ash and mineral contents, including sodium and potassium, which may influence palatability and practical dietary inclusion levels (Makkar et al., 2016). However, these effects are likely species-specific and are also influenced by processing method, diet composition, and supplementation rate.

Digestibility responses were likewise influenced by seaweed species and processing method. Studies evaluating preserved brown seaweed have demonstrated that ensiling and other processing approaches can alter feed characteristics, nutrient composition, and ruminal fermentation responses, potentially affecting nutrient utilization. However, the magnitude and direction of these effects appear to depend on the seaweed species and preservation conditions employed (Campbell et al., 2020; Navarro Marcos et al., 2023; Yen et al., 2024).

For example, ensiling studies with Saccharina latissima reported reductions in crude protein (CP), ash, neutral detergent fiber (NDF), and acid detergent fiber (ADF) concentrations under specific preservation conditions, together with more extensive lactic acid fermentation and lower silage pH, indicating improved fermentation quality (Campbell et al., 2020). In contrast, Fucus vesiculosus exhibited greater nutrient stability during ensiling but a less extensive fermentation process, characterized by higher silage pH and lower lactic acid production. These findings suggest that although Saccharina latissima may produce better-preserved silage from a fermentation perspective, this may occur at the expense of greater nutrient losses, whereas Fucus vesiculosus better preserves its nutritional composition despite less efficient fermentation. Similarly, alternative ensiling methods applied to Saccharina latissima altered chemical composition and in vitro ruminal fermentation characteristics, highlighting the importance of preservation strategy when evaluating the nutritional value of brown seaweed (Navarro Marcos et al., 2023). Collectively, these responses indicate that both seaweed species and preservation conditions influence silage fermentation quality and the nutritional value of brown seaweeds intended for ruminant feeding.

In contrast, Silva et al. (2022) reported linear increases in dry matter (DM), organic matter, and crude-protein digestibility with increasing Ascophyllum nodosum meal inclusion, whereas NDF and ADF digestibility were unaffected. More recent studies evaluating processed brown-seaweed products, including hydrolysates and fermentates, have similarly suggested that processing method may influence nutrient availability, fermentation characteristics, and nitrogen-utilization responses (De Bhowmick et al., 2025).

An important distinction across the literature is whether brown seaweed is used as a low-dose supplement or incorporated as a larger dietary ingredient. Responses observed at supplementation levels such as those evaluated by Antaya et al. (2019) and Silva et al.7 (2022) are unlikely to affect fiber utilization and digestibility to the same extent as studies in which seaweed constitutes a substantial proportion of the diet. Consequently, intake and digestibility responses appear to depend not only on inclusion level but also on seaweed species, processing method, preservation strategy, delivery system, basal diet composition, and production environment (Reynolds et al., 2025; Casey et al., 2026a; De Bhowmick et al., 2025).

Overall, the available evidence indicates that brown seaweed supplementation can influence feed intake, digestibility, and nutrient utilization under specific conditions. However, responses remain inconsistent across studies, and current findings do not support broad generalizations regarding the effects of brown seaweed supplementation on feed intake or nutrient digestibility across cattle-production systems.

Production parameters

Reported effects of brown seaweed supplementation on productivity have generally been modest and inconsistent across studies. Some investigations reported small increases in milk yield, whereas others detected no significant effects on production performance (Antaya et al., 2019; Muizelaar et al., 2023; Bošnjaković et al., 2024). Positive responses were most often observed under specific management or environmental conditions, such as grazing systems, heat stress, or combined feed-additive interventions, rather than as a consistent consequence of brown seaweed supplementation alone (Lee et al., 2020). Similarly, more recent studies evaluating seaweed mixtures, extracts, and alternative supplementation strategies reported variable production responses across feeding systems and management conditions (Reynolds et al., 2025; Casey et al., 2026a). Collectively, current evidence does not support a consistent milk-yield response to brown seaweed supplementation in dairy cattle.

Milk-composition responses have also varied among studies. Muizelaar et al. (2023) reported increased milk yield and lactose-related measures following supplementation with Saccharina latissima, whereas Thorsteinsson et al. (2023) did not observe similar responses. In the study by Muizelaar et al. (2023), increased lactose output appeared to be primarily associated with greater milk production rather than an independent effect on lactose synthesis. In general, milk-fat concentration remained largely unchanged across studies, and brown seaweed supplementation produced limited and inconsistent effects on milk-composition variables, including protein and lactose concentrations (Thorsteinsson et al., 2023).

Changes in milk mineral composition and iodine concentrations have also been reported following supplementation with Ascophyllum nodosum (Newton et al., 2023). Variable responses in milk fatty-acid and mineral composition have likewise been reported following supplementation with seaweed mixtures, further emphasizing the influence of seaweed species, inclusion level, and feeding context on productive outcomes (Reynolds et al., 2025).

Among productivity-related responses, changes in milk mineral composition, particularly iodine concentration, have been among the most consistently reported findings. Studies evaluating Ascophyllum nodosum supplementation have frequently reported increased iodine concentrations and transfer into milk (Sorge et al., 2016; Silva et al., 2022; Newton et al., 2023). Silva et al. (2022) also reported increased arsenic excretion in feces and urine with increasing kelp supplementation; however, the study did not determine whether the excreted arsenic was present in inorganic or organic forms. The authors noted that arsenic in Ascophyllum nodosum is predominantly present in the organic form, which is generally considered less toxic than inorganic arsenic, whereas arsenic transfer into milk remained limited. These findings highlight the importance of evaluating mineral metabolism and excretion alongside productive responses. More recent studies have similarly emphasized the need to monitor iodine transfer efficiency, feed-to-milk iodine transfer, and milk mineral concentrations when incorporating brown seaweed into dairy-cattle diets (Lamminen et al., 2026).

Compared with dairy systems, evidence related to beef-cattle productivity remains limited. Although some studies evaluating alternative delivery systems and supplementation approaches reported effects on rumen fermentation and selected performance-related variables under specific feeding conditions (Casey et al., 2026a), available evidence remains insufficient to support strong conclusions regarding improvements in beef-cattle productivity (Costa et al., 2021; Hodge et al., 2024).

Overall, productivity responses to brown seaweed supplementation appear to depend on seaweed species, inclusion level, processing method, supplementation strategy, feeding conditions, and study design. While some studies reported modest improvements in milk yield or selected milk components, these responses have not been consistently reproduced across production systems. In contrast, alterations in milk mineral composition, particularly increased iodine transfer, represent one of the most repeatable responses reported in dairy cattle. Consequently, current evidence suggests that productive benefits of brown seaweed supplementation are variable, whereas mineral-transfer and safety-related responses are more consistently observed.

Factors contributing to variability in productivity responses

The variability in productivity responses reported across studies suggests that brown seaweed supplementation does not exert uniform effects across cattle-production systems. Instead, observed responses likely reflect interactions among seaweed species, inclusion level, basal diet composition, animal physiological status, processing method, and supplementation strategy (Campbell et al., 2020). Consequently, currently available evidence remains insufficient to support universal inclusion recommendations across brown seaweed species and production conditions.

Processing and preservation methods appear to be important sources of variation in brown seaweed responses. Hydrolysis, fermentation, ensiling, drying, and extract preparation can alter nutrient composition, mineral availability, and the availability of bioactive compounds, thereby influencing digestibility, ruminal fermentation characteristics, microbial activity, and productive responses (Campbell et al., 2020; Navarro Marcos et al., 2023; Yen et al., 2024; De Bhowmick et al., 2025). However, reported effects likely reflect interactions between seaweed species and processing conditions rather than the effects of processing alone.

Basal diet and feeding system may also influence productivity responses. Although direct experimental comparisons remain limited, available evidence suggests that intake, rumen fermentation, nutrient utilization, and production responses may differ among grazing, pasture-based, and total mixed ration systems. For example, Antaya et al. (2019) reported productivity-related responses in a grazing-based Jersey-cow system, whereas observations regarding feeding-system effects in other publications are largely interpretative rather than derived from controlled comparisons (Dhakal et al., 2024). More recent studies evaluating seaweed mixtures, extracts, and alternative delivery approaches have similarly reported variable responses across feeding systems and management conditions (Reynolds et al., 2025; Casey et al., 2026a). Consequently, suggestions that certain production systems respond more favorably than others should currently be viewed as preliminary observations rather than established conclusions.

Animal physiological status may represent another important source of variation. Responses to supplementation may differ according to stage of production, metabolic status, environmental conditions, and management practices. For example, improvements in milk production and antioxidant-related measures were reported in a combined feed-additive study conducted under heat-stress conditions; however, these responses could not be attributed specifically to brown seaweed supplementation alone (Lee et al., 2020). More recent physiological evaluations have likewise suggested that productivity-related responses are influenced by metabolic status, environmental conditions, and supplementation strategy in addition to seaweed inclusion (Bošnjaković et al., 2024).

Effects of brown seaweed on reproductive-related parameters

Compared with CH₄-related outcomes, reproductive responses to brown seaweed supplementation have received considerably less attention. Most available studies have focused on endocrine and metabolic indicators rather than direct measures of fertility or reproductive performance, and the majority of evidence originates from studies evaluating Ascophyllum nodosum. Consequently, the current evidence base remains relatively small and does not allow firm conclusions regarding reproductive benefits in cattle.

Hormonal responses

A summary of hormonal responses associated with brown seaweed supplementation is presented in Table 3. Several studies have reported changes in endocrine-related variables following supplementation; however, the magnitude and consistency of these responses vary considerably among studies. In transition Holstein cows supplemented with 2–4% Ascophyllum nodosum by-products, Hong et al. (2015) observed changes in selected hormonal measures, including higher mean estrogen and progesterone concentrations at certain sampling times. However, these findings were derived from a single study, several effects were modest or observed only as statistical trends, and the results do not provide direct evidence of improved fertility or reproductive performance. Moreover, the mechanisms underlying these hormonal responses remain uncertain. Current evidence does not demonstrate that brown seaweed exerts direct phytoestrogenic effects; rather, the observed endocrine changes may be secondary to alterations in metabolic status, nutrient utilization, or iodine-mediated changes in thyroid function, all of which can influence endocrine regulation. Additional studies are needed to clarify the physiological pathways responsible for these responses.

Table 3. Summary of reported endocrine-, physiological-, and iodine-related responses associated with brown seaweed supplementation in cattle.

DM, dry matter; E₂, estradiol; P₄, progesterone; T₃, triiodothyronine; T₄, thyroxine.

Most reported responses relate to endocrine, physiological, or metabolic indicators rather than direct measures of reproductive performance. Consequently, these findings should not be interpreted as evidence of improved fertility, conception rate, pregnancy success, or reproductive efficiency. Several responses were derived from short-term studies, mixed feed-additive interventions, or indirect physiological measurements rather than direct reproductive outcomes. Reported endocrine and physiological responses should therefore be interpreted cautiously until validated by additional long-term studies evaluating reproductive performance under commercial cattle-production conditions.

¹ Silva et al. (2022) reported increased arsenic excretion with increasing Ascophyllum nodosum supplementation, whereas arsenic transfer into milk remained limited. The study did not determine the chemical speciation (organic versus inorganic) of arsenic in excreta. Newton et al. (2021) emphasized the importance of monitoring iodine and heavy-metal concentrations when evaluating the safety of brown seaweed supplementation.

Thyroid-related responses have also received attention because many brown seaweed species, particularly Ascophyllum nodosum, contain high concentrations of iodine. Some studies reported changes in triiodothyronine (T3) or thyroxine (T4) concentrations following supplementation, whereas others detected no significant effects (Sorge et al., 2016; Antaya et al., 2019; Silva et al., 2022). These inconsistent findings may reflect differences in iodine intake, supplementation level and duration, animal characteristics, dietary composition, and baseline iodine status. In addition, thyroid hormone synthesis is regulated by physiological autoregulatory mechanisms. Moderate increases in iodine intake generally support thyroid hormone production, whereas excessive or prolonged iodine exposure may transiently suppress thyroid hormone synthesis through the Wolff–Chaikoff effect, a protective mechanism that limits excess hormone production until thyroid adaptation occurs (Leung and Braverman, 2014). Consequently, variation in iodine exposure among studies may contribute to the inconsistent T3 and T4 responses reported following brown seaweed supplementation. Collectively, the available evidence suggests that brown seaweed supplementation can influence iodine exposure and selected thyroid-related variables under certain conditions; however, the biological significance, consistency, and long-term implications of these responses remain unclear.

Changes in stress- and antioxidant-related indicators have also been reported. For example, some studies observed reduced cortisol concentrations or improvements in antioxidant-associated measures following supplementation. However, several of these responses were reported in studies evaluating brown seaweed as part of combined feed-additive systems, making it difficult to attribute the observed effects specifically to brown seaweed supplementation alone (Lee et al., 2020).

Overall, current evidence indicates that brown seaweed supplementation may influence selected endocrine and metabolic variables in cattle. Nevertheless, the available literature remains insufficient to determine whether these changes consistently translate into improved fertility, reproductive efficiency, reproductive performance, or other biologically meaningful reproductive outcomes under commercial cattle-production conditions.

Fertility and estrous outcomes

Direct evidence linking brown seaweed supplementation to improved reproductive performance remains very limited, representing one of the major knowledge gaps identified in the current literature. Compared with CH₄-related outcomes, relatively few studies have evaluated reproductive endpoints such as conception rate, pregnancy rate, days open, calving interval, estrous regularity, or overall reproductive efficiency. Instead, most available studies have focused on endocrine, physiological, or metabolic indicators that may be associated with reproductive function (Antaya et al., 2019; Lee et al., 2020; Silva et al., 2022).

Although changes in estrogen, progesterone, thyroid-related measures, oxidative status, and cortisol concentrations have been reported, these responses have not been consistently observed across studies and are often derived from single experiments or mixed feed-additive interventions. Furthermore, several reported responses may reflect indirect effects on metabolism, mineral status, or physiological adaptation rather than direct reproductive mechanisms. Consequently, the available evidence is more appropriately interpreted as indicating potential endocrine and physiological responses rather than confirmed improvements in fertility or reproductive performance.

A notable limitation of the current literature is the scarcity of well-controlled studies evaluating direct reproductive outcomes under commercial cattle-production conditions. As a result, it remains unclear whether observed endocrine and physiological responses translate into meaningful improvements in conception success, pregnancy maintenance, reproductive efficiency, days open, calving interval, or overall herd-level reproductive performance.

Overall, current evidence remains insufficient to conclude that brown seaweed supplementation consistently improves reproductive outcomes in cattle. Additional long-term studies evaluating direct reproductive endpoints under commercial production conditions are needed before practical recommendations can be established.

Proposed mechanisms and influencing factors

Several mechanisms have been proposed to explain reproductive-related responses associated with brown seaweed supplementation, including endocrine modulation, reduced stress-related responses, altered mineral exposure, and improvements in metabolic status. Although these mechanisms are biologically plausible, supporting evidence in cattle remains limited and is derived primarily from studies measuring hormones, oxidative markers, thyroid-related variables, or metabolic indicators rather than direct reproductive outcomes. Consequently, mechanistic interpretations should be considered preliminary and should not be viewed as evidence of confirmed fertility improvement (He et al., 2018).

Among the proposed mechanisms, reduced cortisol concentrations have received particular attention because chronic stress can influence hypothalamic–pituitary–gonadal function. However, the relationship between cortisol responses and reproductive performance remains indirect within the current brown seaweed literature, and much of the mechanistic support originates from studies outside cattle systems (He et al., 2018). Therefore, reproductive interpretations of cortisol-related responses should be approached cautiously.

Mineral composition may also contribute to observed physiological responses associated with brown seaweed supplementation. In particular, iodine-rich species such as Ascophyllum nodosum may alter iodine exposure and influence thyroid-related variables (Sorge et al., 2016; Silva et al., 2022). More recent studies have demonstrated increased iodine transfer from feed to milk following brown seaweed supplementation, emphasizing the importance of considering both animal responses and food-safety implications when evaluating brown seaweed use in dairy systems (Newton et al., 2021, 2023; Lamminen et al., 2026). Consequently, iodine should be viewed not only as a potential contributor to thyroid-related responses but also as one of the principal practical considerations associated with brown seaweed supplementation.

Recent physiological and metabolic evaluations further suggest that responses to brown seaweed supplementation may depend on animal status, mineral exposure, diet composition, and feeding conditions rather than on direct reproductive mechanisms alone (Bošnjaković et al., 2024). Collectively, these findings indicate that reproductive-related responses are likely influenced by multiple interacting physiological pathways. However, the extent to which these responses affect fertility, reproductive efficiency, or long-term reproductive performance remains unclear.

Integration of findings and practical applications

Taken together, the available literature suggests that brown seaweed supplementation has the potential to influence CH₄ production, physiological responses, nutrient utilization, milk composition, and mineral transfer in cattle systems. However, responses remain highly variable among studies, and the current evidence base does not support broad practical recommendations across all seaweed species, production systems, or supplementation strategies.

CH₄ mitigation remains one of the primary motivations for evaluating brown seaweed as feed additives. Nevertheless, CH₄ responses reported in cattle studies have generally been less consistent than those observed under laboratory conditions. While some studies reported modest reductions in enteric CH₄ emissions, others found little or no effect, particularly under practical production conditions (Muizelaar et al., 2023; Thorsteinsson et al., 2023). More recent studies evaluating seaweed mixtures, extracts, and alternative delivery systems have similarly reported variable responses depending on seaweed species, feeding system, supplementation strategy, and management conditions (Reynolds et al., 2025; Casey et al., 2026a, c). Consequently, practical expectations regarding CH₄ mitigation should remain cautious and species-specific.

Similar variability has been reported for reproductive and productivity outcomes. Although some studies observed changes in endocrine variables, milk yield, milk composition, nutrient utilization, or physiological indicators, these responses have not been consistently reproduced across studies. In contrast, one of the more repeatable findings across the literature has been altered mineral transfer, particularly increased iodine concentration in milk following supplementation with iodine-rich species such as Ascophyllum nodosum.

Practical implementation of brown seaweed supplementation therefore requires consideration of multiple factors, including seaweed species, inclusion level, processing method, delivery strategy, animal physiological status, production system, economic feasibility, and safety. At present, available evidence suggests that responses are highly context-dependent and that practical use should be guided by both biological efficacy and safety considerations rather than CH₄-mitigation potential alone.

Safety and regulatory considerations

Safety considerations represent one of the most important aspects of brown seaweed supplementation because seaweed contains variable concentrations of minerals, trace elements, and bioactive compounds. Among these, iodine has emerged as the most consistently documented concern in cattle studies and represents a key link between biological responses and food-safety considerations. Brown seaweed, particularly Ascophyllum nodosum, often contains high iodine concentrations, and multiple studies have reported increased iodine concentrations in milk following supplementation (Sorge et al., 2016; Newton et al., 2021; Silva et al., 2022; Newton et al., 2023). More recent dairy-cattle studies have further emphasized the importance of monitoring iodine transfer efficiency and milk iodine accumulation when brown seaweed is incorporated into feeding programs (Lamminen et al., 2026).

Other trace elements, including arsenic and cadmium, also warrant consideration, although evidence for their transfer into milk appears less consistent than that reported for iodine (Newton et al., 2021). Consequently, evaluation of brown seaweed safety should extend beyond CH₄-mitigation potential and include assessment of mineral composition, contaminant concentrations, seaweed source, harvest environment, and processing method.

Compared with CH₄-, reproductive-, and productivity-related outcomes, safety-related observations have generally been more consistent across studies. One of the clearest conclusions emerging from the literature is that practical implementation of brown seaweed supplementation should evaluate safety and product quality alongside biological efficacy rather than treating them as secondary considerations. Recent reviews have similarly emphasized the need to balance CH₄-mitigation potential with safety, compositional variability, economic feasibility, implementation challenges, and regulatory requirements when evaluating seaweed-based feeding strategies for cattle (Liu et al., 2025).

Although several studies suggest limited transfer of certain contaminants into milk, current evidence remains incomplete, and rigorous sourcing, quality control, and monitoring remain necessary (EFSA, 2014; Newton et al., 2021). Furthermore, regulatory approval and commercial adoption of seaweed-based feed ingredients continue to be influenced by variability in chemical composition, contaminant profiles, and differences among seaweed species, harvest locations, and processing methods (Liu et al., 2025). Therefore, safety assessment should remain a central component of future research and practical implementation efforts.

Implementation

The practical adoption of brown seaweed supplementation in cattle systems will depend not only on biological efficacy but also on factors such as supply consistency, processing requirements, preservation method, delivery strategy, economic feasibility, and regulatory acceptance. Although brown seaweed has shown potential to influence CH₄ production, physiological responses, nutrient utilization, milk composition, and mineral transfer, current evidence remains insufficient to support widespread implementation across production systems. Large-scale adoption is further complicated by variability in seaweed composition, preservation requirements, seasonal availability, uncertain economic returns, and inconsistent responses reported among studies (Craigie, 2011; Navarro Marcos et al., 2023). Consequently, expectations regarding commercial implementation should remain cautious until additional biological, economic, and regulatory data become available (Hristov et al., 2022; Beauchemin et al., 2022; Liu et al., 2025).

Recent studies have highlighted the importance of supplementation strategy and delivery system in determining the effectiveness of brown seaweed supplementation. Controlled dosing and delivery approaches have demonstrated that supplementation method may substantially influence the consistency of CH₄-related responses under practical cattle conditions (Muizelaar et al., 2026). Similarly, studies evaluating seaweed mixtures, extracts, and water-based delivery systems in dairy and beef cattle reported variable CH₄, fermentation, and production responses depending on feeding conditions, supplementation strategy, and management system (Reynolds et al., 2025; Casey et al., 2026a, c). These findings suggest that successful implementation may depend not only on the seaweed itself but also on how it is delivered within the production system.

Preservation and processing methods represent additional practical considerations. Ensiling, hydrolysis, fermentation, extract preparation, and other processing approaches can influence nutrient composition, mineral availability, and the availability of bioactive compounds, thereby affecting biological responses (Campbell et al., 2020; Navarro Marcos et al., 2023; De Bhowmick et al., 2025). Although some preservation methods, such as formic-acid treatment, have shown promise under controlled research conditions, their application under commercial farm conditions may be constrained by handling requirements, infrastructure needs, and implementation challenges. Therefore, preservation and processing strategies should be evaluated not only for their biological effectiveness but also for their practicality, scalability, and economic viability.

Overall, successful implementation of brown seaweed supplementation will require integration of biological efficacy, safety, economic feasibility, supply-chain reliability, regulatory compliance, and practical farm-level management considerations. Until these factors are more comprehensively evaluated, widespread adoption should be approached cautiously and on a species-specific basis.

Research gaps and future research priorities

Despite growing interest in brown seaweed supplementation for cattle, several important knowledge gaps remain. A major limitation of the current literature is the imbalance between promising experimental findings and validation under practical cattle-production conditions. Much of the available evidence is derived from short-term studies, small animal populations, or in vitro fermentation systems, whereas comparatively few studies have evaluated long-term responses under commercial production conditions. In addition, substantial variation in seaweed species, harvest source, processing method, preservation strategy, delivery system, inclusion level, chemical composition, and measured outcomes limits direct comparison among studies and contributes to the inconsistent responses reported across the literature.

Several priorities should be addressed in future research. First, additional long-term in vivo studies are needed across diverse dairy and beef production systems to evaluate the consistency of CH₄, productivity, physiological, and safety-related responses under commercial conditions. Second, reproductive research should focus on direct fertility outcomes, including conception rate, pregnancy rate, days open, calving interval, and overall reproductive efficiency, rather than relying primarily on endocrine or metabolic indicators. Third, studies should clearly distinguish between brown seaweed-only interventions and combined feed-additive systems to improve attribution of observed responses. Fourth, standardized reporting of seaweed species, harvest source, processing method, chemical composition, inclusion level, and delivery strategy would improve reproducibility and facilitate comparison among studies.

Additional research is also needed to evaluate the effects of processing and preservation methods, including hydrolysis, fermentation, extract preparation, and ensiling, because these approaches may substantially influence biological activity, nutrient availability, and consistency of response (De Bhowmick et al., 2025). Furthermore, greater attention should be directed toward understanding the mechanisms underlying observed responses, including interactions among bioactive compounds, rumen microbial populations, animal physiology, and production environment.

Future studies should integrate CH₄ emissions, productivity, reproductive performance, safety, implementation feasibility, and economic outcomes within the same experimental framework to provide a more comprehensive assessment of the practical value of brown seaweed supplementation in cattle systems (Beauchemin et al., 2022; Campbell et al., 2020; Orzuna-Orzuna et al., 2024; Hristov et al., 2022; Liu et al., 2025). Such integrated approaches will be essential for determining whether brown seaweed supplementation can be adopted as a sustainable and practical strategy within commercial cattle-production systems.

Conclusion

Brown seaweed supplementation has attracted considerable interest as a nutritional strategy for cattle because of its potential to influence CH₄ emissions, rumen fermentation, nutrient utilization, mineral transfer, and selected physiological responses. However, the available evidence indicates that these responses are highly variable and depend on factors including seaweed species, inclusion level, processing method, supplementation strategy, diet composition, and production system.

Although several studies have reported reductions in CH₄ emissions, these effects have generally been less consistent under in vivo cattle conditions than in laboratory-based systems and remain more modest than those reported for certain red seaweed species. Likewise, evidence supporting consistent improvements in productivity and reproductive performance remains limited and inconsistent. Most reported responses are derived from short-term studies, indirect physiological indicators, mechanistic fermentation systems, or combined feed-additive interventions rather than long-term evaluations of animal performance under commercial production conditions.

In contrast, increased iodine transfer into milk is among the most consistently reported responses and represents an important consideration for food safety, regulatory compliance, and practical implementation in dairy production systems. These findings highlight the importance of evaluating brown seaweed supplementation within a broader framework that considers biological efficacy, food safety, product quality, regulatory requirements, economic feasibility, and practical implementation.

Overall, the current evidence suggests that brown seaweed supplementation is a promising, yet still emerging, nutritional strategy rather than a consistently validated solution for CH₄ mitigation or performance enhancement in cattle. Future progress will depend on well-designed, long-term in vivo studies, improved characterization and standardization of brown seaweed products, and integrated evaluation of efficacy, safety, economic feasibility, and practical implementation under commercial production conditions. Until such evidence becomes available, recommendations for practical application should remain cautious, species-specific, and guided by both biological efficacy and food safety.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fanim.2026.1918542/full#supplementary-material

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Footnotes

Edited by: Chaichana Suriyapha, Prince of Songkla University, Thailand

Reviewed by: Maharach Matra, Mahasarakham University, Thailand

Sukruthai Sommai, Buriram Rajabhat University, Thailand