New Perspectives on reducing Methane Emissions from Beef and Dairy Cattle Production

Increasing atmospheric concentrations of greenhouse gases (GHG) is a major worldwide concern. It is estimated that, globally, direct emissions from animal agriculture account for 7-10% of human derived GHG emissions (O’Mara, 2011), with this estimate increasing to 14.5% if indirect sources, such as feed production and deforestation for pasture or crop expansion are accounted for (Gerber et al., 2013). Beef and dairy production contribute directly to GHG emissions due to methane (CH₄) generated during feed digestion (i.e., enteric CH₄), and CH₄ and nitrous oxide (N₂O) from manure, as well as indirectly through N₂O and CO₂ emissions from crop production and fuel use, as well as carbon loss/gain in soil due to changes in land use. However, the largest single GHG source from ruminant production is enteric CH₄. Methane is also a loss of potential energy from the animal with up to 12% of the total gross energy consumed by ruminants converted to CH₄ and released via the breath. There has been a lot of interest in recent years in developing mitigation options to reduce GHG emissions from livestock production, and several excellent comprehensive reviews commissioned by the Food and Agricultural Organization (FAO) have recently been published (animal: Hristov et al., 2013a,b; manure: Montes et al., 2013). The present review summarizes the curent knowledge in the area of enteric CH₄ reduction with a focus on the most promising dietary mitigation options available, or soon to be available, to livestock producers in Canada. 

Methane accounts for 40–75% of total GHG emissions from beef and milk production in Canada when estimated using a “cradle-to-farm gate” life cycle analysis (Beauchemin et al., 2010; Mc Geough et al., 2012). This type of life cycle approach accounts for all CH₄, N₂O, and CO₂ emissions arising from producing and transporting inputs used on the farm, livestock production on the farm, manure management, and growing and producing feed for the animals. Using a whole systems approach to evaluate a prospective mitigation practice ensures that a change in management aimed at reducing enteric CH₄ production does not lead to an increase in emissions from other parts of the production cycle.

Methane arises primarily from enteric fermentation in the rumen and to a lesser extent, from manure storage. Life cycle assessment of confinement meat (beef) and milk (dairy) production systems indicate that more than 75% of the total CH₄ is from enteric fermentation and less than 25% is from manure management (Beauchemin et al., 2010; Mc Geough et al., 2012). Emissions of CO₂ from fossil fuel use are typically 5–10% of total GHG emissions. The remaining emissions (20–55% beef; 15–47% dairy) are N₂O largely from denitrification of nitrogen in soils arising from manure and inorganic fertilizers (and urinary deposits on pasture), and to a lesser extent from sources of nitrogen resulting from leaching, runoff and volatilization.

Enteric CH₄ emission is proportional to the animal’s dry matter (DM) intake (DMI) (Figure 1), so expressing emissions on the basis of DMI, as g CH₄/kg DMI or as a percentage of gross energy intake (GEI), accounts for differences in intake amongst animals. Emissions typically range from 200 to 500 g/d for lactating dairy cows and from 75 to 300 g/d for beef cattle, corresponding to 15 to 35 g CH₄/kg DMI. The International Panel on Climate Change (IPCC, 2006) Tier 2 approach calculates CH₄ emissions (55.6 MJ/kg) as 6.5 ± 1% of GEI except if diets contain >90% grain, in which case 3 ± 1% of GEI is lost as CH₄. 

Figure 1. A meta-analysis of the relationship between enteric methane emissions and dry matter intake (DMI) for dairy cows and beef cattle (each point is a treatment mean from the literature; Escobar, Beauchemin and Oba, unpublished data).

The aim in reducing GHG emissions from livestock is to reduce the total CO₂ equivalents (CO₂e) per kilogram of milk or meat produced (referred to as GHG intensity or carbon footprint). Total CO₂e are calculated by summing the individual GHGs after accounting for their respective global warming potential (100 year time horizon): CH₄, kg × 25 + N2O, kg × 298 + CO₂, kg (IPCC, 2007). In a global analysis of dairy production using a “cradle-to-farm gate” approach, Gerber et al. (2013) reported that GHG intensity (kg CO₂e/kg fat and protein corrected milk [FPCM]) ranged from 1 to 2 for the industrialized regions of the world to a high of 7.5 for sub-Saharan Africa. The GHG intensity of milk production in Canada is at the very low end of this range at 0.8-1.2 kg CO₂e/kg FPCM (Mc Geough et al., 2012; Vergé et al., 2013). Furthermore, GHG intensity of milk production tends to be lower in the western provinces (0.93 kg CO₂e/L milk) than in the eastern provinces (1.12 kg of CO₂e/L milk) due to climate conditions and management (Vergé et al., 2013).

Estimates of GHG emissions from beef production also vary widely depending upon production system, ranging from 17 to 37 kg CO₂ eq/kg carcass. Globally, GHG intensities are lowest in Canada and the U.S. (17–23 kg CO₂e/kg carcass), where beef production scenarios include a cow–calf suckling/grazing period followed by a growing phase and then by a high-grain finishing period in confined feedlots (Johnson et al., 2003; Beauchemin et al., 2010; Pelletier et al., 2010). Beef production systems that are extensively grass based or subject to climatic stress and feed shortages have much higher GHG intensity because of the slower growth of cattle (i.e, more days on feed, lower feed conversion efficiency) and higher CH₄ emissions from fibrous, low quality feed.

As GHG intensity is a ratio between CO₂ emissions and product output, intensity can be lowered through increased productivity and/or direct reduction of emissions. There are many examples globally whereby increased production has decreased GHG intensity. Capper et al. (2009) reported that the U.S. dairy industry reduced it carbon footprint between 1944 and 2007 by 63% due mainly to increased milk production per cow. Likewise, the Canadian dairy industry reduced its carbon footprint by 29% between 1981 and 2006, mainly due to an increase in milk production per cow and a reduction in animal numbers. Globally, there is tremendous potential to reduce the GHG intensity of meat and milk production through improved animal production. Increased animal productivity can be achieved through improvements in animal genetics, nutrition, health and overall management of the farm. The IPCC (2007) estimates that about 70% of the global GHG mitigation potential from agriculture lies in developing countries, where livestock production is mainly from smallholder farms with low-producing animals, and hence tremendous potential for increased productivity. While animal productivity is expected to continue to increase in North America, further decreases in GHG intensity on an industry-wide basis may be marginal as the limits of biological efficiency are reached. Increasing the productivity of lower producing animals has a relatively large impact on reducing GHG emission intensity, whereas a further increase in the productivity of highproducing animals has a relatively small impact. For example, increased milk production in Canada has occurred at the expense of cow longevity, adding to the need for additional replacement heifers that elevate GHG intensity of milk production because heifers consume feed, produce CH₄, but do not contribute to milk production. We estimate in Canada that farms in the 10th percentile for lowest replacement rate (22%) have potentially a 6% lower GHG intensity of milk production compared with farms with average replacement rate (50th percentile, 37%) (Mc Geough, Little and Beauchemin, unpublished). Improving farm efficiencies, such as reproductive and feed efficiencies, are extremely important in lowering the carbon footprint of milk, especially in developed production systems. Because CH₄ emissions are proportional to DMI, genetic selection of cattle for feed efficiency (e.g., residual feed intake) lowers emissions because feed requirements are reduced (Basarab et al., 2013). In highly productive livestock systems such as those in Canada, there is also opportunity to apply nutritional mitigation options that directly decrease methanogenesis in the rumen. 

Enteric CH₄ is a natural byproduct of microbial fermentation of feed in the rumen. Methane-producing Archaea (methanogens) prevent hydrogen from accumulating in the rumen by using it to reduce CO₂ to CH₄. The rumen microbial populations ferment feed to produce volatile fatty acids (VFA), including acetate, propionate and butyrate. Production of acetate and butyrate liberates metabolic hydrogen whereas propionate serves as a net hydrogen sink. Consequently, diets that increase propionate and decrease acetate in the rumen are often associated with a reduction in CH₄ production. Furthermore, methanogens have a distinct association with protozoa, so defaunation of protozoa can also lower CH₄ production. However, inhibiting methanogens without providing alternative electron acceptors can decrease feed digestibility and animal productivity. Development of strategies that reduce CH₄ production without negative consequences on feed digestion and animal productivity is a significant challenge. Some dietary mitigation strategies that may reduce enteric CH₄ production are listed in Table 1. 

Table 1. Nutritional methane mitigation (CH₄) practices and their expected effect (adapted from Gerber et al., 2013)

Feeding grain-based diets gives rise to less enteric CH₄ (g/d and g/kg DMI) as compared to feeding forage-based diets because starch fermentation promotes propionate production in the rumen, which is a net hydrogen sink (Johnson and Johnson, 1995). Intensifying ruminant production by feeding more grain reduces enteric CH₄ production, but whether net farm GHG emissions increase or decrease as a result of this practice depends on the particular farm. Furthermore, grain feeding ignores the importance of ruminants in converting fibrous feeds, unsuitable for human consumption, to high-quality protein sources (i.e., milk and meat). 

Considerable variation in CH₄ production occurs amongst forages. In most cases CH₄ emissions (g/kg DMI) are lower from ruminants fed legumes than from those consuming grasses (McCaughey et al., 1999; Waghorn et al., 2002), because of the faster rate of passage from the rumen of legumes. However, plant maturity at the time of harvest can confound the impact of forage type on CH₄ emissions. Advancing maturity causes reductions in soluble carbohydrate 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 (Pinares-Patiño et al., 2003). Because reductions in forage quality are frequently accompanied by a reduction in ruminal digestibility, in many cases the amount of CH₄ produced (g/kg DMI or as a percentage of GEI) is not influenced by forage quality (Pinares-Patiño et al., 2003; Molano et al., 2008). Nonetheless, improvements in forage quality usually lower emissions per kilogram of animal product as a result of enhanced animal productivity.

Cereal forages contain higher starch concentration than grass forages, which favours production of propionate over acetate and reduces CH₄ production in the rumen. Corn silage and barley silage are the main cereal forages in eastern and western Canada, respectively, with starch concentrations typically higher in corn silage than barley silage. A study by Benchaar et al. (2014) showed that increasing corn silage proportion in the diet of dairy cows at the expense of barley silage increased dietary starch content (16.6 to 25.6% DM) and lowered NDF content (37.8 to 32.4% DM). Total tract digestibility of DM increased, as did DMI and fat corrected milk yield (Table 2). Although total CH₄ production (g/d) increased due to higher DMI, CH₄ g/kg DMI decreased, likely as a result of increased ruminal propionate proportion. Urinary N losses also decreased as the amount of corn silage included in the diet was increased at the expense of barley silage, reflecting enhanced efficiency of N use by the animal suggesting a low potential for N2O emissions. Thus, the increasing use of corn silage in beef and dairy production across Canada due to increasing availability of high yielding short season hybrids is expected to decrease GHG emissions from livestock. 

Table 2. Effects of increasing corn silage (CS) proportion at the expense of barley silage (BS) in dairy cow diets (Benchaar et al., 2014).

Supplementation of diets with lipids is one of the most effective ways of lowering enteric CH₄ emissions by ruminants, as reviewed by Grainger and Beauchemin (2011). The challenge is to lower CH₄ emissions without impairing animal production, given that adding unprotected lipids to the diet can have negative effects on fiber digestion in the rumen. To prevent negative effects, the amount of lipid added to the diet must be limited to 2–4% of the dietary DM, such that total lipid content does not exceed 5-6% of dietary DM. The mechanism by which feeding lipids reduces CH₄ production is multi-faceted. Replacing dietary carbohydrates with lipid decreases the amount of organic matter fermented in the rumen, lipids decrease protozoa numbers and associated methanogens, some fatty acids are toxic to rumen methanogens, and the biohydrogenation of fatty acids functions as a hydrogen sink.

When examined across studies with dairy cows, beef cattle and sheep, using different diets and experimental conditions, the reduction in CH₄ (g/kg DMI) ranges from 2.5-6% per 1% dietary lipid, as summarized by Giger-Reverdin et al. (2003), Beauchemin et al. (2008), and Grainger and Beauchemin (2011). Furthermore, the reduction in CH₄ is usually sustained with continued feeding of lipid. Thus, feeding lipids can be an effective CH₄ mitigation practice but responses are variable depending upon the level of supplementation, fatty acid profile of the lipid source, form in which the lipid is administered (i.e., refined oil, full-fat oilseeds), and the type of diet (high forage diets are less responsive). Fat sources can be costly so improved animal performance is necessary to offset higher costs associated with feeding lipids for CH₄ mitigation. 

A number of plant bioactive compounds, specifically tannins, saponins, and essential oils, have been explored for their ability to suppress CH₄ emissions. Condensed tannins (CT) are secondary phenolic compounds in plants that vary widely in chemical structure, but have high affinity for protein. Tannins bind with proteins forming tannin-protein complexes, which reduce degradation of plant protein in the rumen, thereby enhancing the flow of feed protein to the intestines. For diets containing excess nitrogen, the protein-binding effect of CT can be beneficial for the environment as reduced nitrogen digestion reduces the excretion of nitrogen in urine, the form that is highly volatile.

However, reductions in CH₄ emissions with CT appear to depend on the tannin source and ruminant species. Studies with sheep (Carulla et al., 2005; Hess et al., 2006) have shown decreased CH₄ (g/kg DMI) (up to 20%) with supplemental CT, but our research in beef cattle with CT from quebracho trees (Beauchemin et al., 2007) and sainfoin (Chung et al., 2013) has found no reduction in CH₄ with up to 2.7% dietary CT. It appears that the level of CT needed to reduce CH₄ may be considerably higher than the level needed to decrease protein digestibility, and thus care must be taken to ensure that CH₄ suppression using CT does not occur because of a reduction in feed digestion or feed intake. It is important that CH₄ is not reduced at the expense of animal productivity, such as in the study conducted by Grainger et al. (2009) with dairy cows on pasture supplemented with grain. In that study, CH₄ emission was reduced by up to 30%, but milk production of the cows was also reduced by about 10%.

Saponins are of interest for CH₄ mitigation because they inhibit rumen ciliate protozoa. The two major commercial sources of saponins are the desert plants Yucca schidigera (steroidal saponins) from Mexico and Quillaja saponaria (triterpenoid saponins) from Chile. Many in vitro studies report a decrease in CH₄ production when saponins are added, but the in vitro effects are rarely seen in vivo. Holtshausen et al. (2009) reported that feeding powdered Y. schidigera (containing 6% saponin) or Q. saponaria (3% saponin) to lactating dairy cows at 10 g/kg diet DM did not affect CH₄ production, even though the same products reduced CH₄ production in vitro. The differences between results in vitro and in vivo may indicate that the rumen microflora adapt to saponins. Clearly, additional animal feeding studies are necessary to establish the efficacy of saponins as a CH₄ mitigation option.

Essential oils are the volatile aromatic compounds in plants that are used for their medicinal and antimicrobial properties. While many essential oils (e.g., garlic, cinnamon, oregano) have been shown in vitro to decrease CH₄ production (as reviewed by Benchaar and Greathead, 2011), very few animal feeding studies have been conducted to substantiate these results. The favorable responses attained to date in vitro may not be as marked in vivo as a result of microbial adaptation. Furthermore, many of the concentrations of essential oils that elicit favorable responses in vitro are too high for in vivo applications, due to issues with palatability and possible toxicity. We found that feeding a commercial blend of essential oils to beef cattle had no effects on CH₄ emissions (Beauchemin et al., 2006). However, recent studies with cashew nut shell liquid, the liquid obtained by pressing the shells of cashew nuts, show promise both in vitro and in vivo. The oil contains the alkenyl phenol anacardic acid, which has antibacterial activities against gram-positive bacteria. Cashew nut oil has been shown to inhibit CH₄ production in vitro (Watanabe et al., 2010), but more importantly, in two small studies with three dairy cows, cashew nut decreased CH₄ (g/kg DMI) by 19 to 38%. Methane reduction was accompanied by a decrease in the acetate to propionate ratio in rumen fluid (Shinkai et al., 2012). Larger scale and longer term studies are needed to substantiate these results. 

Commercial yeast products sold in Canada are from Saccharomyces cerevisiae and are generally categorized as active dry yeast which contains viable cells (>10 billion colony forming units per gram [cfu/g]) and yeast cultures that contain yeast plus culture. Many reviews have examined the beneficial effects of feeding yeast on milk production of dairy cows and weight gain of beef cattle (e.g., Beauchemin, 2012). Not unexpectedly, the results vary among studies, even when the same product is tested. However, when examined as a whole, there is an increasing body of literature that demonstrates their beneficial effects on animal productivity. However, their effects on methanogenesis are likely very small. We evaluated two commercially available strains of active dry yeast for beef cattle and reported no effects on CH₄ production (McGinn et al., 2004). Subsequently, we fed a commercial strain of yeast and a new strain of yeast (selected for its ability to increase in vitro rate of fibre digestibility) to cattle and reported that enteric CH₄ (g/kg DMI) was reduced by 10% using the new strain (Chung et al., 2011). The mechanism whereby yeast can decrease CH₄ production is thought to be related to the increase in bacterial numbers that typically occurs as a result of added yeast. The partitioning of degraded carbohydrate between microbial cells and fermentation products may alter the production of hydrogen, thereby decreasing CH₄ yield. While it may be possible to select yeasts that lower CH₄ emissions, the commercial products currently available have not been selected for this purpose.

A wide variety of bacterial DFM are also registered for feeding to cattle in Canada. Those containing Propionibacterium strains are of interest because they may offer a means of increasing ruminal propionate production at the expense of CH₄ production. As propionate is the major precursor for gluconeogenesis in ruminants, an increase in propionate production would be expected to increase feed conversion efficiency or animal production. We conducted a study (Vyas et al., 2014) in which beef heifers were assigned to a control treatment, or one of three Propionibacterium strains (P. acidipropionici P169; P. acidipropionici P5, and P. jensenii P54) dosed into the rumen once daily at 5 × 109 cfu. The heifers were fed a diet containing 70% forage. There were no treatment differences for total VFA, molar proportion of individual VFA, or CH₄ production (g/d), although g CH₄/kg DMI was reduced by 12, 8, and 13% with P169, P5, and P54 as compared to Control, respectively. The reduction in CH₄ intensity (g CH₄/kg of DMI) was a function of improved DMI with Propionibacterium strains, rather than a reduction in CH₄. The relative abundance of Propionibacterium P169 in rumen fluid was greater 3 h post-dosing; however, the levels returned back to pre-treatment levels by 9 h post-treatment, suggesting lack of persistence of the supplemented bacterial strains. We concluded that these strains have a low capacity to survive and integrate into the ruminal microbial community. For bacterial additives to play a role in CH₄ reduction, the challenge is to improve their survival and function within the rumen microbial community. 

Rumen fermentation converts carbohydrates into VFA and produces metabolic hydrogen. The main hydrogen sink in the rumen is methanogenesis; the methanogens reduce CO₂ to CH₄ which utilizes hydrogen (Janssen, 2012). However, there are also other pathways in the rumen that ensure that hydrogen does not accumulate; sulfates and nitrates are reduced to sulfides and ammonia, and unsaturated fatty acids are saturated. Synthesis of microbial cells also provides a sink for hydrogen.

When nitrate is present in the rumen, nitrite and ammonia formation are favoured over CH₄ production (Ungerfeld and Kohn 2006). Thus, feeding nitrate can be a means of reducing CH₄ production. Nitrate reduction has the added benefit of contributing to the rumen ammonia pool, which is often limiting in ruminants fed low quality diets. There is now overwhelming data to support the concept that feeding nitrate reduces CH₄ emissions (Nolan et al., 2010; van Zijderveld et al., 2010, 2011; Hulshof et al., 2012; Li et al., 2012; El-Zaiat et al., 2014). In a dairy study, cows (33.2 kg/d of milk; 104 days in milk at the start) were fed a total mixed ration containing either urea or nitrate (21 g of nitrate/kg DM) during 4 successive 24-d periods (van Zijderveld et al., 2011). At the start of the study, the cows were adapted to the treatments over 4 weeks. Nitrate decreased CH₄ production by 16% (g/d, g/kg DMI, % GEI), and the reduction persisted for the entire experiment. There were no negative effects on diet digestibility, but the energetic benefit from the decreased CH₄ production did not appear to benefit the animal, as milk production and energy balance were not affected.

The major concern when feeding nitrate is the potential for toxicity if the nitrite level in the rumen exceeds the capacity of the microbes to convert it to ammonia. In that case, nitrate and nitrite are absorbed into the bloodstream and the nitrite causes toxicity by combining with hemoglobin to form methemoglobin. Thus, ferrous iron (Fe2+) in hemoglobin is transformed into ferric iron (Fe3+), rendering the hemoglobin molecule incapable of transporting oxygen to the tissues. If high levels of methemoglobin are formed (30– 40%), the animal begins to suffer from oxygen starvation, and must be treated immediately with methylene blue. The key is to adapt the animals to nitrate slowly so the population of nitrite-reducing bacteria increases in size and capacity to reduce nitrite. Another critical point is that the animals should not be restrictively fed. Feeding ad libitum decreases the rate of feed intake and helps ensure nitrate is converted to ammonia, without a build-up of nitrite in the rumen (Lee and Beauchemin, unpublished data). We are currently conducting studies to examine the potential benefits of using an encapsulated form of nitrate as a means of reducing its rate of availability in the rumen. With further evaluation and development of protocols to reduce toxicity risk, nitrate feeding could become a viable strategy to lower CH₄ emissions. However, use of nitrate products for CH₄ reduction would first need to be approved by Health Canada. 

Some inhibitory compounds such as bromochloromethane, 2-bromo-ethane sulfonate, chloroform and cyclodextrin, have been shown to reduce CH₄ emissions, but their safety as feed additives is a major limitation to their commercial use (McAllister and Newbold, 2008). Recently, the compound 3- nitrooxypropanol (NOP) from DSM Nutritional Products (Kaiseraugst, Switzerland), was shown to reduce CH₄ emissions (Duval and Kindermann, 2012), but unlike other inhibitors, preliminary studies indicate that NOP has a high degree of safety although further study is needed. NOP is a methyl-coenzyme M analog, and thus is thought to inhibit the last step of methanogenesis.

3-Nitrooxypropanol has been shown to reduce CH₄ production (g/kg DMI) when supplemented to sheep (Martinez-Fernandez et al., 2013), lactating dairy cows (Haisan et al., 2013, 2014; Reynolds et al., 2014), and growing beef cattle (Romero Perez et al., 2014) with no signs of animal illness or intoxication (Table 3). Although NOP reduced CH₄ production in all studies, the magnitude of the response was variable (from 4 to 60% reduction), depending upon animal type, diet, and in particular, method of dosing the product. Either no effect, or very minor effects, on DMI were reported. As those studies were short term (< 1 month feeding periods), we recently examined the effect of feeding NOP over a 4 month period in beef cattle with NOP included in the total mixed ration (Romero Perez et al., unpublished data). Methane production was consistently reduced by 60% with no signs of adaptation. We are currently conducting a larger scale feedlot trial to evaluate whether CH₄ is lowered both during the backgrounding and finishing period, and to establish effects on animal performance and safety. The results to date for NOP have been very promising, but obviously, further research is required to fully establish its efficacy and safety before it can be registered for animal feeding in Canada. 

Table 3. Summary of the effects of the methane inhibitor 3-nitrooxypropanol (NOP; DSM Nutritional Products, Kaiseraugst, Switzerland)

Interventions to reduce CH₄ emissions from beef and dairy production currently available in Canada are limited to mainly those practices that improve production efficiency at the animal and herd levels. Improved diet formulation, herd health, and animal management can increase product output, which reduces enteric emissions relative to the quantity of meat/milk produced by the farm. These types of improvement in management can also increase farm profitability. It is also possible to apply mitigation strategies that directly reduce methanogenesis, such as the use of high starch forages and lipid supplementation of diets. In the future, it may be possible to use feed additives to reduce methanogenesis, as a number of very promising compounds have been identified. However, additional animal studies are needed to confirm these products are effective and safe. Although substantial research is required (especially long-term studies) to bring these feed additives forward, the potential for reducing the carbon footprint of meat and milk in Canada, and globally, is a tremendous incentive. 

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