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The purpose of the review was to examine enteric methane emissions, quantification methods, and mitigation strategies in grazing beef systems.
Sources
Peer-reviewed literature and conference abstracts were the main sources of information for this review.
Synthesis
Methane emissions (CH4) can be reduced by improving forage quality by including more cool-season forages and legumes and rotationally grazing animals. Including forages with beneficial secondary compounds such as condensed tannins and saponins also has CH4-mitigation potential. Providing nutritional supplements that improve the nutritional status of the animal and the efficiency of feed energy use has the potential to reduce CH4 emissions from grazing cattle. Genetic selection has shown some viability in reducing herd emissions, but heritability estimates are low for CH4 yield. More research is needed to understand the potential. Soil methanotrophy may partially offset CH4 emissions when animals are stocked moderately but soil CH4 uptake rates are relatively low in most grazing ecosystems. A new metric to quantify the global warming potential of CH4, GWP*, may allow future models to more appropriately consider the behavior and effects of CH4 in the atmosphere.
Conclusions and Applications
Methane mitigation strategies in grazing environments are limited, but producer decisions that improve the nutritional status of animals, the quality of the forage base, and supplementation of known CH4-mitigation compounds can reduce CH4 production. Now that less expensive, easier to use quantification tools exist, researchers need to conduct more long-term monitoring experiments and focus on reducing CH4 production of grazing animals where potential for reduction is largest.
The environmental impact of the beef industry has received increased public attention due to its perceived effects on climate change. A recent International Panel on Climate Change (IPCC;
IPCC. 2019. Climate Change and Land 2019. An IPCC Special Report on Climate Change, Desertification, Land Degradation, Sustainable Land Management, Food Security, and Greenhouse Gas Fluxes in Terrestrial Ecosystems. Accepted Draft. Intergov. Panel Climate Change, Geneva, Switzerland.
) report estimated that 23% of global anthropogenic greenhouse gas (GHG) emissions were from agriculture, forestry, and other land uses. These sources contribute an estimated 44% of all methane (CH4) emissions (4.5 ± 1.4 Gt of CO2 equivalents∙y−1), with enteric CH4 from ruminants responsible for 46%, of the 4.5 ± 1.4 Gt of CO2 equivalents∙y−1 or 2.1 Gt of CO2 equivalents∙y−1 (
IPCC. 2019. Climate Change and Land 2019. An IPCC Special Report on Climate Change, Desertification, Land Degradation, Sustainable Land Management, Food Security, and Greenhouse Gas Fluxes in Terrestrial Ecosystems. Accepted Draft. Intergov. Panel Climate Change, Geneva, Switzerland.
IPCC. 2013. Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. T. F. Stocker, D. Qin, G.-K. Plattner, M. Tignor, S. K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex, and P. M. Midgley, ed. Cambridge Univ. Press, Cambridge, United Kingdom.
). Anthropogenic GHG have been increasing since the start of the industrial revolution and will continue to grow with increased fossil fuel (e.g., coal, gas, and oil) combustion (
Invited review: Contemporary environmental issues: A review of the dairy industry’s role in climate change and air quality and the potential of mitigation through improved production efficiency..
In the United States, the agriculture sector contributes about 9% of total GHG emissions, whereas the transportation, industry, and electricity sectors produce the majority of emissions (about 79% of total;
). The Paris Climate Accord funded a special IPCC climate report in 2018 that recommended that developed countries should reach zero-emission targets in an “as soon as possible” window and reduce global emissions by 45% by 2030. Thus, although the United States agriculture emission footprint is considerably less compared with other US sectors and the world, the zero-emission target implies mitigation from all sectors barring marked improvement in carbon (C) sequestration technologies (
Rogeli, J., D. Shindell, K. Jiang, S. Fifita, P. Forster, V. Ginzburg, C. Handa, H. Kheshgi, S. Kobayashi, E. Kriegler, L. Mundaca, R. Séférian, and M. V. Vilariño. 2018. Mitigation pathways compatible with 1.5°C in the context of sustainable development. In Global Warming of 1.5°C. An IPCC Special Report on the Impacts of Global Warming of 1.5°C Above Pre-Industrial Levels and Related Global Greenhouse Gas Emission Pathways, in the Context of Strengthening the Global Response to the Threat of Climate Change, Sustainable Development, and Efforts to Eradicate Poverty. V. Masson-Delmotte, P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P. R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J. B. R. Matthews, Y. Chen, X. Zhou, M. I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield, ed. World Meteorol. Org., Geneva, Switzerland.
). Of the 9.1% attributed to US agriculture production, about 60% is due to animal agriculture, and about 60% of that is attributed to biogenic enteric CH4 emissions from all domesticated ruminants (3.2% of total US emissions;
IPCC. 2014. Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. O. Edenhofer, R. Pichs-Madruga, Y. Sokona, E. Farahani, S. Kadner, K. Seyboth, A. Adler, I. Baum, S. Brunner, P. Eickemeier, B. Kriemann, J. Savolainen, S. Schlömer, C. von Stechow, T. Zwickel and J. C. Minx, ed. Cambridge Univ. Press, Cambridge, United Kingdom.
). Agriculture contributes about 37.8% of domestic CH4 emissions from the combined sources of enteric, manure management rice cultivation and field residue burning, with enteric CH4 emissions being the largest agricultural source at 27% of US CH4 emissions (
). Because of methane’s contribution to the overall agriculture emission footprint, addressing mitigation opportunities is essential to reach reduced emission targets.
Recent life-cycle assessments have highlighted the need to focus on the grazing sectors of the US beef industry, with the cow-calf and stocker cattle components contributing approximately 70 to 80% of total GHG from the US beef sector (
). The reason for this is 2-fold. First, cattle consuming a high-forage diet have increased CH4 emissions, and second, breeding stock live on the land continuously and produce one calf per year (
). Considering the contribution of the grazing sector to enteric CH4 emissions, the primary objectives of this review were to focus on mitigation strategies from a grazing perspective and to explore how soil–plant–animal interrelationships can be manipulated and enhanced to reduce CH4 emissions and improve ecosystem functioning and overall system sustainability.
SOURCES OF METHANE EMISSIONS
Enteric Methanogenesis
Enteric CH4 is a natural by-product of the anaerobic fermentation process in the reticulo-rumen and hindgut in ruminants (
; Figure 1). The rumen is an anaerobic environment where large numbers of symbiotic bacteria, protozoa, and fungi derive their energy from consumed feedstuffs. The digestion end products of their digestion are primarily microbial cell protein and VFA (primarily acetate, butyrate, and propionate) that the host animal uses to meet its own metabolic needs (
). This symbiotic relationship allowed ruminants to evolve across a multitude of biomes to fill an ecological niche using complex carbohydrates, chiefly cellulose, that most mammalian species cannot digest (
). This evolution led to the rise of a complex microbial community that include methanogenic species that differ from methanogens in other populations because it lacks cytochrome proteins responsible for electron transfer (
). In addition to the VFA and protein produced during the fermentation process, gaseous CO2 and H2 are produced. These serve as the primary substrates for methanogenic archaea to produce CH4, typically through the hydrogenotrophic pathway (
This process of cellular respiration by methanogens uses the H2 to produce CH4 and H2O, thereby preventing metabolic hydrogen from accumulating in the reticulo-rumen. Hydrogen removal is crucial for healthy ruminal fermentation as accumulation limits the ability of microbial populations to oxidize the cofactors responsible for electron transfer, thereby reducing carbohydrate degradation, microbial growth rate, and synthesis of microbial cell protein (
). Feeding cattle a high-concentrate diet compared with a diet high in cell wall fiber results in less dietary energy lost as CH4 through effects on ruminal pH, shifting microbial populations, and a decrease in the acetate:propionate ratio (
). The production of acetate is greatest in cattle fed high-fiber diets, and its production increases CH4 production by increasing the amount of metabolic H2, whereas propionate acts as a hydrogen sink:
[2]
[3]
Numerous other factors including forage processing and quality, lipid content, forage secondary compounds, and dietary additives also alter CH4 production. Due to the myriad of factors influencing enteric CH4 production, energy losses in the form of CH4 can range from 2 to 12% of GE intake (
). Therefore, when including its effects on global climate change, taking strides to mitigate CH4 production is both economically and environmentally beneficial. Figure 1 displays the role of enteric CH4 in the biogenic C cycle. Carbon dioxide is fixed in plants via photosynthesis, and this C is then converted into CH4 after consumption of plant material by ruminants and expelled out of the mouth of the animal. This CH4 has a residence time of about 9 to 12 yr in the atmosphere before being broken back down into CO2.
Regional Emissions
An inherent complexity when discussing global emission mitigation is the regional specificity of emissions and the societal cost–benefit relationship at the more localized level. Therefore, it is important to compare regional emission rates to determine where improvement is needed and why some regions favor lower CH4 intensity (CH4 per unit of product, typically kg of carcass weight in beef cattle) compared with others. Globally, 2.8 Gt of enteric CH4 GHG are annually produced, with cattle being responsible for 77% of the total (
FAO. 2013. Tackling Climate Change Through Livestock: A Global Assessment of Emissions and Mitigation Opportunities. Food Agric. Org. United Nations, Rome, Italy.
). However, the emission rate is not equal across countries, with developing countries contributing as much as 75% of total global GHG emissions from ruminants (
). In terms of CH4 emission intensity (kg of CO2 equivalents/kg of carcass weight), sub-Saharan Africa and southern Asia have the greatest with 41 and 50 kg of CO2 equivalents/kg of carcass weight (
Gerber, P. J., H. Steinfeld, B. Henderson, A. Mottet, C. Opio, J. Dijkman, A. Falcucci, and G. Tempio. 2013. Tackling Climate Change Through Livestock—A global Assessment of Emissions and Mitigation Opportunities. Food Agric. Org. United Nations (FAO), Rome, Italy.
Gerber, P. J., H. Steinfeld, B. Henderson, A. Mottet, C. Opio, J. Dijkman, A. Falcucci, and G. Tempio. 2013. Tackling Climate Change Through Livestock—A global Assessment of Emissions and Mitigation Opportunities. Food Agric. Org. United Nations (FAO), Rome, Italy.
Gerber, P. J., H. Steinfeld, B. Henderson, A. Mottet, C. Opio, J. Dijkman, A. Falcucci, and G. Tempio. 2013. Tackling Climate Change Through Livestock—A global Assessment of Emissions and Mitigation Opportunities. Food Agric. Org. United Nations (FAO), Rome, Italy.
). In developing countries, greater CH4 emission intensities are driven by poor feed digestibility, low slaughter weights, greater age at slaughter, and poor animal husbandry (
Gerber, P. J., H. Steinfeld, B. Henderson, A. Mottet, C. Opio, J. Dijkman, A. Falcucci, and G. Tempio. 2013. Tackling Climate Change Through Livestock—A global Assessment of Emissions and Mitigation Opportunities. Food Agric. Org. United Nations (FAO), Rome, Italy.
). In developed countries, emission rates are low due to improved grazing management leading to greater diet digestibility, more intensive feeding practices, and temperate conditions (
Gerber, P. J., H. Steinfeld, B. Henderson, A. Mottet, C. Opio, J. Dijkman, A. Falcucci, and G. Tempio. 2013. Tackling Climate Change Through Livestock—A global Assessment of Emissions and Mitigation Opportunities. Food Agric. Org. United Nations (FAO), Rome, Italy.
). All of these result in improved dietary quality throughout the life span of the animal, thereby reducing days on feed and emitting less enteric CH4 emission per unit of feed consumed (
Gerber, P. J., H. Steinfeld, B. Henderson, A. Mottet, C. Opio, J. Dijkman, A. Falcucci, and G. Tempio. 2013. Tackling Climate Change Through Livestock—A global Assessment of Emissions and Mitigation Opportunities. Food Agric. Org. United Nations (FAO), Rome, Italy.
Gerber, P. J., H. Steinfeld, B. Henderson, A. Mottet, C. Opio, J. Dijkman, A. Falcucci, and G. Tempio. 2013. Tackling Climate Change Through Livestock—A global Assessment of Emissions and Mitigation Opportunities. Food Agric. Org. United Nations (FAO), Rome, Italy.
). Overcoming the economic, political, and educational complexities of developing countries is essential for these regions to reduce their enteric CH4 emissions and improve their C footprint. Beef production in these countries is not focused primarily on food production and nutrient security as in developed countries (
). In developing countries livestock provide wealth, draft power, fuel, and religious significance, which are not as important in developed countries (
). It is estimated that livestock production plays a major role in the livelihoods of more than 1 billion people in Africa and Asia and that two-thirds of the livestock managers are women (
). Mitigation strategies for these countries must balance the improvements in efficiency with the underlying complexities of the local populations and their cultures and not be detrimental to human health and environmental sustainability.
QUANTIFICATION METHODS
The gold standard for measuring enteric CH4 production is the respiration chamber and, similarly, the head-box method that quantifies enteric CH4 production by multiplying airflow through the chamber by the difference in CH4 concentration in and out measured by a gas analyzer (
). However, restricted movement of animals in the respiration chambers and head-boxes creates an artificial environment that does not reflect a normal production environment and could limit feed intake (
). Additionally, an issue in estimating farm-scale emissions is that grazing cattle are selective grazers, forming food preferences over time, which play a critical role in meeting their nutritional needs (
). This can result in animals selecting a higher quality diet than the average of the available forage base; this adds uncertainty to applying confinement emission quantification methods to grazing animals. Additionally, the artificial environment created within the chamber can result in lower DMI (
). The respiration chamber or head-box was used in the vast majority of studies used to develop emission equations and C accounting. Therefore, there are potential flaws in that these methods may not adequately capture farm-scale emissions when comparing changes in management and environment. In an examination of chamber data,
found that using these methods underestimated DMI compared with that found in grazing studies and gave lower total enteric CH4 emission estimates. Current methods to quantify enteric CH4 production in grazing environments are the sulfur hexafluoride tracer technique (SF6), the GreenFeed gas quantification system (GEM; C-Lock Inc., Rapid City, SD), various field-level emission quantification methods, and animal models (
The first method to demonstrate repeatable accuracy in quantifying emissions in an open-air environment, such as grazing, and allow for natural forage selection was the SF6 system (
). A halter with a stainless steel collection vessel and a capillary tube attached to a collection canister is then placed around the animal’s head to collect respired air from the animal (
). The enteric CH4 emission rate can then be determined by gas chromatography using the ratio of CH4:SF6 multiplied by the standard SF6 release rate and corrected for background SF6 concentration (
). The limitations of the SF6 method may explain these, at times unacceptable, error rates including the curvilinear release rate of the standard SF6 gas from the intraruminal bolus over time, labor, and the inability to collect emissions from the hindgut (
Vlaming, J. B. 2007. Quantifying variation in estimated methane emission from ruminants using the SF6 tracer technique. PhD Thesis. Animal Sci., Massey Univ., Palmerton North, New Zealand.
). The method relies on a presumably continuous and constant release rate of SF6 from the permeation tubes. Yet measured pre- and post-experiment release rates of SF6 from permeation tubes displayed a curvilinear release rate in the laboratory (
Vlaming, J. B. 2007. Quantifying variation in estimated methane emission from ruminants using the SF6 tracer technique. PhD Thesis. Animal Sci., Massey Univ., Palmerton North, New Zealand.
Vlaming, J. B. 2007. Quantifying variation in estimated methane emission from ruminants using the SF6 tracer technique. PhD Thesis. Animal Sci., Massey Univ., Palmerton North, New Zealand.
Vlaming, J. B. 2007. Quantifying variation in estimated methane emission from ruminants using the SF6 tracer technique. PhD Thesis. Animal Sci., Massey Univ., Palmerton North, New Zealand.
). Additionally, hindgut CH4 emissions that are not absorbed into the blood stream and respired out of the lungs may account for 1 to 11% of CH4 emissions (
Vlaming, J. B. 2007. Quantifying variation in estimated methane emission from ruminants using the SF6 tracer technique. PhD Thesis. Animal Sci., Massey Univ., Palmerton North, New Zealand.
Based on the previously outlined limitations of the SF6 system, the GEM system was developed by using spot measurements to estimate enteric CH4 production (
). Similar to SF6, this method is able to estimate CH4 emissions from free-grazing animals but is less intrusive and relies on spot measurements to estimate daily production (
). The GEM system uses a portable head-box either in a freestall or on a trailer that uses bait feed to entice animals to place their head in the head-box (
). When an animal places its head in the hood, the volume of air that is drawn from around the animal’s head and shoulders is measured, and a subsample is analyzed by nondispersive near infrared gas analyzers for CO2 and CH4 concentrations (
). These results are then compared with background gas concentrations from before the animal entered the hood to determine gas emission rates. Spot estimates are then averaged over the course of the sampling period to estimate each animal’s daily gas production (
). Previous research indicates the GEM can result in highly variable data sets due to greater within-day and within-animal variability that is more difficult to capture with a spot measurement system (
Methane emissions from cattle: Estimates from short-term measurements using a GreenFeed system compared with measurements obtained using respiration chambers or sulphur hexafluoride tracer..
reported that GEM in a grazing experiment was not able to capture treatment differences that were evident in respiration chambers using 4-d sampling periods with an average of 1.6 visits per day. However, recent studies on the duration of sampling and number of adequate visits may explain these differences as more visits may be required to detect treatment differences during short sampling periods (
Methane emissions from cattle: Estimates from short-term measurements using a GreenFeed system compared with measurements obtained using respiration chambers or sulphur hexafluoride tracer..
). Additionally, these studies focused on meal-fed cattle, which have a larger diurnal variation than grazing animals, which emphasizes the importance of the timing of visits to the head-box (
). Last, because this system is less expensive than chamber methods and requires less labor than the SF6 method, larger samples sizes (more experimental replicates) are possible compared with other sampling techniques, which aids in efforts to minimize experimental and sampling errors (
). This method has typically been used with grazing sheep and has shown moderately high correlations with the respiration chamber method (r = 0.71); it can also identify relative emission changes without scaling up to daily CH4 production (DMP) estimates (
). These chambers are boxes that are open at the bottom and clear sided, and they seal at the bottom with high-density foam rubber. The chambers have 3 sampling ports located on the superior, posterior, and lateral walls, and the chamber is placed on industrial-grade rubber (
, in a study comparing respiration chambers with PAC using sheep, found R2 = 0.42 to 0.48 for 2-h sampling periods and 0.39 to 0.43 in 1-h sampling periods. Measuring ewes for 40 to 60 min,
determined that these estimates had a repeatability of about 0.47 once adjusted for BW and ADG but did not alter the animal’s intake as respiration chambers do. Researchers using the PAC method have noted that repeatability can be affected by time of sampling and feeding schedules, which can cause issues with repeatability by altering the diurnal pattern of DMP (
). This causes issues when scaling up to estimate DMP, which is less of an issue when using the GEM because spot samples are obtained throughout the day if the animals use the head-box (
On the field level, micrometeorological techniques including flux-gradient, eddy covariance, and inverse dispersion models are being developed to provide herd-scale emission estimates (
). These methods measure atmospheric CH4 concentrations and meteorological variables to estimate herd-level, animal-group, or farm-level emission rates. Measuring emissions on a farm scale allows researchers to have a better understanding of how mitigation strategies are affecting the operation and to test strategies on a large number of animals (
). These methods have an advantage over individual animal sampling techniques because they do not require animal handling or bait feeding, which may alter animal behavior and diet selection, and allow for long-term comparison between management strategies (
). However, spatial variability and low intensity of emissions from grazing animals have limited the adoption of these methods beyond feedlot and confinement operations and may require animals to be grazed at high densities to estimate emissions (
). There are numerous different micrometeorological methods each having its own set of advantages and disadvantages and proper field configurations, which are outside the scope of this review but have been reviewed in depth by
Methane production in grazing environments is a combination of many different factors that interact to influence individual animal intake, performance, and emission rates. Dietary quality (e.g., digestibility) plays an important role in the production of CH4 (
). Feeding highly fermentable carbohydrates, such as the starch in high-concentrate diets, results in lower enteric CH4 production per unit of feed DM consumed by ultimately shifting microbial populations to favor propionate production and increased ruminal rate of passage (
). However, feeding all cattle only diets high in digestible carbohydrates would reduce the advantage of the ability to convert complex carbohydrates with high fiber content and untillable land into useable end products, chiefly milk or meat (
). Therefore, the question posed to scientists, and ultimately producers, should be how to leverage soil–plant–animal interrelationships to meet productivity goals of the present and future, without compromising social and ecologic outcomes (
Proper management of the forage base, the type of forage being grazed, and the stage of forage maturity can affect CH4 emissions and productivity of cattle and can serve to improve or degrade the land base (
conducted a meta-analysis examining CH4 production of C3 and C4 grasses and cold- and warm-season legumes (all legumes use the C3 photosynthetic pathway). Grasses using the C3 photosynthetic pathway are typically considered cool or temperate grasses, and those using the C4 pathway are considered warm or tropical grasses (
). The authors indicated that cattle fed C4 grasses had greater CH4 production than cattle fed C3 grasses and both warm and cold legumes. Cattle fed warm-season legumes produced the least CH4 per kilogram of DMI and per kilogram of OM intake compared with the other diets (
, who compared 2 C4 species and a C3 grass and found that C4 grasses produced 23% more CH4 than a C3 grass grown under the same subtropical conditions. These results also align with established literature about CH4 production from forages. Grasses that use the C3 photosynthetic pathway are normally considered higher quality than C4 grasses because they are typically lower in fiber, including decreased lignin production, and greater in protein contents (
found that first-calf heifers grazing a mixture of alfalfa–meadow bromegrass (Medicago sativa L. and Bromus biebersteinii) lost less energy as CH4 compared with heifers grazing a 100% meadow bromegrass pasture at 7.1 versus 9.5% of gross energy intake (GEI) lost as methane, respectively.
examined the CH4 production of young ram lambs fed different forage rations: a mixture of ryegrass–white clover pasture, lucerne, sulla, chicory, red clover, lotus, and mixtures of sulla and lucerne, sulla and chicory, and chicory with red clover. They found that lambs consuming the ryegrass–white clover pasture produced the most CH4 (g/kg of DMI) as compared with the other 10 treatments and reported a 10-fold range in overall CH4 emissions. These results clearly highlight the wide range of emissions that occur even in high-quality forages. However, these studies were all conducted with monoculture or a simple forage mixture. Studies using diverse forage bases resulted in reduced enteric CH4 production, as diverse forage bases allow animals to select plants and plant parts and build their own diet, when animals grazed at lower stocking densities (
Provenza, F. D., and J. J. Villalba. 2006. Foraging in domestic herbivores: linking the internal and external milieu. Pages 210–240 in Feeding in Domestic Vertebrates: From Structure to Function. V. L. Bels, ed. CABI Publ., Oxfordshire, UK.
The quality of a forage diet is not solely determined by the type of forage being offered. Forage maturity affects enteric CH4 production by altering the nutrient density and digestibility of the forage, thereby lowering its quality. As forages mature, fiber content increases and lignin deposition in the cell wall increases as the plants shift from primary cell wall growth to secondary cell wall thickening, resulting in decreased digestibility and intake (
examined 3 different forage diets classified as high quality (legume–grass mixed hay, 61.5% in vitro OM disappearance), medium quality (grass hay, 50.7% in vitro OM disappearance), or low quality (grass hay, 38.5% in vitro OM disappearance) and found that DMI, digestible OM digestibility, and GEI decreased. However, CH4 emissions as a percent of GEI were not affected by stage of maturity, but the reductions in DMI and digestibility resulted in CH4 emissions per unit of digestible OM consumed being highest on the low-quality diet. Similarly,
measured CH4 production of Charolais cows grazing timothy grass monocultures at 4 differing stages of maturity: early vegetative, heading, flowering, and senescence. Organic matter intake was greatest at heading compared with other stages of maturity, but the GEI lost as CH4 was not affected by maturity. The production of CH4 did decrease, however, with increasing fiber digestibility.
reported that when emissions were reported per unit of milk yield in dairy cows grazing pastures of differing forage quality, the less-mature, higher-quality pasture resulted in a reduction in CH4 emissions. Similarly, in a study comparing low and high stocking rate systems in France with Holstein-Friesian heifers, CH4 emissions per unit of digestible feed intake were decreased with increasing digestibility, and heifers consuming the higher-quality forage in the high stocking rate treatment were heavier at the end of the grazing season (
). These studies highlight that when considering the effects of forage quality and maturity on enteric CH4 emissions, production may best be expressed on an intake or animal-production basis rather than GEI lost (
). However, the literature is inconsistent on the short- and long-term effects of different grazing management strategies. Ruminants selectively graze and form food preferences over time, which play a critical role in meeting their nutritional needs (
Matches, A. G., and J. C. Burns. 1995. Systems of grazing management. Pages 179–192 in The Science of Grassland Agriculture. R. F. Barnes, D. A. Miller, and C. J. Nelson, ed. Iowa State Univ. Press, Ames.
agreed that continuous set stocking allows for maximum diet selection, which results in the capture of short-term gains in animal performance. However, long-term effects on enteric CH4 production between continuous and rotational systems is an area of recurrent inconsistency.
, in a comparison of continuous versus rotational stocking systems in Brazil, indicated that continuous stocking resulted in lower emission intensity (CH4/kg of ADG) compared with a rotational system. Yet, by refining the rotational system,
later showed that rotating cattle among paddocks at a target residual forage height of 11 cm resulted in decreased CH4 production compared with that of a traditional rotational system, which had targeted pre- and postgrazing sward heights of 25 and 5 cm, respectively.
determined that when incorporating a rotational system consisting of best management practices, with periodical fertilization and animals being rotated frequently, annual enteric CH4 production was reduced by 22% compared with a continuous stocking system. Conversely,
compared enteric CH4 production and voluntary intake of steers grazing in 1 of 2 grazing systems: continuous or 10-paddock rotational stocking system with animals moved based on forage availability, at 2 stocking rates: 1.1 or 2.2 steer/ha. They found that neither voluntary intake nor CH4 production were affected by management system.
Considering that sustainability is a multipronged goal, perhaps the benefits of a grazing management system occur beyond short-term animal performance. One hypothesis suggests that the variable results in enteric CH4 production in different systems are the result of animals in the studies being able to select high-quality diets within the treatments and perform well. However, some literature argues that the effects of management are more crucial to the long-term performance of the supporting environment. One suggestion is that the use of continuous stocking results in imbalanced patch grazing pressure, which can result in overgrazing of preferred forages and ecosystem process impairment (short- or long-term damage to normal ecosystem function). This leads to a reduction in plant diversity, increases the proportion of undesirable or low-quality forages, and ultimately leads to soil erosion (
). Additionally, the loss of high-quality forages from overgrazing will result in a lower amount of solar energy captured and, thus, reduce the amount of solar energy converted to forage and then to useable end products (e.g., meat and milk).
Forage Secondary Compounds.
Grazing ecosystems are complex and affect emissions through other mechanisms beyond intake and forage quality. In grazing lands containing a forage base of differing forages at different stages of growth, cattle may capture synergies due to primary and secondary compounds present in the plants resulting in improved performance (
). Plant secondary compounds or metabolites consist of a broad spectrum of compounds including flavonoids, tannins, pectins, glycosides, terpenoids, and sesquiterpene lactones, just to name a few. With a wide range of compounds comes a wide range of ecological functions performed by the compounds, but the compounds of interest from a grazing perspective are those that act as a defense against herbivory (the consumption of plants by animals;
). Some of these defense compounds can be toxic to the animal or act as a deterrent through taste, and animals learn either by experience or animal-to-animal learning which plants to consume and how to minimize the effects of toxic compounds (
). However, some of these compounds have shown promise to improve animal productivity and health and mitigate enteric CH4 production. Compounds of interest are saponins, condensed tannins, and essential oils (
found that grazing cattle on diverse pastures containing different legumes has divergent effects on CH4 production, potentially due to differences in secondary compounds. They noted that cattle grazing a diverse pasture with saponin-containing alfalfa had greater CH4 emissions per unit of BW gain than cattle on pastures with tannin-containing sainfoin or tannin-containing birdsfoot trefoil, as well as improved animal performance. In a study feeding alfalfa hay or sainfoin hay, beef heifers produced less CH4 on an OM basis when fed the tannin-containing sainfoin hay (
Supplementation with whole cotton seed causes long-term reduction of methane emissions from lactating dairy cows offered a forage and cereal grain diet..
found that dairy cows grazing ryegrass supplemented with 163 or 326 g/d of condensed tannins had reduced enteric CH4 emissions, although the condensed tannins also caused a corresponding decrease in milk yield and intake. Tannins are able to reduce enteric CH4 because of its affinity to bind protein and carbohydrates in the rumen, which decreases ruminal fiber digestion. However, its inclusion at high dietary concentrations can decrease voluntary feed intake.
suggested that tannins affect rumen protein degradation when consumed at 20 to 45 g of condensed tannins/kg of DM, and >55 g/kg of DM can inhibit voluntary feed intake and forage digestibility. Tannins also may serve to improve animal performance through bloat control and providing some antiparasitic properties (
). However, tannin research is difficult and inconsistent due to difficulties in isolating tannins and because different forages may contain different structural forms of tannins.
Mitigation Strategies Directed Toward Animals
Mitigating enteric CH4 emissions by acting directly on the animal and not the forage base is problematic in grazing environments partially due to the difficulty in estimating DMI of grazing animals (level of intake is a major driver of enteric CH4 production), the infrequency of supplementation, and the variable level of individual animal supplement intake (
). Common mitigation strategies are supplementation to increase animal performance (thereby decreasing emission intensity, although not always done specifically to decrease emissions, e.g., protein supplementation), supplements that directly alter ruminal fiber digestion or methanogens, breeding for more efficient animals, and vaccinating against methanogens (
). Other strategies such as nitrate supplementation do not seem practical because of issues in getting the inhibitor to the animal in useful quantities or risks of toxicity (
hypothesized that improving the efficiency of feed energy use has the greatest potential for mitigating enteric CH4 production. Outside of improving the forage base, providing supplementation to meet the nutritional needs of different classes of grazing animals is a mitigation option. Producers have long supplemented grazing ruminants to improve performance during times of nutritional deficiencies in the forage crop, such as the late summer slump or winter dormancy that affects many forage bases across the United States (
). Similar to estimating emissions of grazing animals, accurate and precise supplementation is difficult because energy requirements for grazing animals are not the same as for confinement-fed cattle (
). The most common forms of beef cow supplementation are energy supplementation, when intake of C skeletons are the limiting nutrient for ruminal microbial growth, and protein supplementation when low-quality forages (e.g., CP <7%) are the primary diet. Energy supplementation can come in many forms, such as corn grain or fat supplementation, depending on the goals of the producer. For example, when cattle are grazing wheat pasture that is high in water and protein contents, but low in fiber, corn supplementation can improve animal performance (
Hogan, J. P. 1982. Digestion and utilization of proteins. Pages 245–257 in Nutritional Limits to Animal Production from Pastures. J. B. Hacker, ed. Commonwealth Agric. Bureaux, Slough, UK.
). Protein supplementation does not directly affect rumen methanogens, unlike energy supplementation strategies, and can come in a multitude of forms including pelleted feed, limit grazing of cool-season forage, and lick blocks, among others (
in a crossover design using ruminally cannulated steers reported that cottonseed meal (37.9% CP) fed at 800 g/head per day improved forage use by improving IVDMD and animal performance. Unfortunately, there is limited literature on the effects of supplemental protein on enteric CH4 emissions. Decades ago,
Methane production by ruminants—Literature review of I. Dietary manipulation to reduce methane production and II. Laboratory procedures for estimating methane potential of diets..
suggested that this strategy could be particularly beneficial in developing countries where animal nutrition is potentially poor. They also suggested that using targeted supplementation in these countries could reduce enteric CH4 emissions by improving the nutritional balance in the rumen.
, using the SF6 technique, reported that heifers grazing ryegrass ad libitum had one-tenth of the CH4 emission intensity compared with animals limit grazing ryegrass for 1 h during the spring but that limit grazing for 4 h resulted in similar enteric CH4 emissions.
compared grazing steers supplemented with rolled barley grain at 2, 4, and 4 kg/head per day during the early, mid, and late grazing season, respectively, with steers grazing alfalfa and meadow grass pastures without supplementation. They reported that supplementation reduced forage intake and increased total OM consumed but did not affect overall enteric CH4 emissions. It was hypothesized from the results that the greater forage quality and quantity were the major factors influencing animal responses. However, considering the high forage quality available to the steers, one should not apply these results to cattle grazing lower quality forage where supplementation might improve the nutritional status of the rumen. Considering the heavy reliance on forage quality by governing bodies when estimating CH4 emissions (
), it would be beneficial to do more long-term monitoring because of the variation among different production systems, changes in forage quality throughout the year, and supplementation strategies that exist to improve animal performance. Considering the advancements in modern emission quantification techniques, along with the lowering cost of monitoring, long-term studies should be conducted.
Lipid supplementation is a plausible approach to directly reduce enteric CH4 emissions (
). There is a wealth of literature exploring the effects of fats on ruminal CH4 production. Lipid supplementation apparently alters emissions through reductions in ruminal fiber digestion, although there is some evidence for a suppressive effect on bacteria and protozoa activity (
). An additional mode of action is UFA acting as a H2 sink during the process of biohydrogenation in the rumen. This reduces the H2 pool available to methanogenic archaea; however, this process appears to have minimal effect and was suggested to only account for 1 to 2% of metabolic H2 (
). A group of lipids, including animal-based tallow and grease, oils from plants (e.g., soybean oil and cottonseeds), and high-fat by-products (such as distillers grains), can alter ruminal CH4 production (
). The most likely mode of action for these supplements is coating fiber particles to protect them from ruminal microbes. When supplemented at appropriate dietary concentrations, these lipid sources can reduce CH4 production without altering DM digestibility or forage intake (
). There has been extensive research done with dietary lipids, but unfortunately, the majority were conducted with dairy or beef cattle fed TMR with significant proportions of concentrate feeds (
examined the effects of whole cottonseed supplementation and rumen bypass fat or soybean oil on enteric CH4 emissions of steers grazing old world bluestem pastures. In both experiments CH4 production was reduced when steers were offered whole cottonseed or soybean oil. Rumen bypass fat did not alter CH4 production (g/d) but did when expressed as grams per kilogram of BW gain and lowered CH4 yield (9.7 vs. 8.0% for control vs. bypass fat-supplemented cattle).
examined Nellore steers grazing C4 grasses offered no supplement, palm oil, linseed oil, rumen bypass fat, or whole soybeans supplemented at 1% of BW. They found that the linseed oil treatment reduced CH4 emissions per kilogram of BW, but none of the lipid supplements improved animal performance or DMI. The authors hypothesized that a reduction in ruminal fiber digestibility may have been responsible for the lack of improvement in performance. These studies show that different lipid sources can potentially be beneficial both environmentally and economically through effects on performance, but overfeeding lipids can be detrimental, although results are not consistent across different lipid sources as some studies have shown reductions in DMI and productivity (
conducted a meta-analysis and showed that whereas some supplemental lipids do reduce CH4 production, there were significant linear and curvilinear responses in CH4 production from diets containing up to 13% lipids, and dietary fat content should not be greater than 6 to 7%, DM basis (
Methane output and rumen microbiota in dairy cows in response to long-term supplementation with linseed or rapeseed of grass silage- or pasture-based diets..
Supplementation with whole cotton seed causes long-term reduction of methane emissions from lactating dairy cows offered a forage and cereal grain diet..
have resulted in long-term emission mitigation that may be due to animal type or diet variability. More long-term experiments should be conducted with grazing beef cows to determine how effective lipid supplementation could be at the herd emissions level, as well as to explore economic methods of supplementation.
Ionophores are common feed additives provided to both grazing and confinement-fed cattle because of their effects on animal health and efficiency (
). The most frequently used ionophore, monensin, improves energy and N utilization by selective inhibition of gram-positive ruminal bacteria, which favors propionate production (
). Ionophores effectively inhibit gram-positive bacteria by facilitating the transport of ions across the cell membrane, which leads to the disruption of the chemi-osmotic gradient (
). A greater proportion of gram-negative bacteria shifts the acetate:propionate ratio, favoring production of the more energetically efficient propionate, which acts as a H sink, thereby potentially reducing CH4 production (
Place, S. E., K. R. Stackhouse, Q. Wang, and F. M. Mitloehner. 2011. Mitigation of greenhouse gas emissions from U. S. beef and dairy production systems. Pages 443–457 in Understanding Greenhouse Gas Emissions from Agricultural Management. L. Guo, A. Gunasekara, and L. McConnell, ed. ACS Symp. Series, Washington, DC.
reported that including monensin at 32 mg/kg of DM reduced CH4 yield (% GE lost as CH4) by 0.33 ± 0.16% for steers consuming TMR. In an experiment with dairy cows grazing a predominantly ryegrass sward with monensin supplemented at 471 mg/d, no differences were detected compared with cows not receiving monensin (
). A respiration chamber experiment was conducted concurrently with the grazing experiment. In cows receiving the same monensin dose and offered fresh-cut ryegrass no difference on CH4 production was detected from monensin supplementation. Similarly,
Use of monensin controlled-release capsules to reduce methane emissions and improve milk production of dairy cows offered pasture supplemented with grain..
dosed monensin using a controlled-release capsule and reported that enteric CH4 yield of dairy cows grazing ryegrass pasture was not affected by monensin supplementation. In a respiration chamber experiment with steers consuming fresh chopped winter wheat at increasing intake levels and offered monensin (158 g/head per d), CH4 production tended to be reduced for supplemented animals compared with nonsupplemented animals at 115 versus 130 L/d, respectively (P = 0.06;
). There was no difference in GEI lost as CH4 when animals were fed at 1.5×maintenance. However, in an experiment with beef steers and heifers grazing winter wheat offered an energy supplement containing monensin (34 mg/kg of DM),
found that increasing level of supplement intake reduced CH4 emission intensity (g of CH4/kg of BW gain). Whereas cattle fed TMR have displayed long-term reductions in CH4 production, potential explanations for the inconsistency in grazing trials could be variable dietary quality of different forage bases, the rate of intake, and the lack of studies conducted using beef cattle in grazing environments (
Use of monensin controlled-release capsules to reduce methane emissions and improve milk production of dairy cows offered pasture supplemented with grain..
). Even though monensin may result in reductions of CH4 emissions and has proven to improve feed efficiency and reduce bloat, it has been banned in the European Union since 2006, and its classification as an antibiotic could result in increased scrutiny in future years in other regions.
Although not a new feed additive, recent research showed the potential antimethanogenic properties of Asparagopsis species (seaweed) inclusion in the diet, both in vitro and in vivo (
). Asparagopsis microalgae produces bioactive compounds including bromoform and dibromochloromethane, giving it the same potential mode of action as the chemical additive bromochloromethane that decreased enteric CH4 emissions (
tested 10 doses of Asparagopsis inclusion from 0 to 16.7% OM incubated with a grass hay substrate. Methane production was reduced by 99% at 2% OM inclusion level. When included on a 0.5 or 1% OM basis in a TMR,
reported that CH4 emissions from dairy cows sampled with a GEM were reduced by 43% at the 1% inclusion level after adjusting to account for differences in DMI. Methane reduction of this magnitude would make seaweed supplementation with Asparagopsis inclusion one of the most noteworthy mitigation strategies tested in vivo. Additionally,
offered it to sheep fed a high-fiber pelleted diet and reported that ruminal propionate concentrations increased with Asparagopsis supplementation and CH4 production decreased over a 72-d period. Supplementation of Asparagopsis in grazing environments at different intakes and frequencies of supplementation is needed to establish its efficacy in high-forage systems, beyond high-concentrate confinement feeding.
Another possible avenue for enteric CH4 mitigation was the discovery that CH4 emissions might be heritable, suggesting that genetic selection of animals might be a viable mitigation possibility (
Bovine host genetic variation influences rumen microbial methane production with best selection criterion for low methane emitting and efficiently feed converting hosts based on metagenomic gene abundance..
Bovine host genetic variation influences rumen microbial methane production with best selection criterion for low methane emitting and efficiently feed converting hosts based on metagenomic gene abundance..
). Some researchers suggested that the mechanisms controlling the microbiome may be in the interactions with receptors in the rumen wall or salivary antibodies (
hypothesized that the physical structure of the rumen could explain the differences between genetic lines, but their results were not consistent. Additionally, their results implied, although indirectly, sire x environmental interactions, indicating that selecting cattle for reduced CH4 emissions in one environment may not improve performance as the animal moves from a grazing system to the feedlot. In a study analyzing about 3,400 records, along with the records reported by
Genetic parameters of methane emissions determined using portable accumulation chambers in lambs and ewes grazing pasture and genetic correlations with emissions determined in respiration chambers..
reported heritability estimates of 0.23 and 0.13 for CH4 g/d and CH4 yield, respectively, in New Zealand sheep. Estimates in Angus cattle were similar as reported by
. They reported heritability values of 0.27 and 0.29 for CH4 production and CH4 yield from 737 Angus cattle in Australia. Additionally, new research found significant differences between breeds. In a study of Nelore and Angus cattle grazing during the growing period and finishing in a feedlot,
determined that the Nelore cattle produced significantly less CH4 (g/d) than Angus cattle regardless of diet. However, in the finishing period the Angus steers reached finishing weight faster and therefore produced the same amount of CH4 during the finishing phase as the Nelore steers (
). More work to refine the selection technique and characterization of beef herds in other countries could potentially allow for reductions in the carbon footprint of beef production.
Methane production is partially dependent on the quantity of feed consumed, which led researchers to explore selecting animals based on their residual feed intake (RFI;
). Residual feed intake is defined as the difference between actual feed intake and the expected rate of intake needed for that animal to meet its maintenance requirements plus the desired production level (e.g., milk, meat, wool;
Effect of divergence in phenotypic residual feed intake on methane emissions, ruminal fermentation, and apparent whole-tract digestibility of beef heifers across three contrasting diets..
). In a study with low- and high-RFI Limousin × Fresian heifers, enteric CH4 production, DMI, and performance were measured in both grazing and confinement systems (
Effect of divergence in phenotypic residual feed intake on methane emissions, ruminal fermentation, and apparent whole-tract digestibility of beef heifers across three contrasting diets..
). The study indicated that animals with low RFI actually had greater CH4 emissions on the basis of DMI and GEI than high-RFI animals. These results were similar to those of
Grass silage intake, rumen and blood variables, ultrasonic and body measurements, feeding behavior and activity in pregnant beef heifers differing in phenotypic residual feed intake..
, reported low-RFI cattle had lower enteric CH4 emissions. These inconsistent results demonstrate that RFI classifications are not consistent across diets or animals, particularly growing animals, currently limiting the application of RFI as a mitigation tool.
Vaccination Against Methanogens.
Vaccination against ruminal methanogens is another potential strategy, which thus far has produced inconsistent results. This approach is based on the generation of an antibody response that is delivered to the rumen through salivary secretions to neutralize methanogens (
). Such a vaccination potentially would be easy for many producers to implement because they already use some type of annual vaccination protocol; thus, vaccination might be cost effective (
isolated methanogens and provided sheep with a 3-methanogen mixture, a 7-methanogen mixture followed by the 3-methanogen mixture, or adjuvant only. Primary and secondary immunizations were given subcutaneously 153 d apart. Testing CH4 emissions with both respiration chambers and the SF6 technique, they found a significant 7.7% reduction in enteric CH4 production 4 wk after secondary immunization in sheep provided the 3-methanogen mixture.
Development of a vaccine to mitigate greenhouse gas emissions in agriculture: Vaccination of sheep with methanogen fractions induces antibodies that block methane production in vitro..
, although not measuring enteric CH4 emission, did find that antisera selected from methanogen fractions produced a strong antibody response in sheep, with both IgG and IgA responses detected in the saliva. However, in a study using a vaccine attempting to account for 52% of the methanogens present in the rumen,
found that CH4 production actually increased by 18% in sheep after 3 vaccinations, opposite of the effect expected. Minimal research has examined the efficacy of methanogen vaccination in cattle.
, using 5-mo-old male Holstein-Friesian calves, provided s.c. antimethanogen vaccinations isolated from Methanobrevibacter ruminantium M1. They detected a strong IgG response and a moderate IgA response in the serum and saliva of inoculated animals. The authors also took rumen fluid samples and found an antibody presence in the rumen (
, the 5-methanogen species vaccination resulted in increased CH4 production, even with the desired immune response being observed. The authors hypothesized that this was because the vaccine was not targeting the species responsible for most of the CH4 production or that some unknown conditions are necessary to see an abatement response recorded in previous studies (
). It also was noted by authors that any vaccine formulation is diet and environment specific, with a broad spectrum currently out of the reach of current research (
found that most rumen methanogens are difficult to culture and that the majority of species, at that time, had yet to be isolated. For the vaccination approach to show promise in the future, it will be critical to successfully culture highly productive methanogen species and to do so across production systems as methanogen populations can vary widely by region and diet (
). Soils can be either a source or sink of CH4, with mostly obligate aerobic methanotrophs located in the upper levels of upland soils responsible for oxidizing atmospheric CH4 to CO2, but the ecological controls involved are still poorly understood, creating large uncertainties (
IPCC. 2013. Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. T. F. Stocker, D. Qin, G.-K. Plattner, M. Tignor, S. K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex, and P. M. Midgley, ed. Cambridge Univ. Press, Cambridge, United Kingdom.
IPCC. 2013. Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. T. F. Stocker, D. Qin, G.-K. Plattner, M. Tignor, S. K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex, and P. M. Midgley, ed. Cambridge Univ. Press, Cambridge, United Kingdom.
). After a rainfall, CH4 is immediately emitted from the soil, with this emission rate gradually decreasing as soil dries, and more CH4 is oxidized as the diffusion becomes less suppressed as less water fills the pore networks (
). Forest soils with well-developed soil structure typically display the greatest CH4 oxidation rate, followed by immature forests and then native range land, and last, intensively managed agriculture lands have the lowest CH4 sink potential due to increased and frequent disturbance and fertilizer application (
). Temperate forest soils have CH4 uptake rates ranging from 12.8 to 25.6 kg of CH4/ha per year, compared with agriculture soils, having maximum oxidation rates of 1.6 to 3.2 kg of CH4/ha per year (
) Nitrogen fertilizer application has a negative relationship with methanotroph activity because ammonium may have an inhibitory effect on the enzyme system responsible for CH4 oxidation (
that it may take at least 7 yr of fertilizer application before inhibitory effects are seen. Uptake rates also display significant temporal variation between seasons (
). Methane uptake rates are greatest in summer months when drier soil conditions are present and less in cold, wetter months when methanotroph activity is suppressed and methanogen activity is greater (
). Even with considerably lower oxidation rates, some agricultural soils may still be able to offset CH4 emissions from animal excreta and part of enteric CH4 emissions, but few studies have been conducted in this area and results have been inconsistent (
Saggar, S., K. Tate, C. B. Hedley, and A. Carran. 2004. Methane emissions from cattle dung and methane consumption in New Zealand grazed pastures. Pages 102–106 in Proceedings of the Workshop on the Science of Atmospheric Trace Gases. NIWA Technical Report 125. T. S. Clackson, ed. Natl. Inst. Water Atmos. Res., Wellington, New Zealand.
compared CH4 uptake rates in a native pasture, an annually fertilized pasture, and a nonirrigated wheat field and found that N fertilization of the native grassland reduced the CH4 uptake rate by 35% compared with the native pasture. The nonirrigated wheat field resulted in a further decrease of 15% from the fertilized native pasture (
, who reported that long-term application of ammonium-N fertilizer resulted in reduced CH4 uptake by soils. However, in a study on the effects of plant diversity and fertilizer application on CH4 and N2O fluxes in Germany, it was found that CH4 uptake decreased with increasing plant diversity regardless of fertilization (
). It was hypothesized that this was due to increased soil moisture, which lowered the diffusion rate of CH4. However, other potential explanations for the inconsistency are that N concentrations were not high enough to be inhibitory, fertilizer may not have been applied to fields before experiment onset, or water content of the soils was the limiting factor before enzyme inhibition. In a grazing study on 3 different types of steppe in Inner Mongolia, China (meadow steppe, typical steppe, and desert steppe) and 3 different grazing rates (light, moderate, and heavy grazing), it was reported that light grazing did not change CH4 uptake rates, but moderate and heavy grazing reduced uptake by 6.8 to 37.9% (
). However, in a synthesis of 43 studies conducted in China, including the previous study, only heavy grazing rates consistently decreased CH4 uptake (
reviews a depth of literature on uptake rate in grazing ecosystems, there is a lack of research incorporating enteric CH4 production rates and the potential of different systems to offset parts of the enteric CH4 budget.
Carbon Accounting
The Paris Climate Accord set an aggressive goal of keeping global temperature rise below 1.5°C compared with preindustrial levels by the year 2100 (
Rogeli, J., D. Shindell, K. Jiang, S. Fifita, P. Forster, V. Ginzburg, C. Handa, H. Kheshgi, S. Kobayashi, E. Kriegler, L. Mundaca, R. Séférian, and M. V. Vilariño. 2018. Mitigation pathways compatible with 1.5°C in the context of sustainable development. In Global Warming of 1.5°C. An IPCC Special Report on the Impacts of Global Warming of 1.5°C Above Pre-Industrial Levels and Related Global Greenhouse Gas Emission Pathways, in the Context of Strengthening the Global Response to the Threat of Climate Change, Sustainable Development, and Efforts to Eradicate Poverty. V. Masson-Delmotte, P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P. R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J. B. R. Matthews, Y. Chen, X. Zhou, M. I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield, ed. World Meteorol. Org., Geneva, Switzerland.
Rogeli, J., D. Shindell, K. Jiang, S. Fifita, P. Forster, V. Ginzburg, C. Handa, H. Kheshgi, S. Kobayashi, E. Kriegler, L. Mundaca, R. Séférian, and M. V. Vilariño. 2018. Mitigation pathways compatible with 1.5°C in the context of sustainable development. In Global Warming of 1.5°C. An IPCC Special Report on the Impacts of Global Warming of 1.5°C Above Pre-Industrial Levels and Related Global Greenhouse Gas Emission Pathways, in the Context of Strengthening the Global Response to the Threat of Climate Change, Sustainable Development, and Efforts to Eradicate Poverty. V. Masson-Delmotte, P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P. R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J. B. R. Matthews, Y. Chen, X. Zhou, M. I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield, ed. World Meteorol. Org., Geneva, Switzerland.
). Methane production is a large component of the beef industry’s C footprint because it is a potent GHG with a GWP over a 100-yr time frame (GWP100) 28 times that of CO2 according to IPCC AR5 (
IPCC. 2014. Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. O. Edenhofer, R. Pichs-Madruga, Y. Sokona, E. Farahani, S. Kadner, K. Seyboth, A. Adler, I. Baum, S. Brunner, P. Eickemeier, B. Kriemann, J. Savolainen, S. Schlömer, C. von Stechow, T. Zwickel and J. C. Minx, ed. Cambridge Univ. Press, Cambridge, United Kingdom.
). With a supporting population of approximately 32 million head of beef cows in the United States, and greater CH4 emission rates with high-forage diets compared with high-concentrate diets, this results in a C footprint that heavily favors intensive production systems, whereas cow-calf systems contribute 70 to 80% of total of the CO2 equivalents budget (
). The importance of enteric CH4 from grazing ruminants in the biogenic C cycle, however, provides an additional layer of complexity in the discussion of enteric CH4 mitigation. In the United States, CH4 from ruminants has always been a large component of the C cycle during the transition from wild to farmed ruminants (
). Two studies examining pre-European enteric CH4 emissions from the large bison herds and other wild ruminants compared with modern-day farmed ruminants found that emission rates pre-European settlement were similar to modern emission rates (
The metrics being used when conducting current life cycle assessments and monitoring progress of mitigation are in continual discussion, and the benefits of reductions of short-lived pollutants, such as CH4, may be overstated because of this (
). A carbon footprint represents the totality of emissions from a system, with different GHG contributions converted to a CO2-equivalents basis; this may be misrepresenting the burden of enteric CH4 (
). Greenhouse gases are typically converted into CO2 equivalents using a GWP metric over varying time scales, and the time scale that is selected to calculate the equivalent metrics can have significant effect on the final computed C footprint (
IPCC. 2013. Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. T. F. Stocker, D. Qin, G.-K. Plattner, M. Tignor, S. K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex, and P. M. Midgley, ed. Cambridge Univ. Press, Cambridge, United Kingdom.
). For example, if GWP20 (meaning over a 20-yr time frame) is used, CH4 is considered 84 times more potent than CO2, but when this is expressed over 100-yr time frame, then the GWP falls to 28 (
IPCC. 2013. Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. T. F. Stocker, D. Qin, G.-K. Plattner, M. Tignor, S. K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex, and P. M. Midgley, ed. Cambridge Univ. Press, Cambridge, United Kingdom.
). The high heat-trapping potential increases with shorter time frames because CH4 is a short-lived pollutant with an atmospheric half-life of about 9 to 12 yr before being oxidized by hydroxy radicals, compared with CO2, which can remain in the atmosphere for thousands of years (
IPCC. 2013. Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. T. F. Stocker, D. Qin, G.-K. Plattner, M. Tignor, S. K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex, and P. M. Midgley, ed. Cambridge Univ. Press, Cambridge, United Kingdom.
; Figure 1). This has led some to question the use of GWP100 and to suggest other metrics including global temperature potential and GWP*, although both are relatively new and lack policy influence (
IPCC. 2013. Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. T. F. Stocker, D. Qin, G.-K. Plattner, M. Tignor, S. K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex, and P. M. Midgley, ed. Cambridge Univ. Press, Cambridge, United Kingdom.
). The metric GWP* takes a slightly different approach to determine the effect of short-lived pollutants by equating a yearly 1-t increase in the rate of CH4 emission with a one-off pulse release of 100 × GWP100 tonne of CO2 (pulse release to calculate temperature effect of methane over 100 yr;
). This allows CH4 to be considered on a decadal time scale, one that is more appropriate given its atmospheric lifetime. This may allow future models to more appropriately consider the behavior of CH4 in the atmosphere by allowing its warming potential to reach zero, which is not possible with CO2 due to its long atmospheric lifespan, unless sequestration is considered (
). The implication, therefore, is that if cattle numbers remain static, there should be no increase in radiative forcing, and likewise, any decrease in cattle numbers or emission rate should actually have a cooling effect on a 100-yr time frame versus consistent radiative forcing during the same time.
Livestock Environmental Assessment and Performance Partnership (
FAO. 2016. Environmental Performance of Large Ruminant Supply Chains: Guidelines for Assessment Livestock Environmental Assessment and Performance Partnership. Food Agric. Org. United Nations, Rome, Italy.
FAO. 2016. Environmental Performance of Large Ruminant Supply Chains: Guidelines for Assessment Livestock Environmental Assessment and Performance Partnership. Food Agric. Org. United Nations, Rome, Italy.
). Studies that use Tier II methods use a standard conversion factor for enteric CH4 production, 6.5% of GEI is lost as CH4 for grazing cattle and 3.5% of GEI for cattle on feedlot diets, which arise from studies that are not associated with normal grazing patterns and with nonrepresentative DMI based on the CH4 measurement techniques that were used traditionally. Research using new technologies, including the SF6 tracer technique and GEM, indicates that the conversion factor for grazing cattle may overestimate emissions in some systems (
using data collected for production systems in California and the upper Midwest, respectively, both indicated that the IPCC Tier II methods were overestimating emissions by up to 15%. With GHG monitoring technologies evolving rapidly to better sample the complex grazing environment where respiration chamber methods have limitations, more long-term monitoring studies and updated life-cycle assessment calculations are needed to improve the accuracy of the industry’s C footprint in grazing cattle systems.
APPLICATIONS: CURRENT AND FUTURE
Producer decisions affecting the soil–plant–animal interrelationships show promise in reducing the CH4 emission rates from cattle. Improving the quality of the forage base by incorporating high-quality, readily digestible forages and grazing strategies that improve the quality of the forage base, while potentially sacrificing short-term gains in animal performance, can result in the reduction of CH4 production. Additionally, mitigation tools such as lipid supplementation, supplementing Asparagopsis (seaweed), incorporating forages with beneficial secondary compounds, and genetic selection for reduced enteric CH4 production may be viable tools for beef producers to lower their C footprint. Improvements in grazing emission estimates and micrometeorological techniques also will give researchers better insights in future years on how animal management and grazing strategies affect whole-herd emissions and landscapes. In particular, as financial markets for C and ecosystem services develop over the coming decades, these landscape monitoring systems may serve an important role in the future of agriculture and allow producers to benefit from other ecosystem services they provide rather than just provisioning services. However, these systems must be cost effective, accurate, and precise for these services to be fairly compensated. Lastly, changing from the GWP100 system to GWP* may allow researchers to better understand the effects of short-lived pollutants on climate change and be an improved representation of how enteric CH4 production operates as part of the natural C cycle.
ACKNOWLEDGMENTS
The authors acknowledge that this work was partially funded by the Michigan Alliance for Animal Agriculture (East Lansing, MI; grant #AA18-027).
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