The livestock industry faces mounting pressure to reduce its environmental footprint whilst maintaining productivity and profitability. Enteric greenhouse gas emissions, particularly methane (CH₄), represent a significant challenge in ruminant production, contributing approximately 10-12% loss of digested gross energy and accounting for about 75% of agricultural greenhouse gas emissions (Olagunju et al., 2025). This environmental concern has sparked intensive research into sustainable feed additives that can simultaneously reduce emissions and improve animal performance.
Phytogenic feed additives, derived from plant materials rich in secondary metabolites, have emerged as promising alternatives to synthetic additives. These natural compounds offer potential benefits including antimicrobial properties, improved digestion, and reduced methane production without the concerns associated with chemical residues (Lambo et al., 2024). Whilst red seaweed species such as Asparagopsis taxiformis have demonstrated remarkable methane reduction capabilities of up to 80% in field trials (Meo-Filho et al., 2024), their commercial scalability remains limited. Among alternative plant-based additives being investigated, onion peel (Allium cepa) has garnered increasing attention as a sustainable and cost-effective option.
Onion peel represents a significant agricultural byproduct, with approximately 20-30% of total onion weight consisting of outer skins typically discarded as waste. Globally, this generates substantial volumes including 0.6 million tonnes annually in European countries alone (Bains et al., 2023; Celano et al., 2021). Upcycling this abundant waste stream presents an opportunity for valorisation in livestock nutrition.
Methodology and experimental design
Recent research by Olagunju et al. (2025) employed a comprehensive in vitro batch culture approach to evaluate the effects of onion peel inclusion on ruminant fermentation. The study utilised a 2 × 3 × 5 factorial arrangement, examining two distinct dietary substrates: a high-concentrate diet (HC) and a high-forage diet (HF) comprising corn silage. The onion peel treatments included four inclusion levels: 2.5% (OP2.5), 5% (OP5), 7.5% (OP7.5), and 10% (OP10), compared against control diets without additive supplementation.
The experimental methodology involved rigorous preparation protocols, with yellow variety onion peels cleaned, air-dried under shade at 30 ± 2°C for five days, and milled through a 2 mm sieve. Approximately 0.5 ± 0.05 g of treatments were weighed into Ankom fibre filter bags and sealed before insertion into 100 mL serum bottles. The study employed rumen inoculum collected from three Holstein Friesian cannulated cows fed identical substrates to those evaluated in the experiment, ensuring consistency and minimising potential variations.
Gas production measurements were conducted at 6, 24, and 48 hours of incubation using pressure transducers, whilst individual gases including methane, carbon dioxide, ammonia, and hydrogen sulphide were analysed using a portable gas analyser. The researchers also evaluated nutrient degradability parameters including dry matter, neutral detergent fibre, acid detergent fibre, and acid detergent lignin degradation, alongside volatile fatty acid production profiles.
Major findings and results
The study revealed significant additive × diet interactions for most measured parameters, indicating that optimal onion peel inclusion levels vary depending on the basal diet composition. The high-concentrate diet consistently produced more total gas but lower concentrations of methane, carbon dioxide, ammonia, and hydrogen sulphide compared to the high-forage diet. This pattern aligns with established understanding of rumen fermentation dynamics, where concentrate-rich diets typically exhibit higher fermentability due to readily available carbohydrates.
Particularly noteworthy was the methane reduction achieved with onion peel supplementation in high-forage diets. At 48 hours of incubation, the OP5 treatment demonstrated the lowest methane production in the high-forage diet through quadratic effects (p = 0.027). This finding is especially significant given that high-forage diets are typically associated with elevated methane emissions due to increased fibre fermentation by methanogenic archaea (Li et al., 2019).
Nutrient degradability responses varied considerably between diet types and onion peel inclusion levels. The study demonstrated that onion peel linearly increased degradable acid detergent fibre in both diet types across all incubation periods. However, the highest degradable dry matter was observed with the OP7.5 treatment in the high-concentrate diet at 48 hours (quadratic effect, p = 0.038), whilst lower values were noted at different inclusion levels in the high-forage diet.
The volatile fatty acid profiles revealed complex interactions between onion peel inclusion and diet type. At 24 hours of incubation, all onion peel inclusion levels increased acetate and propionate concentrations in the high-forage diet, which is particularly beneficial as these compounds support approximately 70-80% of ruminant energy requirements (Sun et al., 2022). Conversely, the OP5 treatment resulted in the lowest total volatile fatty acids and acetate in the high-concentrate diet.
Discussion and mechanistic insights
The differential responses observed between high-concentrate and high-forage diets can be attributed to the distinct bioactive compounds present in onion peel and their interactions with rumen microbiota. Onion peel contains substantial concentrations of flavonoids (particularly quercetin-3-glucoside), polyphenols (quercetin, kaempferol, and anthocyanins), sulphur-containing compounds (thiosulfates and allicin), saponins, and essential oils (Celano et al., 2021).
The antimicrobial properties of these phytochemicals appear to selectively target methanogenic archaea whilst preserving beneficial fibrolytic bacteria. Allicin, a primary active compound in onion, demonstrates high capacity for inhibiting thiol enzyme reactivation, consequently suppressing methanogenic archaea activity and methane production (Busquet et al., 2005). Additionally, organosulfur compounds may inhibit HMG-CoA reductase, which catalyses isoprenoid unit synthesis in methanogenic archaea membranes, further contributing to emission reduction.
The quadratic responses observed for gas production and methane emissions suggest dose-dependent effects of onion peel supplementation. Lower inclusion levels may enhance microbial activity through beneficial modulation of rumen environment, whilst higher concentrations could exert inhibitory effects due to increased phytochemical concentrations. This phenomenon aligns with previous research demonstrating that excessive levels of bioactive compounds can negatively impact ruminal bacterial populations (Eom et al., 2020).
Fibre degradation enhancement with onion peel supplementation appears linked to stimulation of cellulolytic bacteria populations. Previous research has demonstrated that onion extract increases abundance of key cellulolytic species including Ruminococcus albus, Fibrobacter succinogenes, and Ruminococcus flavefaciens (Eom et al., 2020). Furthermore, onion essential oil has been shown to increase ruminal cellulase, α-amylase, and proteinase activity, alongside enhanced concentrations of Prevotella species that contribute to enzyme production (Yaxing et al., 2022).
Comparative context: red algae as methane inhibitors
Whilst onion peel demonstrates promising methane mitigation potential, it is essential to contextualise these findings within the broader landscape of emerging feed additives. The most extensively researched natural methane inhibitor is red seaweed, particularly Asparagopsis taxiformis and A. armata, which have demonstrated remarkable methane reduction capabilities in both laboratory and field conditions.
Recent research by Meo-Filho et al. (2024) at the University of California, Davis, provides compelling evidence for the effectiveness of A. taxiformis under real-world farming conditions. Their study with twenty-four grazing beef cattle (Wagyu and Angus crosses) demonstrated significant methane emission reductions during optimal and decreasing supplementation phases, with treated cattle producing 115 g/day of methane compared to 185 g/day in control groups. Importantly, this reduction was achieved without affecting animal growth performance or nutrient absorption, with no differences observed in weekly live weight gains or dry matter intake between treatment groups.
The mechanism of action in red algae centres on bromoform, a halogenated methane analogue that specifically inhibits coenzyme M methyltransferase activity, thereby blocking ruminal methanogenesis (Liu et al., 2024). This compound is naturally synthesised and stored in specialised gland cells as a defence mechanism. However, the high bromoform content that makes Asparagopsis so effective also raises safety considerations regarding potential residues in animal products and dose-dependent toxicity profiles that require further investigation.
A comprehensive review by Liu et al. (2024) from the Beijing Academy highlights both opportunities and challenges in seaweed supplementation. The authors emphasise the need to explore alternative species with lower bromoform content, including Bonnemaisonia hamifera, Dictyota bartayresii, Cystoseira trinodis, and Saccharina latissimi. These species may offer sustainable alternatives whilst reducing concerns about heavy metal accumulation (iodine, bromine) and bromoform residues in milk and meat products.
However, red algae supplementation faces significant commercial limitations despite proven efficacy. Although over 12,000 seaweed species exist globally, only 221 have commercial value, and even fewer are intensively cultivated. Current challenges include production scalability, declining efficacy over time due to bromoform volatility, supply chain development costs, and regulatory frameworks (Liu et al., 2024). These constraints highlight the importance of investigating complementary natural additives such as onion peel, which may offer more accessible and practical solutions for widespread implementation.
Comparative efficacy: onion peel versus red algae
When comparing onion peel to red algae supplementation, several key differences emerge that may influence practical implementation strategies. Whilst red algae achieve more dramatic methane reductions (38-62% in field conditions), onion peel offers more moderate but potentially more sustainable benefits. The inclusion levels for onion peel (5-10%) are higher than those required for red algae (typically less than 1%), but onion peel is significantly more abundant and cost-effective as an agricultural waste product.
Critically, onion peel appears to maintain its efficacy over time, unlike red algae where bromoform degradation leads to declining effectiveness after 8-12 weeks of supplementation. Additionally, onion peel demonstrates enhanced effects in high-forage diets, which aligns well with sustainable feeding practices and pasture-based systems where red algae may be more challenging to implement due to supply chain complexities.
Environmental and economic implications
The environmental benefits of onion peel utilisation extend beyond methane reduction to encompass food loss valorisation and circular economy principles. Converting agricultural waste streams into valuable feed additives addresses dual challenges of waste management and sustainable livestock production. The substantial reduction in methane emissions observed with optimal onion peel inclusion levels could contribute meaningfully to climate change mitigation efforts within the agricultural sector.
From an economic perspective, onion peel represents a cost-effective feed supplement that is widely available and typically underutilised. The enhanced nutrient degradability observed, particularly for fibre fractions, could improve feed efficiency and reduce overall feeding costs. The preferential effects on high-forage diets are particularly relevant given that forage-based feeding systems are generally more economical and sustainable than concentrate-intensive approaches.
Research implications and future directions
The in vitro findings presented by Olagunju et al. (2025) provide valuable preliminary insights into the potential of onion peel as a feed additive, but significant research gaps must be addressed before any practical implementation can be considered. The observed methane reduction and enhanced nutrient degradability in high-forage diets represent promising starting points for controlled feeding trials rather than definitive evidence for commercial application.
Priority research areas should include comprehensive in vivo studies with lactating dairy cows to evaluate the effects of onion peel supplementation on milk production, composition, and quality parameters. These studies must assess animal health indicators, feed intake patterns, and palatability acceptance over extended feeding periods to determine whether laboratory observations translate to real-world conditions. The apparent preference for high-forage diets suggests particular relevance for pasture-based systems, but this requires validation under diverse feeding management scenarios.
Safety assessments represent a critical knowledge gap, particularly regarding the long-term effects of sulphur-containing compounds and potential accumulation of phytochemicals in animal tissues or milk. Residue studies should examine whether bioactive compounds from onion peel appear in animal products, addressing both food safety and consumer acceptance considerations. Additionally, the optimal inclusion levels identified in vitro (5% for high-forage diets) require confirmation through dose-response studies in live animals.
Economic feasibility analyses must evaluate the cost-effectiveness of onion peel processing, storage, and distribution compared to conventional feed ingredients. The abundance of onion waste suggests favourable economics, but practical considerations including seasonal availability, transportation costs, and processing requirements need thorough investigation. Integration with existing supply chains and regulatory approval processes represent additional implementation challenges that require systematic evaluation.
The mechanistic understanding of how onion peel bioactive compounds interact with rumen microbiota deserves deeper investigation through molecular techniques including metagenomics and metabolomics. This research could inform optimization strategies and help predict responses across different animal populations and feeding systems, ultimately supporting more targeted and effective supplementation protocols.
Interim conclusions
The research demonstrates that onion peel supplementation represents a promising strategy for sustainable ruminant nutrition, offering simultaneous benefits of methane emission reduction and enhanced nutrient utilisation. The optimal inclusion level of 5% in high-forage diets provides a practical target for implementation whilst maximising environmental and nutritional benefits. This approach aligns with growing demands for sustainable livestock production systems that balance productivity with environmental responsibility.
Whilst red algae remain the most potent natural methane inhibitors currently available under controlled conditions, onion peel offers complementary advantages including abundant availability, cost-effectiveness, sustained efficacy, and enhanced compatibility with forage-based feeding systems. The development of a portfolio of effective natural additives, including both onion peel and red algae, provides flexibility for different production systems and management contexts.
As the industry continues seeking sustainable alternatives to conventional feeding strategies, phytogenic additives such as onion peel offer promising pathways for achieving reduced environmental footprint without compromising animal welfare or production efficiency. The study contributes valuable insights into the potential for agricultural waste valorisation in livestock nutrition whilst addressing critical environmental challenges.
Dario Dongo
Cover art copyright © 2025 Dario Dongo (AI-assisted creation)
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Dario Dongo, lawyer and journalist, PhD in international food law, founder of WIISE (FARE - GIFT - Food Times) and Égalité.
