Nutritional Strategies to Mitigate Enteric Methane Emissions from Dairy Cows: State of Knowledge and New Perspectives

Introduction

Ruminants play a crucial role in food security. They supply 51% of all protein from the livestock sector; of which 67 and 33% are from milk and meat, respectively (Gerber et al., 2013). For many populations, livestock is a primary source of nutrition, and not simply a source of calories. Ruminants have the digestive particularity of being able to digest fibrous material (i.e., forages, agro-industrial by-products and crop residues) that is not edible to humans, and convert it into high-quality products (i.e., meat, milk). Thus, ruminants are able to valorize resources that would otherwise be wasted.

In 2018, The International Panel on Climate Change (IPCC) presented its Special Report on Global Warming of 1.5ºC and concluded that “limiting warming to 1.5°C implies reaching net zero CO₂ emissions globally around 2050 with concurrent deep reductions in emissions, particularly those of CH₄”. The report concluded that “24 to 27% reduction in methane (CH₄) is needed by 2050”.

Ruminant production systems are an important source of anthropogenic CH₄. Reducing CH₄emissions from ruminants is a hot topic because this greenhouse gas (GHG) contributes substantially to global warming. Consequently, there is an urgent need to reduce the concentration of CH₄in the atmosphere in order to contribute to slowing down the global warming of the planet. All sectors, including the agricultural sector, are moving towards reducing their carbon footprints and words like “zero emission”, “carbon neutral”, or “low carbon economy” among others, are part of our daily reality.

The Agriculture GHG emissions accounted for 55 Mt, or 8.2% of total GHG emissions for Canada in 2020. Agriculture accounted for 30% of national CH₄emissions and 75% of national N₂O emissions (ECCC, 2022). In 2020, emissions from enteric fermentation accounted for 43% of total agricultural emissions, and the application of inorganic nitrogen fertilizers accounted for 21% of total agricultural emissions (ECCC, 2022). According to ECCC (2022), 90% of CH₄emissions are from enteric fermentation and the remaining 10% is from manure storage and management. Emissions from enteric fermentation originate almost entirely (96%) from cattle production. Beef cattle are the main contributor to these emissions (81%) followed by dairy cattle (15%), and other species (5%).

Methane has a global warming potential (GWP) 28 times higher than CO₂, when compared over a 100-yr period (Forster et al., 2007). However, CH₄has a much shorter (12 years) lifetime than CO₂ (hundred years) in the atmosphere (Forster et al., 2007). This difference makes CH₄an attractive target for short-term gains in global warming reduction.

“Cradle-to-farm gate” Life Cycle Assessments (LCA) of milk produced in confinement systems in Canada revealed that approximately 40 to 50% of the carbon footprint of milk is from enteric fermentation (Guyader et al., 2017; Little et al., 217; Holtshausen et al, 2021). Therefore, mitigation of enteric CH4 from dairy cows could play an important role in stabilizing and reducing GHG emissions from the dairy sector.

In addition to be a potent GHG, CH₄produced from enteric fermentation represents a loss of energy to ruminants (2 to 12% of gross energy intake; Johnson and Johnson, 1995). These losses vary from 4 to 7% in lactating dairy cows fed forage-based diets (Kebreab et al., 2008). Theoretically, a reduction in energy losses in the form of CH₄should result in an improvement in animal productivity because the recovered energy would be used for production purposes (e.g., milk, meat).

Based on these considerations and given that enteric CH₄is a not only a potent GHG but also of a loss of productive energy to the ruminant, mitigating enteric CH₄emissions from dairy cows has both long-term environmental and short-term economic benefits. Accordingly, several dietary and nutritional strategies have been suggested to mitigate enteric CH₄emissions from dairy cows.

Several comprehensive reviews on dietary options to mitigate enteric CH₄production have been developed over the years (e.g., Hristov et al., 2013; Beauchemin et al., 2020). The objective of this paper is not to review all existing enteric CH₄mitigation strategies, but rather focusing on the potential of specific mitigation options, including those recently investigated.

Enteric methane production

Methane arises primarily from enteric fermentation in the rumen and to a lesser extent in the hindgut. Murray et al. (1976) reported that approximately 87% of the CH₄exhaled from the mouth and nose of the animal originates from the forestomach via eructation and absorption into the blood. Approximately 13% of CH₄is produced in the hindgut, where 89% of that (11% of total CH₄produced) is absorbed into the bloodstream and eliminated via expiration (Ricci et al., 2014). Contrarily to popular beliefs, CH₄is not released through flatulence. In fact, 90-95% of the CH₄produced is emitted via respiration and eructation and a very small amount (1-5%) is released from the rectum.

The key element in ruminal methanogenesis is H2 (Figure 1). Rumen microbes ferment carbohydrates and to a lesser, proteins, to produce volatile fatty acids, mainly, acetate, propionate and butyrate. Production of acetate and butyrate liberates hydrogen, whereas propionate serves as a net hydrogen sink. Consequently, diets such as high-forage diets that increase acetate also increase CH₄production because of the increasing availability of H₂ to methanogens. In contrast, diets that increase propionate production, such as high-grain diets are often associated with a reduction in ruminal CH₄production, given that less H2 is available to methanogens for reducing CO₂ to CH₄.

Figure 1. Production of methane in the rumen

Forage utilization

Forages are an integral part of dairy cow rations as they can represent 50-90% of the diet. Forages are important for the cow’s digestive and animal health. They supply energy and protein for milk synthesis, thereby reducing costs of producing milk.

Cereal-forages (e.g., corn or barley silages) contain high starch concentrations, which favors the production of propionate over that of acetate and reduces CH₄production in the rumen. Furthermore, intake of cereal-forages is often greater than that of legume/grass forages, which reduces ruminal residence time and hence, restricts ruminal fermentation and promotes post-ruminal digestion. Hassanat et al. (2013) reported lower CH₄yield [g/kg dry matter intake (DMI); percentage of gross energy intake (GEI)] and CH₄emission intensity for cows fed cornsilage based diets versus alfalfa silage-based diets. However, replacing alfalfa silage with corn silage increased emissions of CH₄ from stored manure due to increased volatile solids (i.e., organic matter: OM) in manure as a consequence of an inhibition of ruminal degradation of NDF (Massé et al. (2016). In an LCA study, Little et al. (2017) demonstrated that despite the 10% decrease in enteric CH₄yield observed by Hassanat et al. (2013) when replacing alfalfa silage with corn silage, differences in CO₂ emission intensity between the two forage systems were minimal when soil carbon was taken into account. Thus, recommending the use of high-starch forages to mitigate enteric CH₄production must consider possible effects (i.e., increases) on GHG emissions elsewhere in the production system, and this can be addressed using the LCA approach.

Not all cereal-forages are equal in terms of their ability to reduce enteric CH₄production. For instance, Benchaar et al. (2014) showed that replacing barley silage (starch = 14 %; NDF = 52.3%) with corn silage (starch = 32%; NDF = 36.7%) in dairy cow diets decreased CH₄yield (g/kg DMI) and CH₄intensity (g/kg ECM) mainly because of increased DMI, ruminal propionate proportion, and milk production (Table 1). In the same study, urinary losses decreased as the proportion of corn silage increased in the diet at the expense of barley silage, suggesting a better N utilization and low potential emissions of ammonia and N₂O. With the increased availability of high yielding short season cultivars, corn silage may offer opportunity to reduce GHG emissions from dairy cows in Canada, but an LCA is necessary to determine the net emissions of GHG and the carbon footprint of milk.

Table 1. Barley silage (BS) versus corn silage (CS) in the diet of lactating dairy cows (Benchaar et al., 2014)

Choice of forage cultivars offers opportunities to improve forage quality and mitigate enteric CH4 emissions. For example, compared with conventional corn silage (CCS) cultivar, brown midrib (BMR) corn silage is characterized by lower lignin concentration and higher rumen potentially digestible NDF (Ebling and Kung, 2004; Gehman et al., 2008). Hassanat et al. (2017) reported (Table 2) that replacing CCS with BMR in dairy cow diets increased DMI and milk production and lowered CH₄yield (g/kg DMI; % GEI) and CH4 emission intensity (g/kg ECM). Urinary N decreased with the replacement of CCS with BMCS, suggesting an enhanced efficiency of N utilization by the animal. Thus, the use of low-lignin cultivar may represent an option to mitigate enteric CH₄emissions. However, field survivability of low-lignin forage cultivars and their ability to support higher milk production still need to be investigated.

Table 2. Conventional corn silage (CCS) versus brown midrib corn silage (BMCS) in the diet of lactating dairy cows (Hassanat et al., 2017)

At a similar physiological stage of maturity, legume forages contain less (NDF) and more non-structural carbohydrates than grasses. Thus, substituting grass forages with legume forages in dairy cows diets may represent an interesting means to mitigate enteric CH₄production. Indeed, feeding legumes versus grasses increases DMI, which lowers CH₄yield (g/kg DMI) because of faster passage rates from the rumen. In addition, very often feeding legumes versus grasses is associated with improved milk production, which is expected to lower enteric CH₄emission intensity (g/kg milk). However, caution must be taken as plant maturity at the time of harvest can confound the impact of forage species (e.g., legumes versus grasses) on CH₄production. Advancing maturity decreases the soluble carbohydrates content and increased lignification of plant cell walls, which promote the production of acetate in the rumen, thereby increasing the amount of CH₄ produced per unit of forage digested.

Other forage-based strategies such as using high-sugar grasses or tannin-containing forages (e.g., sainfoin, and birdsfoot trefoil) may have the potential to reduce CH₄emissions (Guyader et al., 2016).

Feed additives

Several feed additives have been suggested to mitigate enteric CH₄production. Among them, ionophores (e.g., monensin), plant bioactive compounds (e.g., condensed tannins, saponins, essential oils), yeast, direct-fed microbials, hydrogen sinks (e.g., nitrate), and inhibitors (e.g., 3- nitrooxypropanol: NOP).

3-nitrooxypropanol: NOP

In recent years, 3-NOP has attracted much attention as several studies have proven its effectiveness in reducing enteric CH₄emissions from dairy cows. The 3-NOP is a structural analog to methyl-coenzyme M, a cofactor involved in the terminal step of ruminal methanogenesis. The 3-NOP is supposed to bind to the active site of the methyl-Co A reductase, causing an inhibition of CH₄synthesis in the rumen. Hristov et al. (2015) observed a sustained (12 weeks) decrease (up to 30%) in emissions (g/d), yield (g/DMI), and intensity (g/kg ECM) of enteric CH₄when 3-NOP was fed at 40, 60, and 80 mg/kg DM to lactating dairy cows. In that study, there were no effects on DMI, milk yield or milk composition. It has been reported (Thiel et al., 2019a, b) that 3-NOP is metabolized in the rumen to very low concentrations of nitrate, nitrite and 1,3-propanediol and presents low risks for human health (i.e., not detected residues in milk). To the best of our knowledge, 3-NOP is not currently commercially available in Canada because it is not approved by Health Canada, which considers inhibitors as a drug and not a feed additive.

Industrial production processes and transportation of feed additives may also contribute to GHG emissions and therefore, these emissions should be taken into account to determine the efficacy of a given feed additive to improve the carbon footprint (CO₂e/kg of milk) of milk production.

Seaweed (Macroalgae)

Recently, seaweeds (i.e., macroalgae) have been investigated for their potential to manipulate rumen microbial fermentation in a manner that inhibits ruminal methanogenesis. Of particular interest are red and brown algae. The main secondary metabolite produced by these algae species is bromoform (CHBr3) and this compound has been shown to exhibit antimethanogenic properties (Machado et al., 2016). Bromoform interferes with methanogenesis by inhibiting the cobamide-dependent methyl transferase at the terminal step of the methanogenic pathway (Denman et al., 2007). Other seaweeds can contain polysaccharides, proteins, peptides, bacteriocins, lipids, phlorotannins, saponins, and alkaloids that have the potential to inhibit methanogenesis (Abbott et al., 2020). The concentration of CHBr3 varies considerably depending on algae species (Table 3).

Table 3. Bromoform levels in brown, red and green seaweeds (from Abbott et al., 2020 based on Carpenter and Liss, 2000)

Extensive research on the antimethanogenic effect of seaweed has been conducted in Australia with native red algae species such as Asparagopsis taxiformis and Asparagopsis armata. These seaweeds have been shown to decrease ruminal methanogenesis in vitro Kinley et al., 2016) and in vivo (Roque et al., 2019). Roque et al. (2019) reported that feeding Asparagopsis armata to dairy cows at 1% of the diet decreased enteric CH₄production by 67%. However, in that study, feed intake and milk production dramatically decreased (11 and 38%, respectively) upon feeding the seaweed. Bromoform can potentially be harmful, particularly at high-concentrations and long-term oral exposure of animals to high concentrations of bromoform can cause liver and intestinal tumors (ATSDR, 2016). High levels of bromoform could pose risks to human health and Heath Canada (2020) considers this compound as a possible human carcinogen. In the study of Roque et al. (2019), milk bromoform concentrations of cows fed Asparagopsis were in the range of 0.11-0.15 μg/L, which much lower that the level of 100 μg/L set by Health Canada (2020) for drinking water. Because of high levels of inorganic minerals in Asparagopsis, accumulation of iodine and bromide in milk (Stefenoni et al., 2021). Thus, it would probably be necessary to process the algae in order to eliminate/reduce the concentration of inorganic minerals to achieve iodine concentrations in milk not exceeding the upper limits recommended by Health Canada.

Based on this information, seaweeds may represent an effective means to mitigate enteric CH₄emissions from dairy cows. However, importing these algae into Canada requires farming, processing and shipping of the product, which will contribute to more GHG emissions and may, therefore, offset the gain achieved via the mitigation of enteric CH₄emissions. Thus, seeking alternative Canadian seaweeds is likely to be beneficial. Also, more work is needed to determine the balance between the extent of reducing enteric CH₄, the cost, and safety of these supplements for animals and humans. Regardless of the source, the provenance (i.e., transporting), and the mode of production (i.e., harvesting, growing, processing, storing) of the algae, it is important to carry out an LCA to determine the net impact on the GHG intensity of milk production.

Biochar

Recently, biochar has been suggested as a means to reduce enteric CH₄production. Biochar is a co-product obtained via pyrolysis by heating (350–600°C) plant biomass (agricultural or forestry) under oxygen-free or oxygen-limited conditions (Lehman and Joseph, 2015). The product obtained has a very porous structure, giving it a great capacity to absorb gases (e.g., CH₄) and liquids. Biochar may be beneficial for animal health due its detoxifying (Villalba et al., 2002), antidiarrheal (Watarai et al., 2008), and anthelmintic (Van et al., 2006) properties. Biochar has been reported to enhance biofilm formation and H₂ transfer among members within microbial communities (Chen et al., 2014). The effects in the rumen are highly dependent upon the biomass and condition of pyrolysis (e.g., temperature) used to produce the biochar, although in a recent in vitro study (Benchaar et al., unpublished), CH₄production was not affected by biochar produced from different sources of biomass used to produce the biochar. The lack of an effect of biochar on enteric CH₄production has also been reported in vivo (Terry et al., 2019). Most of the literature data available to date suggest that biochar is not a viable option to mitigate enteric CH₄emissions from dairy cows (Honan et al., 2021).

Fat supplementation

There is a general consensus among the scientific community that diet supplementation with unsaturated fat is a potentially effective strategy for mitigating enteric CH₄emissions (Beauchemin et al., 2020). A wide range of unsaturated fat supplements were evaluated for their potential to reduce enteric CH₄production. Their effects are variable depending on many factors.

These include the level of fat supplementation, the form of the fat supplement (e.g., oils vs. seeds); seed processing (e.g., (extruded > whole seeds); the fatty acid (FA) composition of the supplement: medium (e.g.,12:0; 14:0) and long-chain FA (e.g., 18:3) very effective and the composition of the basal diet: high-grain diets (i.e., high-starch) are more responsive than high-forage diets. Feeding high amounts of fat may impair diet digestibility and animal productivity. Beauchemin et al. (2020) suggested that a supplementation level of added fat lower than 4% of diet DM can reduce enteric CH₄emissions by up to 20% without impairing animal productivity, although the authors warned that the effect may vary. Benchaar et al. (2015) showed that adding 4% linseed oil (LO) in a red clover silage-based diet reduced enteric CH₄emissions and yield by 10%, with no negative effects on DMI, OM digestibility and milk production (Table 4). The same LO supplementation level in a corn silage-based diet markedly reduced (25%) enteric CH₄emissions, but impaired DMI, OM digestibility and milk production. As a consequence of changes in the quantity and the composition of manure upon LO supplementation, CH₄emissions from manure increased (Hassanat and Benchaar 2019).

Table 4. Including linseed oil (LO) at 4% of dietary DM in lactating dairy cows diets based on red clover or corn silage (Benchaar et al., 2015).

One of the limitations of the use of fat to mitigate enteric CH4 is the high cost. Alternative low-cost lipid sources are dry distillers’ grain with solubles (DDGS), a by-product of the ethanol industry. Adding DDGS at 30% of dietary DM decreased enteric CH₄yield (% GEI) by 14%, but adversely affected OM digestibility (Benchaar et al., 2013), which increased volatile solids in manure and increased (14%) fugitive CH4 emissions from stored manure (Massé et al., 2014). Thus, the gain achieved in enteric CH₄emission was completely offset by higher emissions from stored manure, suggesting that caution must be taken when using DDGS to mitigate enteric CH₄emissions from dairy cows. A limitation to using DDGS to reduce CH₄ emissions is that it may contribute to increase nitrogen excretion, which could potentially increase ammonia and N₂O emissions (Benchaar et al., 2013).

Considering the potential impact of fat supplementation on CH₄emission from manure, an LCA is necessary to determine the effectiveness of this mitigation option to improve the carbon footprint of milk.

Combination of mitigation strategies

It has been suggested that combining mitigation strategies may allow to achieve larger reductions (Beauchemin et al., 2020). However, this will be only achieved if the effects of the combined strategies are additive. The effectiveness of combining individual strategies may be further increased if the strategies have different mode of action (e.g., direct and indirect effects). Additive effects between fat and starch supplementation were observed (Benchaar et al., 2015; Hassanat and Benchaar, 2021). Additive effects of diet supplementation with fat on CH₄mitigation were reported when canola oil was combined with 3-NOP (Zhang et al., 2021) and linseed oil with nitrate (Guyader et al., 2015). There is a lack of information about the effect of combining more than two dietary strategies to mitigate enteric CH₄emissions from dairy cows. It is important to ensure that the combination of individual enteric CH₄ strategies does not have a negative effect on animal productivity and does not lead to increased GHG emissions (e.g., N₂O).

Perspectives/Conclusions

A number of dietary strategies have been suggested to mitigate enteric CH₄production from dairy cows. Fat supplementation, forages (i.e., legumes vs. grasses; cereal forages versus legumes/grasses; corn silage vs. barley silage; more digestible forages: BMR vs. regular corn), seaweeds (i.e., red); and 3-NOP have been shown to be effective mitigation option. However, focusing only on suppressing enteric CH₄production would not ensure that the carbon footprint of milk would be improved because DMI, nutrient digestibility and animal productivity may be impaired when high levels of fat, starch, or seaweed are included in the diet. Manure CH₄emissions and other potential emissions (e.g., N₂O, ammonia) may also increase if the implementation of these strategies increases volatile solids and/or change the chemical composition of manure.

Production of forages is greatly affected by several factors, including the climate conditions (e.g., rainfall, water availability, temperature), the agronomic conditions (e.g., soil quality, use of fertilizers), management practices (e.g. harvest, preservation), soil carbon sequestration. Such factors affect not only the yield/quality of forages, animal productivity, and manure nutrient excretion but also GHG emissions. The use of forages to mitigate enteric CH₄emissions needs to account for net GHG emissions, including soil carbon sequestration in the farming system.

The use of seaweeds, particularly the red ones (e.g., Asparagopsis) may represent an opportunity. However, these algae species are non-native to Canada and importing them from countries as far away as Australia, would not only increase the cost of their use, but would contribute to increased GHG emissions associated with processing, packing and transportation. The production at large scale of this type of seaweed in Canada requires their cultivation, growth, harvest, processing, storage, etc., which may increase sale prices. The identification of native seaweeds that contain required concentrations of bromoform to inhibit ruminal methanogenesis may represent a viable option if it is cost-effective. Because bromoform can be harmful (carcinogenic) at high concentrations, more work is warranted to provide more information on the extent of its transfer into milk and meat.

The feed additive 3-NOP is apparently very effective in mitigating enteric CH₄, but its use could be limited because of its high cost and its lack of improving animal productivity (i.e., milk production). Therefore, financial incentives (e.g., governmental) may be necessary to encourage its adoption by the dairy farmers. The environmental impact associated with manufacturing feed additives and their use along the supply chain for dairy cows must be also considered when evaluating the potential of a given feed additive to reduce enteric CH₄emissions. This can be addressed using LCA based on guidelines developed for this purpose by the FAO (2020).

Combination of individual strategies with relatively small potential mitigation may help to achieve larger decreases in enteric CH₄emissions. However, as for any diet intervention, a LCA is necessary to account all emissions from the dairy production system.

The adoption of any mitigation strategy by dairy producers would only be possible if it is accompanied by an increase in milk production. Low-CH₄diets are not usually low-cost and therefore financial incentives are needed to motivate producers to adopt mitigation. Consumers have a negative perception towards the use of feed antibiotics and chemical additives in dairy cow diets and therefore, alternatives to these substances (e.g., plant-extracts) are needed.

It is difficult to indicate the extent of decrease in enteric CH₄emissions for a given dietary mitigation because there is no “one-size-fits-all approach” to emissions reduction. Several mitigation approaches exist, being developed and others will be developed. This will allow the agriculture sector to play an important role in curbing GHG emissions, thereby helping Canada to achieve Canada to 30% reduction by 2030 and a pledge to become net-zero by 2050.

Presented at the 2022 Animal Nutrition Conference of Canada. For information on the next edition, click here.