Strategies to Mitigate Enteric Methane Emissions in Ruminants: A Review

Abstract

Methane is the main greenhouse gas (GHG) emitted by ruminants. Mitigation strategies are required to alleviate this negative environmental impact while maintaining productivity and ruminants’ health. To date, numerous methane mitigation strategies have been investigated, reported and suggested by scientists to the livestock industry. In this review, the authors will focus on the commonly practiced and available techniques expanding the knowledge of the reader on the advances of methane mitigation strategies with a focus on the recent literature. Furthermore, the authors will attempt to discuss the drawbacks of the strategies in terms of animal health and performance reduction as well as the concept of feed and energy loss, adding an economic perspective to methane emission mitigation which is in the farmers’ direct interest. As a whole, many factors are effective in reducing undesired methane production, but this is definitely a complex challenge. Conclusively, further research is required to offer effective and efficient methane production mitigation solutions in ruminants worldwide, thus positively contributing to climate change.

1. Introduction

The agricultural sector is a major emitter of greenhouse gasses, and animal husbandry is one of the largest sources therein, particularly concerning ruminant breeding carried out on feedlots with feed concentrates and/or on pastures where animals graze for forage. Ruminants obtain their nutrients from plant-based, difficult-to-digest lignocellulosic food by fermenting it in a specialized stomach (the multi-chambered “rumen”) before digestion. The process is termed “enteric fermentation”, a digestive process in which carbohydrates are broken down by microorganisms into small molecules. Ruminal anaerobic fermentation of nutrients is a common pathway to reduce carbon dioxide and hydrogen ions (H⁺) to methane [1]. Approximately 200 species of both domesticated and wild ruminants, including cattle (dairy and beef), goats and sheep, exist in the world. Methane (CH₄), together with nitrous oxide (N₂O) and carbon dioxide (CO₂), constitute the major greenhouse gases (GHG) produced by ruminants. Yet, there is growing concern over their negative effect on global climate change, see Figure 1.

Cattle breeding and production, due to their size and number (~1.5 billion cattle globally), is the main source of methane emission compared to other ruminants such as sheep. Global demand for livestock products is expected to double by 2050, mainly due to improvements in the worldwide standard of living [2]. According to Rojas-Downing et al., animal husbandry not only contributes to climate change but is also affected by it. Climate change threatens livestock production because of the “impact on quality of feed crop and forage, water availability, animal and milk production, livestock diseases, animal reproduction, and biodiversity” [2]. Prasad et al. [3] reviewed the ways in which climate change impacts livestock production, concluding that increasing temperatures, drought, flooding and variation in rainfall trends all negatively affect the livestock industry.

Apart from causing climate change, at least in part, methane emission from ruminants is also a direct cost factor for farmers due to the energy loss from feed. According to the FAO, up to 12% of the energy contained in feed is usually lost to CH₄ production [4]. Under anaerobic conditions, the microbial population in the animals’ digestive system produces volatile fatty acids (VFA) through the fermentation of nutrients. Major VFAs include acetate, propionate and butyrate, which can both act as sources of energy for the animal and as sources of CH₄ and CO₂ [5]. Fermentation is an oxidative process, during which reduced cofactors (NADH, NADPH and FADH) are re-oxidized (NAD⁺, NADP⁺ and FAD⁺) through dehydrogenation reactions releasing hydrogen that is then used by methanogenic archaea, a microbial group distinct from eubacteria, to reduce CO₂ into CH₄ [6]. The microbial fermentation of fodder in the rumen yields different end products and, therefore, is different in terms of hydrogen production efficiency. From the production of acetate and butyrate, pure hydrogen is released, while the formation of propionate creates a competitive pathway for H⁺ use in the rumen. Thus, metabolic pathways in hydrogen production and utilization are among the main factors that need further attention in the development of ruminant methane mitigation strategies, as well as addressing the methanogenic community composition.

The properties of the ration have a direct effect on the production level and rumen metabolism of the animal. Enteric methane emission is greatly affected by diet composition, of which fiber is the most important. There is a positive correlation between enteric methane emission and fiber content, while this correlation is negative with dietary lipid content [7]. Gaviria-Uribe et al. [8] reported that dry matter intake (DMI) has a high correlation with CH₄ emissions. Moreover, treatments with higher nutritional quality and higher DMI resulted in lower CH₄ emission and, thus, lower energy loss. In the following sections, the authors will discuss the main dietary manipulation strategies: forage-to-concentrate ratio and dietary additives with methane production reducing or inhibiting properties.

Over the past few years, review papers and meta-analyses have been published on how different mitigation strategies influence CH₄ production in ruminants. However, given the importance of the topic and the fast pace of growing knowledge in the area, in this article, the authors attempt to focus on bringing together and discussing the most recent findings as well as new methane mitigation strategies in ruminants.

2. Background and Significance

Anthropogenic greenhouse gas (GHG) emissions [9] have detrimental effects on the climate [10]. Natural methane emissions from wetland ecosystems constitute a relevant part of the global CH₄ amount. There are many factors which influence unwanted CH₄ production by ruminants: level of feed intake, type and quality of feed, energy consumption, animal size, growth rate, level of production and environmental temperature [11]. The methane emissions in dairy cows range from 151 to 497 g per day [11]. Lactating cows produce more CH₄ (354 g per day) [11]. In a study by Harper et al. [12], pasture-grazing cattle produced 0.23 kg CH₄ per animal per day, which corresponded to the conversion of 7.7 to 8.4% of gross energy into CH₄. The same cattle fed with a highly digestible, high-grain diet produced 0.07 kg CH₄ per animal per day, corresponding to a conversion of only 1.9 to 2.2% of the feed energy to CH₄ [12]; hence, four times more CH₄ was produced when low-grade feed was used.

Methane emissions from ruminants can be measured using either respiration chambers (RC) or sulphur hexafluoride tracer (SF₆) techniques [13]. Another method is laser spectroscopy, carried out by line-averaging sensors [14]. Wu et al. reported measurement techniques in cattle buildings [15]. To determine ruminants’ CH₄ emissions, in vitro or in vivo trials can be carried out. Rossi et al. described an in vitro volatile fatty acids (VFA) production-based method for the assessment of CH₄ emission using high energy, high protein and forage feedstuffs (fermented in vitro using rumen fluid from dairy cows as inoculum) [16].

The atmosphere contains approximately 1.9 ppm of methane, whereas the respiration air of cattle is roughly 1000 ppm [17]. The National Oceanic and Atmospheric Administration’s (NOAA) recent findings [18] surface that the annual increase in methane level in the atmosphere in 2021 was 17 parts per billion (ppb). This rate constitutes the largest annual increase measured since the beginning of systematic recordings. Atmospheric methane levels averaged 1.898 ppm last year, which is approximately 162% greater than pre-industrial levels. Figure 2 illustrates the increasing trend in atmospheric CH₄ levels.

As seen in Figure 2, over the past three centuries, the atmospheric methane burden has grown 2.5-fold, reaching levels unprecedented in at least 650,000 years [19]. Marielle Saunois et al., in the “Global Methane Budget 2000–2017” seminar [20], stated that “domestic ruminants such as cattle, buffalo, sheep, goats, and camels emit methane as a by-product of the anaerobic microbial activity in their digestive systems. The very stable temperatures (about 39 °C) and pH (6.5–6.8) values within the rumen of domestic ruminants, along with a constant plant matter flow from grazing (cattle graze many hours per day), allow methanogenic archaea residing within the rumen to produce methane. Methane is released from the rumen mainly through the mouth of multi-stomached ruminants (eructation, 87% of emissions) or absorbed into the blood system.

3. Strategies to Reduce CH₄ Emissions

There are several routes to reduce CH₄ emissions from livestock operations. Ricci et al. [21] carried out a systematic investigation of methane emissions from beef and dairy, where they attempted to quantify the effects of physiological stage and diet characteristics, lactating or nonlactating status, local conditions and available fodder. Nutrition plays a critical role [22] in CH₄ emissions. Difford et al. [23] found that the host genome and rumen microbiome play a critical role in CH₄ emissions and that these two factors were independent. The authors suggested addressing these two points separately. Zang et al. [24] summarized mitigation and adaptation strategies for developed versus developing countries (Figure 3).

Wallace et al. [25] stated that the rumen microbial metagenome of cattle is associated with high methane production. Microorganisms in the rumen of dairy cows are the cause of methane production in these animals [26]. There are many strategies devised by scholars to reduce enteric methane emissions. Although there is a common goal for breeders and those seeking to protect the climate, it can be difficult to convince animal breeders to deploy them [27]. Better feed utilization efficiency in ruminants not only accelerates animal production and lowers the farmers’ costs but also reduces the environmental impact [28]. Both the farmer and the environment would benefit substantially from so-far-elusive measures to reduce enteric methane production [19]. Hence, strategies on how to roll out the best methane reduction methods so that farmers can be reached and motivated are essential.

3.1. Genetic Selection

There is increasing evidence that feed conversion efficiency (FCE), as well as methane emission, are heritable traits of dairy cows [29], and for determination of these traits, the composition of the rumen microbiome shall be considered. Correspondingly, to exploit the genetic factor, those with lower CH₄ emissions shall be selected [30].

3.2. Ratio of Forage to Concentrate (F:C)

Of all the enteric methane emission mitigation strategies suggested and practiced, forage-to-concentrate ratio (F:C) management, introduced in many studies, may be the most common one. Concentrates (supplements) are often administered to cattle and other ruminants in grazing systems when the availability and/or quality of pasture is limiting animal performance [31]. Supplementary feeding covers the animals’ needs for protein, energy, roughage and minerals.

Table 1. Typical tropical animal supplements for critical seasons.

Animal Nutrient Needs	Supplement	Critical Season
Energy	Grains, molasses	Dry
Protein	Urea	Dry
Roughage	Silage, hay	Dry and wet
Minerals	Phosphorus	Wet

Data in the table is extracted from Suybeng et al. [31].

shows a typical example of supplements (concentrates) for the tropical region in Northern Australia.

High-forage diet-fed dairy cows produced 35% more methane in comparison to the period when they were fed a high-concentrate diet [32]. Among carbohydrates, cellulose fermentation produces the highest amount of methane [6]. High-concentrate diets produce less methane than high-forage diets [33]. Thakur et al. [34] concluded that increasing the concentrate ratio reduced methane emissions and improved the growth performance of crossbred goat kids. As previously discussed by Barbosa et al. [35], methane emission was reduced linearly by increasing the dietary concentrate ratio. Accordingly, in the study of Alqaisi et al. [36], high-concentrate diet-fed rams exhibited a significant reduction in methane production. According to the study of Li et al. [37], bacterial community structure and methane production in the rumen changed via changing F:C, and lowering the F:C ratio resulted in a decrease in methane production. Forage quality [38], increased levels of concentrates [38], feeding high-forage diets as pelleted form and increased propionic acid production resulted in lower methane formation. Molano and Clark [39] noted that methane emissions per unit of DMI were not associated with diet quality. Chagunda et al. [40] demonstrated that high-quality silage and a high proportion of concentrate in the ration resulted in low methane emissions. Silage is ruminant fodder that is prepared from foliage crops preserved by lactic acid bacteria fermentation.

According to Thompson and Rowntree [41], forage type and forage maturity stage affect methane emissions. Lima et al. [42] indicated that lower hemicellulose content leads to increased methane production. Nascimento et al. [43] observed that higher concentration improved weight gain and the meat quality of lambs. Huhtanen and Huuskonen [44] reported that increasing concentrate levels led to decreased crude protein (CP) digestibility and nitrogen excretion (N) flow from the rumen. There is a negative correlation between concentrate amounts and methane production [45]; therefore, increasing concentrate level in the ration leads to decreasing acetate and increasing propionate. Propionate acts as a hydrogen sink and reduces hydrogen availability to methane production. The main drawback of the strategy is lower milk quality as well as metabolic problems, thus limiting its application in the livestock industry. However, diets containing high amounts of concentrate, due to the high fermentation rate of fermentable carbohydrates, increase the production of volatile fatty acids in the rumen. A sudden increase and accumulation of volatile fatty acids leads to a decrease in ruminal pH and can reduce the ability to absorb volatile fatty acids by damaging ruminal tissue. Therefore, maintaining a balance between the ratio of forage to concentrate is essential to sustain the quantity and quality of production while maintaining animal health. Concentrate is more costly than grazing grass, so a balance needs to be found. Singh and Sharma [46] recorded higher body weight gain in concentrate-fed goat kids. Accordingly, animals were fed concentrate at the early growth stage, followed by green fodder after weaning. Fodder quality and type are decisive factors.

3.3. Addition of Dietary Oils/Lipids to Ration

Using fat as a source of energy in the ration would lead to significant changes in the efficiency of microbial flora and energy usage in the rumen, followed by a reduction in methane production [47,48]. McGinn et al. showed that the utilization of sunflower oil and possibly some yeast products could decrease gross energy (GE) loss as methane from cattle on a high forage diet but would impair fiber digestibility [48]. Vargas et al. [49] concluded that the addition of vegetable oils reduced methane production and increased the formation of propionic acid. The inclusion of lipids in the ruminants’ ration reduces enteric methane emission by inhibiting the methanogen’s growth and decreasing the ruminal ration fermentation capacity [50]. Consequently, this reduces microorganisms’ access to the diet and/or increases the ruminal passage rate. The inhibitory effects of lipids on methane emissions are not transitory but persist over time [51]. Judy et al. [52] showed that the addition of corn oil and calcium sulfate to diets reduced methane production and improved net energy balance in lactating dairy cows. In agreement, Embaby et al. [53] reported that supplementation of hemp and blueberry oils moderately reduced ruminal in vitro methane formation without compromising rumen fermentation and digestibility. Lima et al. [54] similarly reported that the inclusion of soybean oil significantly reduced methane production. Karlsson et al. [55] observed that supplementation of diets with glycerol could increase methane production. Winders et al. [56] reported that the inclusion of corn oil leads to a considerable methane production reduction. It can be concluded that oils and lipids might effectively be used, alone and or in combination with other strategies, for enteric methane mitigation. Five possibilities have been identified for reducing methane production by adding fat to the diet: decreased fiber digestion (especially long-chain fatty acids), reduced feed intake (total dietary fat exceeding 6–7%), decreased methanogens, decreased rumen protozoan populations and increased biohydrogenation process [57]. Pirondini et al. investigated the effect of fish oil supplements on methane emission and reported its potential mitigation effects [58].

3.4. Tannin-Rich Feedstuffs

Tannins (tannic acid derivatives) are a chemically diverse group of water-soluble phenolics which bind to proteins and form soluble or insoluble complexes found on the skin, leaves and roots of most plants [59]. Tannins are known to have direct and indirect effects on rumen microbes, decreasing ruminal protein degradability and methanogenesis as well as biohydrogenation of unsaturated fatty acids [60]. Nevertheless, these effects may partially be due to the antimicrobial properties of the tannins reducing fiber digestion, thus leading to incomplete digestion of the diet by the ruminal microbial population [61]. It is noteworthy that hydrolyzable tannins express toxic effects in ruminants rather than acting as a digestion inhibitor [62]. Molina-Botero et al. [63] observed that the total ruminal microbial population, methanogenic archaea and total protozoa were not affected by the increasing levels of condensed tannins in rations. Petlum et al. [64] found that the efficacy of condensed tannins as ruminal methane production inhibitors is greatly influenced by their molecular weights.

The potency of tannins has also been shown to be dose dependent. Lima et al. [54] reported that methane production was not affected by tannins at a concentration of 30 g/kg DM (dry matter). The findings of Adejoro et al. [65] show the in vitro potency of lipid-encapsulated Acacia extracted tannins in reducing enteric methane production. In the study of Deuri et al. [66], Bauhinia leaves supplementation considerably reduced the production of enteric methane. In another study, the inclusion of sainfoin (Onobrychis viciifolia Scop) and/or hazelnut (Corylus avellana L.) in ruminant rations reduced rumen fermentability, methane production and protein degradability [67].

The effects of condensed tannins on rumen fermentation depend on their sources and concentrations [68]. However, studies have proposed that the chemical structure and molecular weight of these metabolites have a vital role in their efficacy for the manipulation of rumen fermentation and methane mitigation [69]. Decreased ruminal protein breakdown is the most pronounced effect of tannins [70,71]. Hydrolyzable tannins and condensed tannins bind to proteins making those proteases resistant and nutritionally inaccessible to the animal. Tannins prevent the attachment of microorganisms to the cell wall of plants, and this binding is essential for degradation. In addition, the formation of complexes with proteins and carbohydrates prevents microorganisms from accessing nutrients [72]. Tannins are also chelating agents that reduce the availability of metal ions required for the metabolism of rumen microorganisms [73]. Additionally, tannins, due to their inhibitory enzyme activities, react with microbial enzymes and inhibit their activity [74].

3.5. Use of Microalgae and Macroalgae

Another inhibitor of CH₄ production as a substitute for fish oil is microalgae. Similar to fish oil, microalgae are rich in the fatty acids C20:5n−3 and C22:6,n−3. Eicosapentaenoic acid (EPA; C20:5 n−3) and docosahexaenoic acid (DHA; C22:6 n−3) are the primary marine-derived omega-3 fatty acids. In vitro experiments have shown that microalgae are capable of inhibiting methanogenesis, reducing acetate concentration and increasing propionate concentration. Their inhibitory effects can be attributed to their unsaturated fatty acid content, in particular C22:6,n−3 [75]. A recent experiment by Sucu [76] reported that under in vitro conditions, using Chlorella Vulgaris, C. variabilis and/or both not only reduced acetate and increased propionate concentrations but also mitigated CH₄ production. Anele et al. [77] reported a significant reduction in CH₄ production under in vitro incubation by Tetracystic sp., Scenedesmus sp. and Nannochloropsis granulatab compared to Chlorella Vulgaris and Micractinium reisseri. Similar to microalgae, Dictyota bartayresii and Asparagopsis taxiformis algae reduced in vitro methane production after 72 h compared with the control group [78]. Furthermore, Vijn et al. [79] successfully used seaweed for enteric methane mitigation.

These beneficial effects of algae could be due to the existence of various secondary metabolites. The presence of bromoform (CHBr₃) with known antibacterial properties, which is chemically similar to bromochloromethane (CH₂BrCl), in Asparagopsis has been shown to reduce CH₄ [80]. The inclusion of different levels of Asparagopsis taxiformis resulted in a linear reduction of CH₄ production and CH₄/DMI in sheep by up to 81.3% for CH₄ and up to 62.6% for CH₄/DMI compared with the control group [37]. Furthermore, findings on ruminal fermentation patterns showed an increased concentration of propionate and decreased concentration of acetate in the algae-fed group. This VFA profile change suggests that Asparagopsis can lead to the diversion of hydrogen to propionate, making hydrogen less available for methanogenesis [37]. Accordingly, the inclusion of Asparagopsis armata at 0.5 and 1% of organic matter basis led to a reduction in CH₄ production (g/day) by 26.4% and 67.2%, respectively. A decrease in CH₄ and milk yield was observed by 18.2% and 60.1%, respectively, compared with the control group in dairy cattle [81]. Similar to the results found in dairy cattle, Kinley et al. [82] reported that the inclusion of Asparagopsis taxiformis at 0.05, 0.1 and 0.2% (organic matter basis) in the beef cattle diet for 90 days reduced CH₄/DMI by 9, 38 and 98%, respectively. In contrast, algae inclusion in ruminant animal feeds did not affect methane production [83]. Similarly, Choi et al. [84] stated that Sargassum fulvellum (macroalgae) inclusion in ruminants’ diet did not affect methane production in 24 h of incubation. It is noteworthy that the use of many algae species in ruminant nutrition is limited due to the presence of bromoform which is a carcinogenic compound [85].

3.6. Pasture Quality

The quality of the fodder depends on many factors. As stated, enteric methane emissions are directly related to the quantity and type of feed intake, and several studies have reported the positive effect of adding legumes to feed. Dini et al. [86] studied the effect of different levels of pasture quality on CH₄ emissions. The CH₄ emissions were 14% lower, expressed as % of gross energy intake, and 11 % lower, expressed as g CH₄/kg dry matter intake, in the high-quality pasture grazing animals than the control group. Desmanthus, a tropical legume, reduced in vivo methane when used as a nutritional supplement [31]. The addition of herbs to fodder was investigated by Vázquez-Carrillo et al. [87]. The authors conducted research on the antimethanogenic effects of lemon grass (Cymbopogon citratus), chamomile (Matricaria chamomilla) and Mexican aster (Cosmos bipinnatus) on high-in-concentrate diet-fed beef cattle. Lemon grass significantly reduced methane yield (g of CH₄/kg of DMI) by 33%, Mexican aster reduced methane yield by 28% and chamomile had no significant effect. Tea-extracted saponins rich in pentacyclic triterpenes (from seeds, leaves or roots) with antiprotozoal attributes had little effect on the methanogen population in sheep [88]. The authors speculate that the “addition of tea saponins in animal diets may be an effective way to inhibit methanogenesis and hence has implications not only for global environmental protection but also for efficient animal production”. Dey et al. [28] looked into the reduction of enteric methane production from buffalo (Bubalus bubalis) by garlic oil (Allium sativum) supplementation in an in vitro rumen fermentation system.

3.7. Commercial Feed Additives

Antibiotics use in livestock is banned throughout Europe due to their detrimental effects. For instance, the supplementation of ionophore antibiotics and other chemicals has been prohibited by the European Union since 2006 due to the development of microbial resistance. Correspondingly, researchers, due to the presence of such traits in animal products, are looking for alternatives to increase animal production, concomitantly lowering environmental pollution [28]. Many natural and synthetic products (e.g., probiotics, polyphenols) to date have been successfully studied for their effect on CH₄ emission mitigation. For instance, per instruction of the manufacturer, a quarter teaspoon of Bovaer per day suppresses the enzyme that triggers methane production in the rumen and consistently reduces enteric methane emission by up to 90% in cows (30% in dairy cows and 90% in beef cows). In 2021, DSM received full regulatory approval to commercialize Bovaer^® from the Brazilian and Chilean authorities for its application in beef, dairy, sheep and goats. In addition, in 2022, DSM received EU market approval for Bovaer^® for dairy cows, following a positive EFSA opinion which confirms that the product reduces enteric methane emissions from dairy cows and is safe for the animal and the consumer. This is the first time the EU has marketed a product as a feed additive for its environmental benefits [89].

3.8. Other Approaches

Leng et al. [90] studied the effect of biochar and potassium nitrate administration on CH₄ emissions. Biochar at 0.6% of diet DM and potassium nitrate at 6% of diet DM reduced methane production by 22% and 29%, respectively. Effects were additive (41% reduction) for the combination of biochar and potassium nitrate. Rani et al. [91] state that methane-utilizing bacteria are found in methane-rich environments such as natural wetlands, rice fields, livestock rumen, peats and bogs. Finn et al. [92] proposed methanotrophs from natural ecosystems as biocontrol agents for ruminant methane emissions. Methanotrophic bacteria use CH₄ as the sole carbon and energy source in an aerobic process and can be used to obtain single-cell proteins (SCP) as feed and food ingredients and biopolymers [93]. Finn et al. [92] reasoned that since methanotrophs (methane-oxidizing bacteria) act as CH₄ sinks in nature, they can be used to reduce CH₄ levels in rumens. The authors stated that methanotrophs which are native to the rumen, have received little attention and that future work will be required to address the potential for methanotrophs to act as biocontrol agents for ruminant CH₄ emissions.

Fungi might also be deployed to mitigate CH₄ in natural and engineered systems. As per Jason et al. [94], fungi may indirectly influence carbon mineralization and methanogen/methanotroph communities and/or directly oxidize and dissolve gaseous CH_4. Anaerobic fungi were first detected in the rumen of sheep in the late 1970s and, not long after, were observed growing in close proximity to rumen methanogens. An initial meta-analysis showed that anaerobic fungi probiotics-fed cows could reduce enteric CH₄ production, yet the effects vary depending on the experimental conditions. Most of the early fungal probiotics were only used to improve milk production, not specifically to reduce CH₄, yet anaerobic fungi probiotics can specifically enhance acetogenesis and mitigate CH₄ emissions.

Another idea is to capture and/or destroy exhaled CH₄ from the animals, see Figure 4.

The invention in Figure 4 suggests collecting emitted methane for later utilization, whereas the invention in Figure 4 targets oxidizing methane, which is exhaled by ruminants. The inventors claim to reach >50% efficiency. Additionally, in 2004, a vaccine was proposed for the same purpose [96]. The Commonwealth Scientific and Industrial Research Organisation in Perth, Western Australia, purportedly developed a vaccine against archaean microbes that produce methane in sheep rumens.

4. The Economics of CH₄ Abatement from Ruminants

A recent report by DeFabrizio et al. [97] evaluated CH₄ abatement costs and reported that feed additives are seen as one of the costly measures, whereas “animal health monitoring” has an equal impact in terms of CH₄ potential and can even bring cost savings. As stated before, there are more than 1.5 billion cattle all over the globe, and any individual smallholder and cattle breeder of different herd sizes needs to be educated on the application of a set of measure (s) suitable for that specific industry. This task is critical and must be based on the consensus of which levers make the most sense to mitigate CH₄ emissions.

5. Other Ruminants

While the focus of his review has been on cattle, there are other ruminants of importance too, such as buffalo, sheep, goats and camels [20], which are all domesticated. For instance, there are almost 40 million camels on Earth [98]. Emission rates were estimated by ref. [99]: 5 to 8 kg for sheep, 1 to 1.5 kg for pigs, 58 kg for camels, 50 kg for water buffalos, 5 kg for goats, 18 kg for horses and 15 kg for caribous per year.

Let us take Peru as an example. The share of livestock in the countries’ GHG emissions is reported to be 14.5%, of which 64% stem from enteric fermentation and manure emissions from ruminants [100]. Peru is home to 27% of the world’s llama population, 0.75 million animals, and a study using tannins in the diet of these animals found that methane production could be reduced by 30% in llamas [100].

Figure 5 summarizes the daily CH₄ emissions by different mammals as a function of their body mass.

It had been suggested to use alternative animals for meat production with lower specific CH₄ emissions, such as ostriches. In ref. [102], the previously reported low methane emissions from these birds were revisited, and it was concluded that data from juvenile animals, which tend to be lower, cannot be extrapolated to an adult population [101]. Still, it can be worthwhile to consider animals with superior feed conversion efficiency as primary meat producers.

6. Conclusions

Serval approaches were found to be promising in reducing enteric CH₄ formation and emission; however, caution is required when assessing the findings. The effects may be transient, and further studies might be needed to confirm whether such reductions are maintained over longer feeding periods [103]. The main concern of animal nutritionists is to reduce diet energy loss while mitigating the greenhouse gasses emitted in the form of methane and CO₂ without adverse effects on the animals’ health, performance and productivity, along with the quality of meat and milk. In some of the above strategies, ruminal microflora are also affected, and, most likely, ruminal fermentation is reduced, creating a challenge in animal breeding. Therefore, considering the economic situation, the relations of greenhouse gas costs with animal yields, animal feed processing and type of farm management are important. Conclusively, all the strategies mentioned above can effectively reduce methane production, yet the topic is a serious challenge requiring further research and attention. Mitigating CH₄ from cattle has both long-term environmental and short-term economic benefits [48], and it should be fostered. Apart from reducing CH₄ emissions from ruminant livestock, alternative approaches could also be studied, such as non-traditional protein sources. Alternative proteins can be plant-based, such as soybean or pea, or single-cell proteins (SCP), e.g., created using fungi, algae or bacteria. Lab-grown meat and insect proteins are further approaches that could be investigated to provide proteins.