Introduction

Agricultural production is responsible for approximately 21%^{Francesco N. Tubiello et al., “Greenhouse Gas Emissions from Food Systems: Building the Evidence Base,” Environmental Research Letters 16, no. 065007 (2021): 1–15.} of global greenhouse gas (GHG) emissions, and 11 percent^{US Environmental Protection Agency, “Greenhouse Gas Inventory Data Explorer” (2021),
https://cfpub.epa.gov/ghgdata/inventoryexplorer/#iagriculture/allgas/source/all.}of US emissions. Of that, animal agriculture, and especially beef production, is the primary source (Figure 1). In the United States, beef production emits about 213.3 million metric tons (MMT) of carbon dioxide equivalent (CO2e), more than chemical, cement, and metal production combined^{C. Alan Rotz et al., “Environmental Footprints of Beef Cattle Production in the United States,” Agricultural Systems 169 (February 1, 2019): 1–13, https://doi.org/10.1016/j.agsy.2018.11.005.US Environmental Protection Agency, “Greenhouse Gas Inventory Data Explorer.”}

But beef is more than just environmental impacts. The US beef industry produced about $111 billion worth of product in 2019^{US Department of Agriculture, “USDA ERS – Statistics & Information,” accessed August 1, 2021, https://www.ers.usda.gov/topics/animal-products/cattle-beef/statistics-information.aspx.}— making up around 20 percent of global beef production^{Rotz et al., “Environmental Footprints.}— and employs hundreds of thousands of workers, many in rural and semi-rural areas of the country.^{IBISWorld, “Beef Cattle Production in the US – Employment Statistics,” accessed August 5, 2021, https://www.ibisworld.com/industry-statistics/employment/beef-cattle-production-united-states/.}Beef upcycles nutrients from grasses and feeds and provides a significant portion of the United States’ protein consumption.^{Lacey Newlin, “Beef’s Greatest Talent Is Protein Upcycling,” Texas A&M University Department of Animal Science, accessed August 1, 2021, https://animalscience.tamu.edu/2020/02/10/beefs-greatest-talent-is-protein-upcycling/.Stefan M. Pasiakos et al., “Sources and Amounts of Animal, Dairy, and Plant Protein Intake of US Adults in 2007–2010,” Nutrients 7, no. 8 (August 21, 2015): 7058–69, https://doi.org/10.3390/nu7085322.}While per capita beef consumption has declined in the United States since the 1970s,^{US Department of Agriculture, “USDA ERS – Chart Detail,” accessed August 1, 2021, https://www.ers.usda.gov/data-products/chart-gallery/gallery/chart-detail/?chartId=58312.} beef remains a central part of the American diet and an important cultural touchstone.

With rising global demand for beef — driven largely by population growth and increasing wealth — enteric methane emissions (a by-product of cattle digestion) are projected to rise by more than 45 percent between 2020 and 2050.^{Michael A. Clark et al., “Global Food System Emissions Could Preclude Achieving the 1.5° and 2°C Climate Change Targets,” Science 370, no. 6517 (November 6, 2020): 705–8, data S2,
https://doi.org/10.1126/science.aba7357.} Although efforts to shift diets toward low-carbon proteins could dampen the rise in beef demand, in the absence of extreme policy and cultural shifts, beef production is not likely to disappear. Thus, in order to advance global decarbonization, the United States must take a leading role in developing and adopting lower-carbon approaches to beef production.

New technologies and improved production practices have already reduced emissions from the US beef industry.^{Judith L. Capper, “The Environmental Impact of Beef Production in the United States: 1977 Compared with 2007,” Journal of Animal Science 89, no. 12 (December 1, 2011): 4249–61, https://doi.org/10.2527/jas.2010-3784.} However, further environmental improvements are both possible and necessary (Figure 2). ^{Diana Pape et al., “Managing Agricultural Land for Greenhouse Gas Mitigation within the United States” (July 2016), https://www.usda.gov/oce/clima...; Joseph E. Fargione et al., “Natural Climate Solutions for the United States,” Science Advances 4, no. 11 (November 14, 2018), https://doi.org/10.1126/sciadv.aat1869.}

In this report, we outline how changes to grazing, animal feed, manure management, and other practices could make beef more climate-friendly. Collectively, we estimate that business-as-usual reductions in emissions intensity and full adoption of existing low-carbon technologies and practices could reduce beef’s carbon footprint in 2030 by nearly 18 percent, or 42 MMT CO2e per year, more than many common proposals (Figure 3).

Technological breakthroughs, such as feed additives that reduce methane emissions from grazing cattle, could cut beef’s carbon footprint even further. We estimate that fully adopting existing and breakthrough technologies, where possible, would reduce beef’s carbon footprint up to 48 percent from current levels, making it nearly three times as carbon-intensive as pork and four and a half times as carbon-intensive as an Impossible Burger.^{Using a 0.72-pound retail weight per pound of carcass weight, per Poore, J., and T. Nemecek.
“Reducing Food’s Environmental Impacts Through Producers and Consumers.” Science 360 (2018): 987–92.Ben Putman et al., “A Retrospective Assessment of US Pork Production: 1960 to 2015,” 2018, 57, https://cpb-us-e1.wpmucdn.com/...files/2019/01/Final_Report-1mgj3oo.pdf.Sofia Khan et al., “Comparative Environmental LCA of the Impossible Burger With Conventional Ground Beef Burger” (2019), https://impossiblefoods.com/sustainable-food/burger-life-cycle-assessment-2019.} This level of mitigation would be equivalent to 18 percent of emissions from the entire US agricultural sector in 2019.^{US EPA, “Greenhouse Gas Inventory Data Explorer.”}

Achieving even a fraction of this reduction would require overcoming steep technological, scientific, and financial barriers that make widespread adoption infeasible today. It would necessitate not just private, but also public sector research and development (R&D), such as through the US Department of Agriculture (USDA)’s National Institute of Food and Agriculture. It would also be critical to expand USDA conservation programs that help producers adopt sustainable practices; to develop new forms of support such as tax incentives; and to reform regulations that stymie development and commercialization of feed additives and genetically modified crops. Such changes would have large climate benefits and, in some cases, reduce land use, improve water and air pollution, and even improve animal welfare, making “clean,” low-carbon beef production possible.

In the next section, we describe the carbon footprint of US beef. We then assess the mitigation potential of technologies and management changes, such as those that receive substantial policy discussion (e.g., regenerative grazing), have nationally representative mitigation estimates published in peer-review journals or reports from leading nonprofit research groups, or whose estimates could reasonably be scaled up to the national level. We divide our assessment into four sections based on the mechanism for mitigation: (1) how cattle are fed, (2) pasture and rangeland management, (3) animal health, and (4) manure management. In the final section, we outline federal policy options to accelerate progress toward low-carbon beef production.

Beef's Carbon Footprint

Beef production generates approximately 3.2 percent of US GHG emissions^{C. Alan Rotz et al., “Environmental Footprints of Beef Cattle Production in the United States”; US EPA, “Greenhouse Gas Inventory Data Explorer.”} and 5.9 percent of global emissions.^{Helen Harwatt, “Including Animal to Plant Protein Shifts in Climate Change Mitigation Policy: A Proposed Three-Step Strategy,” Climate Policy 19, no. 5 (2019): 533–41, https://doi.org/10.1080/146930.... 2018.1528965.US value calculated only for traditional beef breeds (excluding dairy) using 2013–2017 averages and consistent GWP values. Global estimate circa 2016, including beef from culled dairy cows.} The intensive, feedlot-based, large-scale production common in the US makes American beef less carbon- and land-intensive than beef in most other countries. However, it still has a large footprint compared with other protein sources. US beef has a carbon footprint of about 23.3 kg CO2e/kg product — more than five times larger than that of pork and 10 times that of chicken.^{Ben Putman et al., “A Retrospective Analysis of the United States Poultry Industry: 1965 Compared with 2010,” Agricultural Systems 157 (October 2017): 112, https://doi.org/10.1016/j.
agsy.2017.07.008.Putman et al., “A Retrospective Assessment of US Pork Production,” 42.Rotz et al., “Environmental Footprints,” 10.All values per kg carcass weight. Values for pork and chicken converted from kgCO2e per kg live weight to per kg carcass weight by dividing by dressing percentages of 72 and 71 percent, respectively.}

In the United States, beef calves are typically raised on pasture or rangeland for 6–10 months (Figure 4).^{Beef2Live.com, “Stages of Beef Production,” 2021.} Some are transferred directly to feedlots, but most calves are placed first with a “stocker” or “background” operation, where they graze on grass or other forages, such as hay, for about 4–6 months.^{Judith L. Capper, “Is the Grass Always Greener? Comparing the Environmental Impact of Conventional, Natural and Grass-Fed Beef Production Systems,” Animals 2, no. 4 (April 10, 2012): 132,
https://doi.org/10.3390/ani202...Judith L. Capper, “Is the Grass Always Greener? Comparing the Environmental Impact of Conventional, Natural and Grass-Fed Beef Production Systems,” Animals 2, no. 4 (April 10, 2012): 132, https://doi.org/10.3390/ani202...Rotz et al., Figure 2.} Almost all beef cattle in the United States are then feedlot finished — that is, they live the last 3-10 months of their lives in enclosed feeding operations that utilize grain and other feed to increase weight gain.^{USDA Environmental Research Service, “Cattle & Beef: Sector at a Glance,” https://www.ers.usda.gov/topic....} On average, U.S. beef cattle spend 41% of their 18 month-long lives in feedlot systems when taking finishing and backgrounding operations into account.^{Matthew N. Hayek and Rachael D. Garrett, “Nationwide Shift to Grass-Fed Beef Requires Larger
Cattle Population,” Environmental Research Letters 13, no. 8 (July 25, 2018): 4, https://doi.org/10.1088/1748-9....} Approximately one-fifth of beef is produced from dairy cows, such as bulls raised for beef.^{Rotz et al., “Environmental Footprints,” 9.} However, in this report, we consider only the emissions and mitigation potential for beef produced from beef cattle.

Emissions Sources

The largest share of US beef emissions comes from enteric fermentation (Figure 5).^{Rotz et al., “Environmental Footprints,” Figure 3.} Enteric fermentation is a digestive process in which microbes decompose and ferment food, releasing the potent greenhouse gas methane (CH4). Methane has about 28 times the global warming potential of carbon dioxide over 100 years.^{US Environmental Protection Agency, “Inventory of U.S. Greenhouse Gas Emissions and Sinks:
1990-2019, Chapter 5: Agriculture.”Although traditional metrics, such as global warming potential, may overstate some effects of constant methane emissions and understate the impact of new methane emissions, they are widely used to facilitate comparisons among different GHGs. Please see discussion in the Sixth Assessment Report of the Intergovernmental Panel on Climate Change, Climate Change 2021: The Physical Science Basis (2021): 7–123. https://www.ipcc.ch/report/ar6...} Most of the methane from beef production (about 77 percent) arises when cattle are grazing on pasture and rangeland before being transferred to feedlots.^{Rotz et al., “Environmental Footprints,” 7.} In fact, contrary to popular belief, the majority of all life-cycle GHG emissions from beef production come from grazing cattle, largely from cows used for breeding, rather than from cattle in feedlots.^{Rotz et al., Figure 2.}

Pasture, range, and cropland also account for a large share of beef’s carbon footprint.^{Rotz et al., 9.} Cattle deposition of urine and manure on pasture and the application of fertilizer to crops for animal feed release nitrous oxide,^{Rotz et al., 7.} a gas with about 265–298 times the global warming potential of carbon dioxide over a 100-year timescale.^{US Environmental Protection Agency, “Understanding Global Warming Potentials.”}

Manure, which releases both methane and nitrous oxide, accounts for a smaller, yet significant, portion. Upstream sources such as electricity, fertilizer, and fuel production each account for a small portion, as do anthropogenic CO2 sources such as on-farm fuel combustion.^{Rotz et al., Figure 3a.}

Carbon Footprint in Context

Several researchers have recently suggested that beef’s carbon footprint may be larger than most studies and the US Environmental Protection Agency (EPA) estimate. Studies that measure atmospheric GHG levels above animal operations and model what portion came from animal operations versus other sources (e.g., gas fields) estimate that all farmed animals in the US emit 39–81 percent more methane than EPA estimates.^{Matthew N. Hayek and Scot M. Miller, “Underestimates of Methane from Intensively-Raised Animals Could Undermine Goals of Sustainable Development,” Environmental Research Letters (May 19, 2021), https://doi.org/10.1088/1748-9326/ac02ef.} If methane emissions for beef cattle have been underestimated by the same amount, the carbon intensity of US beef production could be as high as 28.4–33.9 kgCO2e/kg.^{Per-kilogram carcass weight. Assumes that enteric fermentation and manure methane emissions from beef production are 39 to 81 percent higher than estimated by Rotz et al.} However, estimates based on atmospheric monitoring, as with bottom-up estimates such as the EPA’s, contain large uncertainties. Further, little work has been done either to incorporate them into official emissions reporting or to reconcile estimates conducted using different methods.^{Personal communication, C. Alan Rotz (May 2021) and Matthew Hayek (June 2021).}

Nevertheless, the carbon intensity of US beef is lower than that of any other major beef-producing country, in large part owing to its intensive, large-scale, grain-finished production (Figure 6).^{Food and Agriculture Organization of the United Nations, FAOSTAT, “Emissions Intensities” (2019), xtagstartza href="Comparison is made of emissions intensities across top five beef-producing countries. Values from FAOSTAT are for 2017 and include only emissions from enteric fermentation and manure. Values from GLEAM are for 2010 and include emissions from feed production, land-use change, and other sources.} While only 7–13 percent of beef is produced with feedlots globally, almost all US beef is.^{Anne Mottet et al., “Livestock: On Our Plates or Eating at Our Table? A New Analysis of the Feed/Food Debate,” Global Food Security 14 (September 2017): 1, https://doi.org/10.1016/j.gfs.2017.01.001.} Only 3 percent of US beef cattle are grass-finished, that is, fed exclusively on grass and other forage throughout their life.^{Renee Cheung and Paul McMahon, “Back to Grass: The Market Potential for U.S. Grassfed Beef” (Stone Barns Center for Food and Agriculture, 2017), 5, 54, https://www.stonebarnscenter.o...
uploads/2017/10/Grassfed_Full_v2.pdf.} Grain- or feedlot-finished animals are more efficient at turning feed into meat, gain weight faster, and reach a higher overall carcass weight. They thus produce less enteric methane per kilogram of meat produced and have a lower net carbon footprint.^{Marian Swain, Linus Blomqvist, James McNamara, et al., “Reducing the Environmental Impact of Global Diets,” Science of the Total Environment 610–11 (January 2018): 1208, https://doi.org/10.1016/j.scitotenv.2017.08.125.}

US feedlot beef cattle also use less agricultural land than grass-finished beef cattle or those from other countries. Although feeding grain to cattle requires cropland to grow the feed, it reduces the amount of land needed for grass and forage production. In fact, shifting current levels of beef production from grain-fed exclusively to grass-fed production would require additional land and cattle, encroach on native ecosystems, and potentially compete with the human food supply.^{Hayek and Garrett, “Nationwide Shift to Grass-Fed Beef Requires Larger Cattle Population.”}

In addition, beef production requires substantial water use for feed production^{Brian D. Richter et al., “Water Scarcity and Fish Imperilment Driven by Beef Production,” Nature Sustainability 3, no. 4 (April 1, 2020): 319–28, https://doi.org/10.1038/s41893....} and generates air and water pollution.^{Rotz et al., “Environmental Footprints,” 10.} Manure and fertilizer release excess nitrogen in the form of nitrate (NO3) in waterways, causing eutrophication, which can lead to dead zones in fresh and marine water bodies, suffocating fish, and causing economic harm to fisheries. These agricultural fertilizers also emit ammonia (NH3), which contributes to the development of particulate matter, a type of harmful air pollution.

How Cattle Are Fed

Feed has a large impact on the carbon footprint of beef production. What and how cattle are fed influence methane emissions through enteric fermentation, the productivity (and thus lifespan) of cattle, and the level of land use and emissions involved in producing feed.

Despite the well-known problems with concentrated animal feed operations (CAFOs), such as animal welfare and water pollution from manure, by increasing weight gain and reducing the amount of time cattle spend grazing and emitting methane, feedlot finishing results in lower net land use and GHG emissions than finishing cattle just on pasture.^{Capper, “Is the Grass Always Greener?”, 136, Table 2.}

But feedlot finishing requires growing extensive crops for feed. Corn and soy production for beef cattle feed uses about 15 million acres of land.^{Hayek and Garrett, “Nationwide Shift to Grass-Fed Beef Requires Larger Cattle Population,” 6.Calculated by dividing January 2021 PRX estimates of beef consumption of corn and soy meal by crop yields and then multiplying by the ratio of planted-to-harvested acreage (PRX Grain Market Overview, 2021). This estimate is only for production of feed crops for beef cattle and excludes feed production attributable to dairy cattle. This estimate is within the range of data derived from peer-reviewed papers: 12.35 million acres for all cropland for beef (Hayek and Garrett, 2018), 18.28 million acres for all cropland for beef finishing extrapolated from Michigan operations (Stanley et al., 2018), and 17.34 million acres for concentrate feed for beef production, derived by updating the model used in Eshel et al. (2014) with 2007–2017 data.} The fertilizer and fuel used to grow these and other feed crops, such as alfalfa, generate as much as 18 percent of beef’s total carbon footprint.^{Senorpe Asem-Hiablie et al., “A Life Cycle Assessment of the Environmental Impacts of a Beef System in the USA,” International Journal of Life Cycle Assessment 24, no. 3 (March 8, 2019): 450, https://doi.org/10.1007/s11367... calf, and feedlot operations,Rotz et al., “Environmental Footprints.”} Still, reducing the climatic impacts of beef would mean improving how cows are fed.

We highlight four pathways for reducing GHG emissions through feed: improving the production of feed crops, changing from conventional feed and developing novel feeds; utilizing feed additives that reduce emissions; and breeding cattle for improved feed efficiency. Although some of these mitigation practices, such as cover cropping, are feasible today, others require future breakthroughs such as the development of crops with enhanced root systems that could sequester more carbon in the soil.

Cutting Emissions from Conventional Feed Crop Production

As shown above, feed crop production has a large environmental footprint. Even so, improving farming practices and adopting new technologies could reduce the emissions caused by feed production.

Although beef cattle in the United States consume mostly grass and other forage, corn and soy account for about 11 percent of their diet and have an outsized environmental impact. In fact, up to 15 percent of feed consumed in beef production derives from sources that could also be used for human consumption.^{Rotz et al., 7.} Feed crop production involves substantial irrigation and pesticide application, and contributes to air pollution, water pollution, soil erosion, and greenhouse gas emissions.^{Asem-Hiablie et al., “A Life Cycle Assessment,” Figure 3.} Corn production alone accounts for about 3 percent of beef’s total carbon footprint, stemming largely from nitrous oxide (N2O) emissions from fertilizer use.^{We assume that all grain concentrate is corn feed, and all other feed (besides grazed forage, harvested forage, and grain concentrate) is distillers grains from corn ethanol. Because some of the grain and other feed are not actually from corn, this calculation is conservative, giving us an upper estimate of emissions from corn.Rotz et al., “Environmental Footprints,” 7, Tables 1 and 2.}

One effective approach for reducing N2O emissions is to apply fertilizer when and where plants need it and to find ways for plants to need less nitrogen. Precision farming tools, such as nitro-gen sensors, enable farmers to better match fertilizer application to crop needs and thereby reduce excess nitrogen levels. Nitrification and urease inhibitors can also help. These compounds can be mixed with fertilizer to reduce the amount of nitrogen converted to nitrous oxide and thereby keep more of the nitrogen in the soil to feed the crops. We estimate that full adoption of precision farming equipment and inhibitors on corn and soy farms could reduce feed production emissions in 2030 by about 2 MMT CO2e.^{Calculated by multiplying emissions reduction values from fertilizer management practices in Fargione et al. (2018) and inhibitors from Eagle et al. (2017), adjusted for existing adoption of practices (Fargione et al, 2018; Kanter and Searchinger, 2018) by emissions from fertilizer manufacturing and application, calculated using methods from Fargione et al. and 2018 fertilizer data (Roberto Mosheim, “Fertilizer Use and Price” (October 30, 2019), USDA Economic Research Service. https://www.ers.usda.gov/data-...).}

In addition, widespread planting of cover crops — to cover the soil outside the regular growing season — could sequester nearly another 2 MMT CO2e.^{Calculated by multiplying annual mitigation per hectare (0.32 Mg/ha/year) (Christopher Poeplau and Axel Don, “Carbon Sequestration in Agricultural Soils Via Cultivation of Cover Crops – a Meta-analysis,” Agriculture, Ecosystems and Environment 200 (2015), 33) by planted area of corn and soy for beef production, adjusted for the portion of acreage for corn-for-grain (5 percent), corn-for-sileage
(25 percent), and soy (8 percent) already planting cover crops (Steven Wallander et al., “Cover Crop Trends, Programs, and Practices in the United States,” USDA Economic Research Service Bulletin no. 222 (February 2021), 18). To be conservative, we assume that 15 percent for corn acreage is already planted cover crops, taking the mean of corn-for-grain and corn-for-silage.} No-till agriculture, though it reduces erosion and has other environmental benefits, is unlikely to substantially sequester carbon, according to recent evidence.^{Jean-François Soussana et al., “Matching Policy and Science: Rationale for the ‘4 per 1000 – Soils for Food Security and Climate’ Initiative,” Soil and Tillage Research, December 2017, 4, https://doi.org/10.1016/j.stil...} Grazing animals on cropland or rotating cropland and pasture can also increase soil carbon sequestration,^{Kelsey M. Brewer and Amélie C.M. Gaudin, “Potential of Crop–Livestock Integration to Enhance Carbon Sequestration and Agroecosystem Functioning in Semi-Arid Croplands,” Soil Biology and Biochemistry (Elsevier Ltd., October 1, 2020), 1, 3, https://doi.org/10.1016/j.soil....Sandeep Kumar et al., “Facilitating Crop–Livestock Reintegration in the Northern Great Plains,” Agronomy Journal 111, no. 5 (September 1, 2019): 2145, https://doi.org/10.2134/agronj....} but this practice remains poorly studied with insufficient research to estimate national mitigation potential.^{Brewer and Gaudin, “Potential of Crop-Livestock Integration,” 2.}

Adoption of each practice faces unique barriers. For instance, precision farming equipment and cover cropping may save farmers money in the long run but have high up-front costs. Other practices, like applying nitrification inhibitors, cost farmers additional money without generating substantial revenue. To address these barriers, technical and financial assistance should be tailored to help different types of farmers adopt a variety of practices and technologies.

Technological breakthroughs could also provide greater mitigation. Ensuring all corn and soy feed comes from crop varieties with “enhanced root systems” that sequester more carbon than current varieties could sequester 12 MMT CO2 per year.^{Calculated by multiplying the carbon sequestration per acre value from the center of the range reported in Mulligan et al., subtracting 28 percent based on the need for more nitrogen additions and related emissions to sequester this carbon, and multiplying by planted area of corn and soy for beef production (James Mulligan et al., “CarbonShot: Federal Policy Options for Carbon Removal in the United States,” 2020, 53-54, https://wriorg.s3.amazonaws.co...).} Scientists at the Salk Institute, Penn State, and elsewhere are working to research, develop, and test such crops, though they estimate it will take years if not decades. Further R&D could enable the production of fertilizer using clean energy sources instead of natural gas.

Methane Reducing Animal Additives

Feeding beef cattle substances to reduce their enteric methane emissions is a relatively novel emissions mitigation strategy. These feed additives have been touted as potential breakthrough technologies to improve the environmental impact of agriculture but face barriers related to cost and regulatory approval.

The products can be split into two main categories: methane inhibitors and rumen modifiers. Rumen (methane) inhibitors act on and disrupt the process of methane production in the cattle rumen, whereas rumen modifiers change the makeup of the rumen in ways that limit methane production.^{M. Honan, X. Feng, J. M. Tricarico, and E. Kebreab, “Feed Additives as a Strategic Approach to Reduce Enteric Methane Production in Cattle: Modes of Action, Effectiveness and Safety,” Animal Production Science (2021), https://doi.org/10.1071/AN2029....}

The effectiveness of feed additives depends on the kind of production system in use (Figure 7). Cattle that are fed high-fiber forage diets — which includes cattle in the cow-calf and stocker phases of production — have greater rates of enteric fermentation.^{Breanna M. Roque et al., “Red Seaweed (Asparagopsis taxiformis) Supplementation Reduces Enteric Methane by over 80 Percent in Beef Steers,” ed. James E. Wells, PLoS One 16, no. 3 (March 17, 2021): e0247820, https://doi.org/10.1371/journa....} These high-fiber diets also limit the efficacy of some products. Recent research has indicated that methane inhibitors are about two-thirds as effective in reducing emissions from cattle while grazing than while in a feedlot,^{Roque et al.} though some rumen modifiers may be more effective with such high-fiber diets.^{Roque et al.Personal communication, Ermias Kebreab, May 2021.} Regardless, more research is necessary to study the efficacy of additives in grazing systems as well as how best to provide them to grazing cattle, such as by incorporating them into mineral blocks.

We estimate that feeding Asparagopsis taxiformis, produced with carbon-intensive electricity, to all beef cattle grazing and in feedlots could reduce beef’s total carbon footprint by about 7 and 2 percent, respectively. Feeding all beef cattle higher levels of A. taxiformis—if it is as effective in all operations as it has been in studies—could reduce beef’s carbon footprint as much as 14 percent. Using A. taxiformis produced with low-carbon electricity and with twice the regular bromoform content could reduce emissions as much as 33 percent.^{We used estimates from Rotz et al. (2019) of enteric methane emissions as a percentage of beef production’s total carbon footprint and findings from Roque et al. (2021) for estimates of the efficacy of low (0.25% OM) and high (0.5% OM) amounts of A. taxiformis in reducing methane intensity. For finishing operations, we assume the efficacy observed for low-forage diets. For other operations, we assume the efficacy observed for high-forage diets. We assumed that 35 percent of beef cattle are backgrounded in feedlots, based on personal communication with C. Alan Rotz (May 2021). We assume A. taxiformis production with high-carbon electricity and low-carbon electricity with high-bromoform concentration offsets 60% and 4% of avoided emissions, respectively. Estimates do not include emissions from changes in CO2 emissions from cattle, or from reduced need for feed due to increases in feed conversion efficiency.}

3-NOP

3-nitrooxyproponal, or 3-NOP, is one of the most studied methane inhibitors. 3-NOP works by neutralizing an enzyme in the cattle rumen that is key to the process of methanogenesis. Whereas the application and use of 3-NOP depend in part on the production system, it has been relatively effective across the board without animal health or productivity tradeoffs.^{J. Dijkstra et al., “Short Communication: Antimethanogenic Effects of 3-Nitrooxypropanol Depend on Supplementation Dose, Dietary Fiber Content, and Cattle Type,” Journal of Dairy Science 101, no. 10 (October 1, 2018): 9041–47, https://doi.org/10.3168/jds.20....Alexander N. Hristov et al., “An Inhibitor Persistently Decreased Enteric Methane Emission from Dairy Cows with No Negative Effect on Milk Production,” Proceedings of the National Academy of Sciences 112, no. 34 (2015): 10663–68, https://doi.org/10.1073/pnas.1....} Currently, DSM, a Dutch-based multinational company, produces 3-NOP in Europe under the brand name Bovaer. Still, 3-NOP has yet to gain regulatory approval in the United States or European Union for use as a feed additive.^{Agnieszka de Sousa, “One Answer to Cutting Potent Cow Emissions Is Awaiting EU Nod,” Bloomberg (December 3, 2020), https://www.bloomberg.com/news....}

Asparagopsis taxiformis

Asparagopsis taxiformis—a type of red seaweed—is another well-studied methane inhibitor. A. taxiformis contains bromoform and other chemical compounds that interfere in methanogenesis and reduce cattle methane production. Still, questions remain as to the long-term effectiveness of A. taxiformis — there is concern that cattle rumens could adapt to the halogenic compounds and return to previous methanogenesis levels, especially if cattle are fed A. taxiformis for their entire lives.^{Penn State. “Seaweed Feed Additive Cuts Livestock Methane but Poses Questions.” ScienceDaily. Accessed August 1, 2021. https://www.sciencedaily.com/r....} Optimistically, one recent study found that A. taxiformis can remain effective for at least 147 days, or roughly the length of feedlot finishing.^{Roque et al., “Red Seaweed (Asparagopsis taxiformis) Supplementation,” 12.}

The widespread use of A. taxiformis is limited by a lack of seaweed production in the United States as well as the need for regulatory approval as a feed additive.^{Sandra Vijn, Devan P. Compart, Nikki Dutta, et al., “Key Considerations for the Use of Seaweed to Reduce Enteric Methane Emissions from Cattle,” Frontiers in Veterinary Science 7, no. 597430 (December 2020), 3.} While seaweed cultivation is a large industry in parts of East Asia, A. taxiformis is not currently cultivated at scale.^{Salon, “Can We Grow Enough Seaweed to Help Cows Fight Climate Change?” (June 9, 2019), https://www.salon.com//2019/06...fight-climate-change_ partner/.} Blue Ocean Barns, a US-based company that aims to produce A. taxiformis for use as a feed additive, claims they will be able to produce enough of the seaweed to be used for all beef and dairy cattle in the United States by 2030.^{CBS Sacramento, “Seaweed Solution In Climate Change Fight,” May 21, 2021, https://www.youtube. com/watch?v=8GP5jQuK6A0.} Additionally, emissions from the production and transportation of A. taxiformis could offset a portion of the emissions reduction that results from feeding it to cattle. Life-cycle assessments of A. taxiformis are needed to fully understand its total mitigation potential.

Mootral

Mootral is a blend of essential oils and other ingredients—including garlic and citrus extracts —that works as a rumen modifier to reduce the methane production of cattle. Essential oils work to reduce methanogenesis by changing the chemical makeup of the rumen and limiting the growth of methanogens—bacteria that break down materials in the cattle rumen and release methane.^{Honan et al., “Feed Additives as a Strategic Approach to Reduce Enteric Methane Production in Cattle.”} Research on essential oils has found potential methane reductions from extracts of lemongrass, citrus, garlic, oregano, and other products, but no single essential-oil compound has shown high anti-methanogenic properties across multiple studies. Mootral, on the other hand, is relatively well studied, effective, and being commercialized.

Breeding Cattle for Feed Efficiency

Cattle breeding can play a significant role in reducing the greenhouse gas emissions related to enteric fermentation. From 1961 to 2018, enteric methane emissions per unit of beef decreased in the United States by 36 percent, due in large part to improved breeding as well as management (Figure 8).^{Capper, “The Environmental Impact of Beef Production in the United States.Megan M. Rolf et al., “Genomics in the United States Beef Industry,” Livestock Science 166, no. 1
(2014): 88, https://doi.org/10.1016/j.livs....} Breeding cattle that grow bigger and faster has reduced the number of cattle and the resulting methane emissions needed to produce each pound of beef. Despite the success of past breeding in increasing cattle size and growth rate, there remains a wide range of efficiencies with which individual cattle convert feed to meat; therefore, the potential remains for further feed efficiency gains through breeding.^{Tim Searchinger et al., “Creating a Sustainable Food Future: A Menu of Solutions to Feed Nearly 10 Billion People by 2050” (Washington, DC, 2019), 173, https://wrr-food.wri.org/sites...files/2019-07/WRR_Food_Full_Report_0.pdf.}

Breeding for feed efficiency is necessary because measuring methane emissions directly is time-consuming and expensive. As a result, a large body of research has focused on identifying proxy traits for methane emissions, many of which are metrics of feed efficiency, which are easier to measure and therefore improve through breeding. However, experts disagree, and more research is needed to identify which feed efficiency metrics are the best proxies for methane emissions.^{D. A. Kenny et al., “Invited Review: Improving Feed Efficiency of Beef Cattle – The Current State of the Art and Future Challenges,” Animal 12, no. 9 (2018): 1815–26, https://doi.org/10.1017/S17517...; Jim Ormond, “Geoengineering Super Low Carbon Cows: Food and the Corporate Carbon Economy in a Low Carbon World,” Climatic Change, July 13, 2020, 1–19, https://doi.org/10.1007/s10584....}

The use of a technology called genomic breeding also has the potential to improve cattle breeding by speeding up the rate of genetic change in desired traits and could be used to improve breeding for feed efficiency in order to reduce methane emissions. Genomic breeding involves “reading” the entire genetic code from an individual animal — which comprises thousands of genes — and pairing this information with measurements of traits to allow researchers to identify genes that impact those traits. Once there is a clear link established, breeders can make decisions using genetic information as a proxy for desired traits without having to measure them. This is particularly useful for traits that are time-consuming or expensive to measure, such as the level of enteric methane emissions.

Genomic selection can reduce the time between generations from five or more years to less than two years and increase the rate of genetic change in desired traits by an estimated 12–100 percent, based on research in dairy cattle.^{J. E. Pryce and H. D. Daetwyler, “Designing Dairy Cattle Breeding Schemes under Genomic Selection: A Review of International Research,” Animal Production Science 52, no. 2–3 (2012): 107–114, https://doi.org/10.1071/AN11098.L. R. Schaeffer, “Strategy for Applying Genome-Wide Selection in Dairy Cattle,” Journal of Animal Breeding and Genetics 123 (2006): 218–223.} Genomic breeding technology is already widely used in the US dairy industry, with over half of the artificial inseminations stemming from genomically tested bulls.^{Adriana García-Ruiz et al., “Changes in Genetic Selection Differentials and Generation Intervals in US Holstein Dairy Cattle as a Result of Genomic Selection,” Proceedings of the National Academy of Sciences of the United States of America 113, no. 28 (July 12, 2016): E3995, https://doi.org/10.1073/pnas.1519061113.} In comparison, adoption of both artificial insemination and genomic selection in beef cattle breeding is lagging.

We estimate that if 50 percent of US beef cattle breeders selected for feed efficiency, US beef emissions would fall by about 1.6 percent by 2030 and up to 4.5 percent as breeding efforts advanced over the course of years or decades. If such a program also encouraged the use of genomic breeding for increasing feed efficiency, we estimate a larger decrease of roughly 2.4 percent by 2030.^{We used estimates from Rotz et al. (2019) of enteric methane emissions as a percentage of beef production’s total carbon footprint and an estimate from Alford et al. (2006, 816) that a maximum of 30 percent adoption of breeding for feed efficiency could reduce methane emissions in an individual herd by 15.9 percent over 25 years. For the increase in the rate of breeding progress using genomic breeding, we used the low-end estimate of 50 percent from García-Ruiz et al. (2016, E3999).} Achieving such widespread adoption would likely require federal support; for example, Ireland’s Irish Cattle Breeding Federation program supports genomic breeding.^{B. W. Wickham, P. R. Amer, D. P. Berry, et al., “Industrial Perspective: Capturing the Benefits of Genomics to Irish Cattle Breeding,” Animal Production Science 52 (2012): 172–79, https://doi.org/10.1071/AN11166.} International collaboration and data sharing could also advance the use of genomic sequencing in US beef cattle breeding.^{Wickham et al., “Industrial Perspective.”}

Rangeland and Pasture Management

Beef production involves vast amounts of land for grazing. Nearly 800 million acres are used for livestock grazing in the United States, equivalent to 35 percent of the land area of the continental US.^{Daniel P. Bigelow and Allison Borchers, Major Uses of Land in the United States, 2012, EIB-178, US Department of Agriculture, Economic Research Service (August 2017), 26, www.ers.usda.gov. Cattle are grazed primarily on pasture and rangeland. A small portion of cattle are grazed on cropland and in forested areas.Bigelow and Borchers, Major Uses of Land in the United States, 26.} Rangelands are uncultivated grasslands, shrublands, and other lands on which the native vegetation is suitable for grazing. They are managed primarily by grazing. Pasture, on the other hand, is land that is more intensively managed, for instance, through seeding, irrigation, and fertilization, to grow grasses and other forage plants for grazing animals.^{US Environmental Protection Agency, “Agricultural Pasture, Rangeland and Grazing | Agriculture,” accessed August 7, 2021, https://www.epa.gov/agriculture/agricultural-pasture-rangeland-and-grazing.}

Widespread changes in how this land is grazed, planted, and otherwise managed could technically sequester enough carbon to offset up to 42 percent of beef’s carbon footprint per year by 2030, but any decline in cattle productivity would negate much of the benefit (Figure 9).^{We assume that grazing optimization, legume planting, and silvopasture are not mutually exclusive and that the mitigation potential of each can be summed.} These changes are also costly, difficult, and impractical on most grass and rangeland. Additional research may be able to reduce costs, such as that of establishing trees on pasture. But widespread change will also require new incentives and support for ranchers and land managers, such as financial and technical assistance.

Regenerative Grazing

A variety of grazing practices and regimes, often referred to as “regenerative,” can potentially increase soil carbon (C). For example, adaptive multi-paddock (AMP) grazing involves frequent rotation of animals at high densities between different enclosures of land called “paddocks.” By allowing plants to recover after being grazed, protecting soils, and promoting plant growth, AMP can reduce soil erosion, improve animal productivity, and increase soil C sequestration.^{Paige L. Stanley et al., “Impacts of Soil Carbon Sequestration on Life Cycle Greenhouse Gas Emissions in Midwestern USA Beef Finishing Systems,” Agricultural Systems 162 (2018): 250, https://doi.org/10.1016/j.agsy.2018.02.003; Samantha Mosier et al., “Adaptive Multi-Paddock Grazing Enhances Soil Carbon and Nitrogen Stocks and Stabilization through Mineral Association in Southeastern U.S. Grazing Lands,” Journal of Environmental Management 288 (March 2021): 112409, https://doi.org/10.1016/j.jenvman.2021.112409.} Although rarely discussed, AMP and similar practices can be implemented during the cow-calf phase for cattle that are later sent to a feedlot, in addition to those that are grass-fed their entire lives.

Although individual studies have found high rates of soil C sequestration, we believe that the national mitigation potential of regenerative grazing is too uncertain to estimate at this time. Estimates of soil C sequestration rates from AMP and related grazing approaches vary widely. Some studies have observed rates as high as 1.45 metric tons/hectare per year,^{Stanley et al., “Impacts of Soil Carbon Sequestration on Life Cycle Greenhouse Gas Emissions,” 253.} while others have found no difference in soil carbon stocks between prairie with AMP and light continuous grazing.^{Mimi Hillenbrand et al., “Impacts of Holistic Planned Grazing with Bison Compared to Continuous Grazing with Cattle in South Dakota Shortgrass Prairie,” Agriculture, Ecosystems and Environment 279 (2019): 156–68, https://doi.org/10.1016/j.agee.2019.02.005.} This study did, however, find that paddocks with AMP grazing had other ecological improvements (e.g., lower bare soil levels, higher vegetation biomass, and lower nonnative species abundance) compared with paddocks with light or heavy continuous grazing. Further complications in estimating average sequestration rates or national mitigation potential are that sequestration rates are highly context-dependent and that sequestered carbon can be lost due to management or climatic changes, fires, or conversion of grassland to crop-land.^{Cécile M. Godde et al., “Soil Carbon Sequestration in Grazing Systems: Managing Expectations,” Climatic Change 161, no. 3 (2020): 2, https://doi.org/10.1007/s10584-020-02673-x.} Finally, shifting any amount of land from feedlot to grass-finished production, whether regenerative or not, typically reduces beef production per acre.^{Stanley et al., “Impacts of Soil Carbon Sequestration on Life Cycle Greenhouse Gas Emissions USA Beef”; Hayek and Garrett, “Nationwide Shift to Grass-Fed Beef Requires Larger Cattle Population.”} This is not inherently problematic, given that much rangeland is not suitable for other purposes such as crop production.^{Mottet et al., “Livestock.”} However, to meet constant or growing global beef demand with less production in the United States would require other countries to increase production. New beef production, especially in the tropics, often involves deforestation and other types of land-use change. Therefore, shifting from feedlot production to grass-finished production could have a high carbon cost.^{Searchinger et al., “Creating a Sustainable Food Future,” 2.} More research is needed to determine how this carbon cost compares with the climate benefits from regenerative grazing.

However, it is possible to roughly estimate the sequestration potential of grazing in ways that increase the production of forages, like grasses, that cattle graze on. For example, reducing the number of cattle grazing per acre (the stocking rate) in areas that are overgrazed and increasing it in areas that are undergrazed could increase both forage growth and livestock production (Figure 10). If carried out on roughly one-fifth of grazing land, this adjustment of stocking rates could sequester up to 11 MMT CO2e/year.^{Fargione et al., “Natural Climate Solutions for the United States,” Supplementary Materials, 38, 71.}

Planting Legumes on Pasture

Planting legumes (plants such as clover and alfalfa that add nitrogen to the soil) in pasturelands can marginally increase carbon sequestration. Although seeding some rangeland with legumes is possible, we only consider seeding on pasture because legume seeding often requires active management such as weed control, requires specific environmental conditions such as sufficient rainfall levels not found on much rangeland,^{Devii R. Rao, “Seeding Part 1: Sow, You Want to Seed Your Rangeland or Irrigated Pasture, ” Livestock & Range - ANR Blogs, accessed August 1, 2021, https://ucanr.edu/blogs/blogcore/postdetail.
cfm?postnum=29029.} and risks replacing native plants on range-land.^{Martín Jaurena et al., “The Dilemma of Improving Native Grasslands by Overseeding Legumes: Production Intensification or Diversity Conservation,” Rangeland Ecology and Management 69, no. 1 (January 1, 2016): 35–42, https://doi.org/10.1016/j.rama.2015.10.006.} Although planting legumes on pasture can result in significant carbon sequestration per acre, it also adds nitrogen to the soil and thus can generate additional emissions of nitrous oxide — a potent greenhouse gas.

Accounting for both sequestration and nitrous oxide emissions, planting legumes would result in net sequestration on only 13 percent of planted pasture in the United States — nearly 14 million acres.^{Fargione et al., “Natural Climate Solutions for the United States,” Supplementary Materials, 38, 71.} The mitigation potential of planting legumes is thus relatively low: about 7 MMT CO2e/year.^{Fargione et al., Supplementary Materials, 71.} In addition, the costs associated with seeding, labor, weed control, and machinery generally prohibit ranchers from planting legumes without external financial incentives.^{Fargione et al., Supplementary Materials, 38.}

Silvopasture

The integration of trees into pasture, known as silvopasture, can also sequester a large amount of carbon in trees and soil.^{Mark Baah-Acheamfour et al., “Trees Increase Soil Carbon and Its Stability in Three Agroforestry Systems in Central Alberta, Canada,” Forest Ecology and Management 328 (2014): 131–39, https://doi.org/10.1016/j.foreco.2014.05.031.} Planting trees on the 69 million acres of pastureland that have historically supported forest cover^{Susan C. Cook-Patton et al., “Lower Cost and More Feasible Options to Restore Forest Cover in theContiguous United States for Climate Mitigation,”One Earth3, no. 6 (2020): 739–52, https://doi.org/10.1016/j.oneear.2020.11.013.} could sequester 81 MMT CO2e.^{Mulligan et al., “CarbonShot: Federal Policy Options for Carbon Removal in the United States” (2020), 19, www.wri.org/publication/carbonshot-federal-policy-options-carbon-removal-united-states.Estimate based on a sequestration rate of 1.34 metric tons CO2/acre/year in “moist” areas of the United States, and 0.66 metric tons CO2/acre/year in dry areas (A. Swan et al., “COMET-Planner: Carbon and Greenhouse Gas Evaluation for NRCS Conservation Practice Planning,” Colorado State University (2015): 33, Comet-planner.nrel.colostate.edu/COMET-Planner_Report_Final.pdf); personal communication, Alex Rudee, May 2020. Total mitigation estimate from Mulligan et al. (2020) for silvopasture is 39 percent of the total mitigation estimated for reforestation from Cook-Patton et al. (2020), in part reflecting lower assumed tree cover.} Although estimates of carbon sequestration vary widely — from at least 0.55 to 1.90 metric tons C/hectare per year^{Rattan Lal et al., “The Carbon Sequestration Potential of Terrestrial Ecosystems,” Journal of Soil and Water Conservation 73, no. 6 (2018): 148A, https://doi.org/10.2489/jswc.73.6.145A.} — even the most conservative estimates indicate that silvopasture adoption could sequester more carbon than other grazing practices.

However, widespread silvopasture adoption may have little to no net climate benefit. Planting trees on pasture — though it provides shade and can reduce heat stress for cattle — typically reduces forage and cattle production per acre in the US and other temperate environments.^{Pent, “Over-yielding in temperate silvopastures: a meta-analysis.”} We estimate that the reduction in cattle productivity from conversion to silvopasture would, by shifting production to other regions, actually increase net emissions.^{We estimate change in emissions using the Carbon Benefits Calculator, Version 1.0 with the default COC and 4% discount rate (Searchinger et al. 2020). We use baseline beef yields and production emissions from Pelletier et al. (2010). We assume silvopasture beef yields are 31% lower than baseline yields, the mean difference across studies of beef in Pent (2020), and that there is a +2.9 tCO2e/ha/yr change in carbon stock under silvopasture based upon Mulligan et al. (2020) and Cook-Patton et al. (2020). Future research should also account for the carbon opportunity cost/benefit of timber production from silvopasture.
Searchinger, Timothy D, Stefan Wirsenius, Tim Beringer, and Patrice Dumas. “Assessing the Efficiency of Changes in Land Use for Mitigating Climate Change.” Nature 564, no. 7735 (December 12, 2018): 249–53. https://doi.org/10.1038/s41586-018-0757-z.
Pelletier, Nathan, Rich Pirog, and Rebecca Rasmussen. “Comparative Life Cycle Environmental Impacts of Three Beef Production Strategies in the Upper Midwestern United States.” Agricultural Systems 103, no. 6 (2010): 380–89. https://doi.org/10.1016/j.agsy...} In addition, harvesting the timber, which is often necessary to make silvopasture financially feasible, could reduce carbon storage including in the soil as tree root systems die and decompose.^{Personal Communication, Dr. Diane Mayerfeld, June 6, 2021.} Yet harvesting timber from silvopasture could also reduce timber harvest and deforestation in other countries or places. Lacking an estimate of the potential carbon benefits of this dynamic, we exclude silvopasture from our estimates of total climate mitigation potential.

Financial, logistical, and behavioral barriers also limit adoption. Scaling up silvopasture without external subsidies or incentives is expensive for producers, costing upwards of $40 to $186 per acre, even when accounting for potential income from timber.^{Mulligan et al., “CarbonShot: Federal Policy Options for Carbon Removal in the United States,” 80.} In addition, planting and maintaining trees requires labor, specialized skills, and landowner interest in changing management practices.

Animal Health

Every cattle death due to illness results in unnecessary resource use and environmental impacts. Even for animals that recover from illness, being sick often reduces their growth rate, and thus increases the GHG emissions per unit of the final product. Improvements in veterinary and animal health practices that reduce
illness and death could mitigate the overall GHG emissions of beef production.

Bovine respiratory disease (BRD) is the largest disease-related cause of herd loss in the United States and is responsible for 45–55 percent of all cattle deaths in the feedlot.^{Personal Communication, Dr. Diane Mayerfeld, June 6, 2021.} Even when BRD is not fatal, the disease decreases the average daily weight gain of cattle,^{Mulligan et al., “CarbonShot: Federal Policy Options for Carbon Removal in the United States,” 80.} increases feed consumption,^{Johnson and Pendell, “Market Impacts of Reducing the Prevalence of Bovine Respiratory Disease,” 4.} and ultimately increases enteric methane emissions. We estimate that if the number of cattle that die due to BRD decreased by 50 percent, emissions from US beef production would fall 1 percent.^{We assumed rates of death due to BRD as well as rates of chronic BRD diagnosis as reported by Johnson and Pendell.}

Vaccination for Bovine Respiratory Disease

Current BRD vaccination practices are inefficient and poorly suited to preventing the disease. Although evidence shows that BRD vaccines are efficacious when delivered in the cow-calf phase, only 39 percent of cow-calf operations vaccinate their cattle. Instead, most BRD vaccinations occur either immediately upon arrival at stocker and background operations, or immediately when cattle arrive at feedlot facilities for finishing.^{A. M. O’Connor et al., “A Systematic Review and Network Meta-Analysis of Bacterial and Viral Vaccines, Administered at or Near Arrival at the Feedlot, for Control of Bovine Respiratory Disease in Beef Cattle,” Animal Health Research Reviews 20, no. 2 (December 1, 2019): 143–62,
https://doi.org/10.1017/S1466252319000288.John T. Richeson et al., “Vaccination Management of Beef Cattle: Delayed Vaccination and Endotoxin Stacking,” Veterinary Clinics of North America – Food Animal Practice (W.B. Saunders, November 1, 2019), 576, https://doi.org/10.1016/j.cvfa.2019.07.003.John T. Richeson and T. Robin Falkner, “Bovine Respiratory Disease Vaccination: What Is the Effect of Timing?,” Veterinary Clinics of North America – Food Animal Practice (W.B. Saunders, July 1, 2020), 474, https://doi.org/10.1016/j.cvfa.2020.03.013.} These cattle are often chronically stressed and have suppressed immune systems that make the vaccine less efficacious. Instead of relying on mass vaccination of cattle at feedlots — 90 percent of US feedlots vaccinate cattle upon arrival^{O’Connor et al., “A Systematic Review,” 143.} — the industry should further research and consider vaccinating the majority of cattle at cow-calf operations, where cattle are less stressed and the vaccine can be more efficacious.^{Richeson et al., “Vaccination Management of Beef Cattle,” 578.}

Additional research into vaccine administration could also help to stem the rise of antimicrobial resistance. Many microbes that commonly cause BRD have developed a high level of antimicrobial resistance.^{John E. Ekakoro et al., “Drivers, Alternatives, Knowledge, and Perceptions towards Antimicrobial Use among Tennessee Beef Cattle Producers: A Qualitative Study,” BMC Veterinary Research 15, no. 1 (January 7, 2019), https://doi.org/10.1186/s12917-018-1731-6.} Mass treatment and prophylaxis for BRD represent a large portion of antimicrobial use in cattle.^{Samuel E. Ives and John T. Richeson, “Use of Antimicrobial Metaphylaxis for the Control of Bovine Respiratory Disease in High-Risk Cattle,” Veterinary Clinics of North America – Food Animal Practice (W.B. Saunders, November 1, 2015), 2, https://doi.org/10.1016/j.cvfa.2015.05.008.Keith Edward Baptiste and Niels Christian Kyvsgaard, “Do Antimicrobial Mass Medications Work?
A Systematic Review and Meta-Analysis of Randomised Clinical Trials Investigating Antimicrobial Prophylaxis or Metaphylaxis against Naturally Occurring Bovine Respiratory Disease,” Pathogens and Disease (Oxford University Press, October 1, 2017), https://doi.org/10.1093/femspd/ftx083.} Improving vaccination is an important part of disease prevention and can subsequently prevent growing antimicrobial resistance.^{S. J. LeBlanc et al., “Major Advances in Disease Prevention in Dairy Cattle,” Journal of Dairy Science 89, no. 4 (2006): 1267–79, https://doi.org/10.3168/jds.S0022-0302(06)72195-6.}

Breeding for Disease Resistance

Breeding for disease resistance is another potential approach to reduce the incidence of BRD since some cattle are less susceptible than others to the disease.^{Donagh P. Berry, “Genetics of Bovine Respiratory Disease in Cattle: Can Breeding Programs Reduce the Problem?,” Animal Health Research Reviews 15, no. 2 (December 28, 2014): 151–56, https://doi.org/10.1017/S1466252314000292.} This approach could benefit from genomic sequencing that could identify the genetic markers for disease resistance without necessarily tracking disease susceptibility over the lifetime of many individuals.

Genetic engineering has also proven to be a useful tool to increase disease resistance in livestock, notably with mastitis-resistant cows and porcine reproductive and respiratory syndrome (PRRS)–resistant pigs. ^{Christine Burkard, et al., “Pigs Lacking the Scavenger Receptor Cysteine-Rich Domain 5 of CD163 Are Resistant to Porcine Reproductive and Respiratory Syndrome Virus 1 Infection,” Journal of Virology 92, no. 16 (2018): 1–14, https://doi.org/10.1128/JVI.Robert J. Wall et al., “Genetically Enhanced Cows Resist Intramammary Staphylococcus aureus Infection,” Nature Biotechnology 23, no. 4 (2005): 445–51, https://doi.org/10.1038/nbt1078.} Genetic engineering could help develop cattle that are resistant to BRD. However, the process of creating a new genetically engineered animal can be lengthy, likely taking over a decade. Additionally, for genetic engineering to be a useful tool in cattle breeding, US regulations would have to dramatically change to lower the astronomical barriers to bringing such an animal to market.^{Emma Kovak, “Proposed Regulatory Changes May Usher in New Era for Genetically Engineered Animal Agriculture,” The Breakthrough Institute (2021), https://thebreakthrough.org/issues/food/regulatory-changes-may-usher-in-new-era-for-ge-animal-agriculture.}

Manure Management

Manure from beef cattle operations produces approximately 7.4 percent of emissions from beef production, or roughly 16 MMT CO2e/year.^{Rotz et al., “Environmental Footprints of Beef Cattle Production in the United States.”} Like other emission-intensive aspects of beef production, most manure-related emissions originate from cow-calf and stocker operations, where cattle defecate on pastures, releasing both methane and nitrous oxide into the atmosphere. About 70 percent of manure-related emissions, or roughly 11 MMT CO2e/year, stem from feedlot production systems used at finishing or backgrounding operations, which is where emissions from manure can most easily be reduced.^{Rotz et al., “Environmental Footprints.”} Even so, reducing GHG emissions related to beef cattle manure from feedlots will not be easy, or cheap.

Currently, cattle manure from feedlots usually follows one of two paths: composting, to be sold as soil amendments, or dry-lot storage, followed by direct application on nearby crop fields. Approximately 20 percent of manure from feedlots is composted and 78 percent stored as is until field application.^{These numbers came from calculations based on Senorpe Asem-Hiablie, C. Alan Rotz, Robert Stout, et al. “Management Characteristics of Cow-Calf, Stocker, and Finishing Operations in Kansas, Oklahoma, and Texas,” The Professional Animal Scientist 31, no. 1 (2015), 9; Asem-Hiablie, Rotz, Stout, et
al., “Management Characteristics of Beef Cattle Production in the Northern Plains and Midwest Regions of the United States,” The Professional Animal Scientist 32 (2016), 746; Asem-Hiablie, Rotz, Stout, et al., “Management Characteristics of Beef Cattle Production in the Western United States,” The Professional Animal Scientist 33 (2017), 468; and Asem-Hiablie, Rotz, Stout, et al., “Management Characteristics of Beef Cattle Production in the Eastern United States,” The Professional Animal
Scientist 34, no. 4 (2018), 321.} The remaining 2 percent is managed in lagoons and through other methods.

If barriers to adoption were overcome, composting (or anaerobically digesting beef cattle manure that is not already composted) could moderately reduce emissions related to beef manure on feedlots. Nevertheless, given feedlot manure’s relatively small emissions share, even major shifts in manure management could only marginally reduce beef’s total carbon footprint emissions.^{We used Rotz et al. (2019) for estimates of emissions from manure, and personal correspondences with C. Alan Rotz to disaggregate indirect and direct N2O emissions from manure emissions. Emissions reductions from composting estimates were taken from IPCC (2019) and do not consider emissions related to nutrient runoff from either composted or raw manure field application. We used Bartram and Barbour (2004) for estimating GHG emissions reductions from anaerobic digesters.}

Composting

Composting increases the value of manure as a soil amendment because it makes the nutrients more bioavailable to plants and can be used for more-targeted applications in crop production than simply applying manure to a field.

Composting can mitigate greenhouse gas emissions from manure storage,^{E. Pattey, M. K. Trzcinski, and R. L. Desjardins, “Quantifying the Reduction of Greenhouse Gas Emissions as a Result of Composting Dairy and Beef Cattle Manure,” Nutrient Cycling in Agroecosystems 72, no. 2 (June 2005): 173–87, https://doi.org/10.1007/s10705-005-1268-5.} but its effects depend on weather, climate, and the specific composting practice used, among other factors. In some circumstances, composting can even increase net GHG emissions. However, windrow manure composting, in which manure is piled into long rows along with other organic material and is aerated through manual or automatic turning, emits up to 30 percent less CO2e than typical dry-lot storage, depending on the frequency of aeration.^{IPCC, “2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories, Volume 4: Agriculture, Forestry and Other Land Use: Emissions From Livestock and Manure Management,” Forestry 4 (2019): 67-69, 88-92, 96-98, http://www.ipcc-nggip.iges.or.jp/public/2006gl/index.html.We used Rotz et al. (2019) for estimates of emissions from manure, and personal correspondences with C. Alan Rotz to disaggregate indirect and direct N2O emissions from manure emissions. Emissions reductions estimates were taken from IPCC (2019) and do not consider emissions related to nutrient runoff from either composted or raw manure field application.} Incorporating other materials into the composting process, such as biochar and dried grass, could further reduce emissions.^{Koki Maeda et al., “Mitigation of Greenhouse Gas Emission from the Cattle Manure Composting Process by Use of a Bulking Agent,” Soil Science and Plant Nutrition 59, no. 1 (February 2013): 96–106, https://doi.org/10.1080/003807..., carbon dioxide (CO2}) Beyond reducing emissions related to manure storage, composting manure could have significant soil carbon sequestration benefits when applied to either cropland or rangeland.^{Whendee L. Silver, Sintana E. Vergara, and Allegra Mayer, “Carbon Sequestration and Greenhouse Gas Mitigation Potential of Composting and Soil Amendments on California’s Rangelands,” California Fourth Climate Change Assessment (August 2018).}

Nonetheless, adopting composting practices is difficult. Costs remain high — especially for intensively aerated windrow composting, the practice most effective at reducing emissions.^{IPCC, “2019 Refinement to the 2006 IPCC Guidelines,” 68.} Composting involves up-front investments as well as ongoing operation and maintenance costs. At the same time, successful adoption would require more frequent manure removal from cattle lots to realistically reduce GHG emissions.

Anaerobic Digesters

Anaerobic digestion is another option for reducing manure emissions (Figure 12). Anaerobic digesters break down manure in enclosed environments in the absence of oxygen. They produce methane-rich biogas that can be sold or burned on site to create energy. The remaining nutrient-rich material following the process of digestion, called digestate, may also be used as a soil amendment. Anaerobic digesters reduce emissions from cattle manure by collecting methane and nitrous oxide. Even though biogas from digesters is combusted, the total emissions are less than those from business-as-usual management.^{ICF, “Greenhouse Gas Mitigation Options and Costs for Agricultural Land and Animal Production within the United States ICF International” (2013), 3.1 et seq., https://www.usda.gov/oce/climate_change/mitigation_technologies/GHG_Mitigation_Options.pdf.Deborah Bartram and Wiley Barbour, “Estimating Greenhouse Gas Reductions For a Regional Digester Treating Dairy Manure” (Washington, DC: Wiley Barbour Environmental Resources Trust, 2004), https://www3.epa.gov/ttnchie1/conference/ei13/ghg/bartram.pdf.}

Digesters are used predominantly in dairy operations where frequent manure collection supplies digesters with constant manure.^{Ben W. Thomas et al., “Anaerobically Digested Cattle Manure Supplied More Nitrogen with Less Phosphorus Accumulation than Undigested Manure,” Agronomy Journal 109, no. 3 (May 1, 2017): 836–44, https://doi.org/10.2134/agronj2016.12.0719.} Beef operations, on the other hand, often do not collect manure frequently, meaning that inorganic materials, e.g., bedding, rocks, dirt, or other debris, collect alongside the cattle manure, preventing effective anaerobic digestion. Shifting from current manure management strategies to anaerobic digestion would require altering broader manure collection practices.

Cost is also a significant barrier to adoption. Up-front capital investment costs can remain high,^{US Environmental Protection Agency, “Market Opportunities for Biogas Recovery Systems at U.S. Livestock Facilities” (2018), www.epa.gov/agstar.} and the operation and management of digesters represent both a financial and a practical burden for feedlot operators.

If barriers associated with manure collection, cost, and operation can be overcome, anaerobic digestion could significantly reduce emissions from manure. In such a breakthrough scenario, anaerobic digestion of all manure that is not currently composted could reduce manure emissions by roughly 34 percent.^{We used Bartram and Barbour (2004) for estimates of mitigation from converting to anaerobic digestion.}

Policy Recommendations

From cover cropping to feed additives, substantial opportunities remain to mitigate GHG emissions from US beef production. Yet all these practices require further R&D, regulatory approval, or economic incentives to enable widespread adoption.

Fund Research and Development of Breakthrough Technologies

The federal government has long been a major driver of agricultural innovation through the R&D funded and conducted by the USDA. This federally supported innovation has increased agricultural production while reducing the land use of US agriculture and producing fewer emissions per unit of agricultural product. Although the federal government already funds R&D aimed at improving beef production systems, increasing federal spending on agricultural R&D will be necessary to drive the decarbonization of the sector. Technological breakthroughs would not only reduce the cost and improve the effectiveness of existing practices like applying nitrification inhibitors to cropland but also expand their total mitigation potential, reducing beef’s carbon footprint up to an additional 30 percent.

The federal government should fund R&D targeting the following technologies:

• Enhanced root crops that can sequester more carbon without reducing yields. The National Academies of Sciences, Engineering and Medicine estimates that $40–$50 million per year for at least 20 years would be needed to conduct the research required to commercialize enhanced root crops.^{National Academies of Sciences, Engineering, and Medicine, Negative Emissions Technologies and Reliable Sequestration, (Washington, DC: The National Academies Press, 2019), 124, https://doi.org/ 10.17226/25259}
• Feed additives that can limit methane emissions from enteric fermentation. The USDA National Institute of Food and Agriculture ^{Penn State, “Feed Supplement for Dairy Cows Cuts Their Methane Emissions, ” National Institute of Food and Agriculture,” accessed August 1, 2021, https://nifa.usda.gov/announcement/feed-supplement-dairy-cows.} and the Agricultural Research Service currently fund research on feed additives.^{US Department of Agriculture, “Project: USDA ARS: Improve Nutrient Management and Efficiency of Beef Cattle and Swine,” accessed August 1, 2021, https://www.ars.usda.gov/resea... project/?accnNo=433257&fy=2018.} Expanding research to improve the effectiveness of feed additives in pasture-based systems could dramatically reduce long-term methane emissions from beef cattle.
• Breeding, including genomic breeding, to improve feed efficiency and disease resistance of beef cattle. Existing USDA programs like the Agricultural Genome to Phenome Initiative (AG2PI)^{US Department of Agriculture, “Agricultural Genome to Phenome Initiative (AG2PI),” accessed September 21, 2021, https://nifa.usda.gov/program/genome-phenome-initiative.} could effectively fund and conduct such research. The initiative was authorized for funding in the 2018 Farm Bill but has received minimal funding to date.
• Anaerobic digesters that can be used to break down beef cattle manure despite its low moisture content. Part of this research could investigate designing and demonstrating ways to remove manure from beef cattle facilities, such as by using feed lanes, that would facilitate manure collection and anaerobic digestion.^{Personal communications with C. Alan Rotz, May 2021.}

Support Adoption of Existing of Existing Mitigation Practices

The federal government has several mechanisms for supporting agricultural producers in adopting technologies and practices that improve environmental outcomes. These include the Environmental Quality Incentive Program (EQIP),^{US Department of Agriculture, “Environmental Quality Incentives Program,” accessed September 21, 2021, https://www.nrcs.usda.gov/wps/portal/nrcs/main/national/programs/financial/eqip/.} the Conservation Technical Assistance Program (CTA),^{US Department of Agriculture, “Technical Assistance,” accessed September 21, 2021,
https://www.nrcs.usda.gov/wps/portal/nrcs/main/national/programs/technical/.} and the Conservation Stewardship Program (CSP)^{US Department of Agriculture, “Conservation Stewardship Program,” accessed September 21, 2021, https://www.nrcs.usda.gov/wps/...financial/csp/.} operated by USDA’s Natural Resource Conservation Service (NRCS), as well as tax credits, discounts on federal crop insurance, and regulations.

To incentivize the adoption of low-carbon beef technologies, Congress should:

• Increase funding for NRCS conservation programs like EQIP, CTA, and CSP. NRCS programs already support most low-carbon beef practices, such as the use of nitrification inhibitors, cover crops, and silvopasture, but are oversubscribed.^{Ken Root, “USDA Conservation Programs: Underfunded and Oversubscribed ,” Iowa Agribusiness Network (July 3, 2017), https://www.iowaagribusinessradionetwork.com/usda-conservation-programs-underfunded-and-oversubscribed/.} Increasing top-line funding could also open incentive funds for practices like AMP grazing and cropland grazing practices, effectively increasing existing NRCS conservation incentives aimed at beef production. ^{US Department of Agriculture, “Develop Grazing Plan to Treat Resource Issues, Improve Soil Health,” accessed August 8, 2021, https://www.nrcs.usda.gov/wps/portal/nrcs/detail/?cid=NRCSEPRD1376423.}
• Establish a federal manure management program to award grants or other support for non-anaerobic digester manure management practices that reduce GHG emissions. Such a program could be modeled after the California Department of Food and Agriculture’s Alternative Manure Management Program,^{California Department of Food and Agriculture, “Alternative Manure Management Practices (AMMP),” accessed September 21, 2021, https://www.cdfa.ca.gov/oei/ammp/.} which has been successful at incentivizing shifts in manure management at dairy facilities.
• Support pilot or experimental anaerobic digester projects on beef cattle operations. This can be done through multiple paths: reestablishing an investment tax credit for anaerobic digesters and other nutrient recovery projects, as proposed in Senators Brown and Roberts’ Agricultural Environmental Stewardship Act;^{Senate.gov. “Brown, Roberts Introduce Bipartisan Legislation to Expand Biogas Production, Help Create a New Market for Farmers” (September 24, 2019), https://www.brown.senate.gov/newsroom/press/release/brown-roberts-introduce-bipartisan-legislation-to-expand-biogas-production-help-create-a-new-market-for-farmers.} creating a manure transport program to ensure that smaller operations can transport manure to nearby digesters, similar to an existing program in Maryland; and expanding coverage of clean fuel standards, similar to California’s Low Carbon Fuel Standard, which incentivize investment in the renewable natural gas that anaerobic digestion can produce.^{Caroline Grunewald, “How to Save US Dairy,” The Breakthrough Institute (2020), 10, https://thebreakthrough.org/articles/save-us-dairy.}
• Establish a rebate program to incentivize the purchase of precision farming equipment such as variable-rate applicators that could reduce emissions from feed crop production. A rebate program could help extend incentives to producers who either require greater financial assistance or have been unable to procure support from existing programs due to oversubscription. This could be modeled after programs like California’s Funding Agricultural Replacement Measures for Emissions Reductions (FARMER) program.^{Alex Smith, “Stimulus Through Farm Conservation,” The Breakthrough Institute (2020),http://s3.us-east2.amazonaws.com/uploads.thebreakthrough.org/Farm_Conservation_Programs.pdf.}

Reform Agency Review of Low-Carbon Innovations

Methane-inhibiting animal products and genetic modification of plants or animals face high regulatory barriers to commercialization.

The US Food and Drug Administration (FDA) has classified products that reduce methane emissions from animals, such as 3-NOP, as animal drugs rather than food additives,^{Corbin Hiar, “New Powder Could Cut Cow Burp Emissions. Will FDA Say OK?,” E&E News
(June 14, 2021), https://www.eenews.net/stories/1063734619.} which typically include enzymes, probiotics, and other feed supplements.^{Gita Cherian, “Feed Additives,” in A Guide to the Principles of Animal Nutrition, 1st ed. (Oregon State University, 2020), https://open.oregonstate.education/animalnutrition/chapter/chapter-19/.} The FDA’s review of animal drugs is typically more extensive, costly, and time-consuming than for additives. The agency’s review process for drugs typically takes 7–10 years,^{Stacy Sneeringer, Maria Bowman, and Matthew Clancy, The U.S. and EU Animal Pharmaceutical Industries in the Age of Antibiotic Resistance, ERR-264, US Department of Agriculture, Economic Research Service (2019): 12.} with companies spending $30.5 million on average to receive approval.^{“Approval and Regulation of Animal Medicines,” Animal Health Institute, accessed June 17, 2021, https://ahi.org/approval-and-regulation-of-animal-medicines/.} For instance, the FDA process to review 3-NOP has taken more than four years and is expected to last several more.^{Hiar, “New Powder Could Cut Cow Burp Emissions.”}

Instead, the FDA should:

• Regulate methane-reducing animal products as feed additives/ingredients, as the EU currently does.^{Hiar, “New Powder.”} The FDA would still review them to ensure that the products are safe for animals, workers, and consumers but require fewer years of research into how effective the products are at reducing methane emissions. The approval process for such additives typically takes three to five years.^{Treena Hein, “US: More Efficient Approval of New Feed Ingredients,” All About Feed,
  https://www.allaboutfeed.net/m...ficient-approval-of-new-feed-ingredients.} The FDA may have the authority to make this change. A drug is defined as a product “intended to affect the structure or any function of the body of…animals,”^{US Food and Drug Administration, “Is It a Cosmetic, a Drug, or Both? (Or Is It Soap?),” accessed August 1, 2021, https://www.fda.gov/cosmetics/....} and methane inhibitors technically affect the microbes in the rumen. Nevertheless, congressional action to create a separate FDA track for environmentally beneficial feed products would provide researchers and product developers with greater regulatory certainty, helping foster innovation.
• Prioritize review of products meant to reduce methane emissions. The FDA has the discretion to prioritize types of applications. For example, it is currently prioritizing additives meant to mitigate African Swine Fever.^{Hein, “US: More Efficient Approval of New Feed Ingredients.”}

The USDA also regulates the approval of genetically engineered crops and animals, which may be needed to develop enhanced root crops or to breed low-methane cattle.
The USDA should:

• Regulate genetically engineered crops based on the risk they pose. Although the USDA has loosened regulation for some types of genetically engineered crops, regulation remains too tied to the type of genetic modification rather than the risks it poses.
• Create product-based regulation of genetically engineered animals that determines the level of risk and regulation based on the familiarity of animal, trait, and intended environment.

Additional reforms will be necessary to improve other issues in the livestock and meat processing industries, such as worker safety, local pollution, and animal welfare. However, through a combination of support for research, regulatory reform, and producers adopting various practices and technologies, policy makers have the power to make lower-carbon-emitting, “clean” beef production a reality.