Beef production, which is one of the largest industries in the agriculture sector, is a significant source of greenhouse gas (GHG) emissions. The main source is from a process known as enteric fermentation, whereby the microbial decomposition of feed in the forestomach, or the rumen, of the animal results in the production and release of a substantial quantity of methane (CH₄). Other sources of GHGs associated with beef production include manure storage and land application, which result in CH₄ and nitrous oxide (N₂O) emissions and the production of feed crops, which results in N₂O and carbon dioxide (CO₂) emissions. Carbon dioxide emissions are primarily due to the manufacturing and operation of farm machinery and vehicles, the manufacturing of fertilizers and agrochemicals, as well as the manufacturing of farm buildings and electrical power generation. Additional emissions are associated with a change in land management practices, which can influence carbon stored in the soil, resulting in either CO₂ emissions or CO₂ sequestration, as soil organic carbon. Land use change can also be a significant source of CO₂ as a result of the loss of soil carbon, as well as above-ground biomass associated with land degradation and/or deforestation. To consider the overall impact of CH₄, N₂O and CO₂, it is essential to take into account the respective global warming potential of these gases. The magnitude of the emissions of these gases can then be reported in terms of carbon dioxide equivalents (CO₂e), which is typically expressed over a 100 year time horizon [1] as (1)

Greenhouse gas emissions, expressed in terms of a mass of CO₂e, are accepted as the reporting unit for the carbon footprint.

Carbon footprint estimates should include all direct and indirect sources and sinks of GHGs associated with a product or service. If the carbon footprint of a product or service contributes significantly to the overall environmental impact and no pollution swapping occurs, then reducing the carbon footprint is one way to minimize the environmental impact. Improved farming practices usually lead to an increase in production efficiency, resulting in less GHG emissions per unit product [2]. The methodology for estimating the carbon footprint, which is still evolving [3], is based on Life Cycle Assessment (LCA) guidelines such as ISO 14044 [4] and PAS2050 [5], typically combined with GHG emission algorithms recommended by the IPCC [6]. In the case of the carbon footprint of beef cattle, a common functional unit is a kg of live weight (LW), which is the weight of the animal at the farm gate. Shrunk live weight (SLW), which is the weight of cattle at the entrance of the slaughterhouse, is also useful if the carbon footprints of other co-products are of interest. Alternatively, some studies report the GHG emissions per kg of carcass weight (CW), which does not include the hide, head, feet and guts. The CW:LW ratio varies substantially (0.68–0.45) depending on a range of factors including breed, sex, time of last feeding, and cold versus warm carcass weight. Cows tend to have a ratio near 0.50 with steers near 0.60. The objective of this review was to present and discuss the carbon footprint of beef cattle for some of the major beef exporting countries in order to compare the carbon footprint of cattle at an international scale. Because of the interest in deriving the carbon footprint of beef cattle hides, we also present the carbon footprint of beef cattle co-products at the exit gate of a slaughterhouse in Canada based on an economic allocation.

The global beef cattle population was about 1.4 billion animals in 2010. About 25% was in Latin America, 28% in Asia, 20% in Africa, 10% in North America, 6% in the EU, 3% in Oceania and 6% in the Middle East. India and Brazil accounted for about 30% of the global beef population [7]. The ratio of beef production to the cattle population is a good indicator of the beef production efficiency. In 2010, North America (Canada, the United States and Mexico) with about 10% of the global cattle population produced nearly 25% of global beef supply. The United States was the largest beef producer in the world, with about 12 million tonnes produced on a stock of 94 million head. The high production efficiency has been attributed to the low-cost corn from the Midwest of the United States which is used for finishing cattle in feedlots [8]. The situation in western Canada is similar to the United States except that barley is used as a finishing ration. In Mexico, beef production is more extensive and feedlots are not as common. In 2010, the EU-27 produced 13% of the beef with 6% of the cattle population [7]. Other world leaders in beef export, such as South America and Oceania grow and finish beef cattle mainly on grass [8]. In 2010, Brazil was the second largest beef producer after the United States, while Argentina and Uruguay were also major players on the global beef market, particularly Argentina which was the world’s fourth largest beef exporting nation. Brazil, despite its low beef productivity [9], was the largest beef exporter, followed by Australia, the United States and Argentina [7].

Cattle producers are continuously improving production efficiency through breeding and improved management strategies. The degree of their success depends on many factors such as whether the cattle are raised intensively or extensively, whether the cattle are grass or grain finished, the quality and type of pasture, the management of feed crops and their location, the amount and type of chemical inputs, and the amount and type of energy used for farm operations. Animal productivity or the rate of weight gain is an important determinant of the carbon footprint of beef production. Grass-finished cattle tend to reduce their carbon footprint by consuming grasses grown requiring less fertilizer and chemical inputs. This effect is however, usually nullified from a carbon footprint perspective because their rate of weight gain is less than a grain-fed, feedlot finished animal, and they must spend more time on pasture, consuming feed and producing CH₄ in order to reach market weight [10]. The fact that the CH₄ emission factor per unit of feed consumed is substantially smaller for grain-finished cattle compounds this effect [11].

In the following section, we will present the carbon footprint of beef cattle for some of the major beef producing nations of the world. The carbon footprints of major cattle rearing (India) and beef producing (China and the Russian Federation) nations are not included in this review because of a lack of data. However, based on management strategies that predominate in these countries and the rate of beef production per head [7] leads us to believe that the carbon footprint of beef cattle in these countries is likely to be greater than the carbon footprint of most of the nations discussed in this study.

The magnitude of the carbon footprint associated with the production of any product varies depending on the extent, or boundary, of the system selected, which defines the up and downstream processes that are included in the assessment. In this study, we define the boundary conditions as the ‘cradle to farm gate’, meaning that all GHG emissions occurring prior to cattle leaving the farm gate will be included. The IPCC [6] methodology is the basis for the carbon footprint estimates; it relies on a tiered system which is based on the availability of emission factors associated with activity data. Tier I relies on default empirical emission factors published by the IPCC [6], and is the simplest approach available. Tier II is still empirical in nature, as the emission factors are usually derived from experimental data specific to a country [12]. Tier III is the most complex method and relies on process-based models [13]. Moving to a higher Tier of estimation is considered a good practice.

Methane emissions are the primary factor influencing the carbon footprint of cattle. They have been reported to account for 55% [14] to 92% [15] of the carbon footprint, with the majority of the CH4 emissions arising from enteric fermentation. Depending on a suite of factors including feed quality, management practices, animal activity, and animal genetics, 3% [11] to 9.5% [16] of the energy consumed by cattle is lost as CH4. Adopting feeding practices that minimize the loss of CH4 improves animal productivity and reduces the carbon footprint of cattle.

Greenhouse gas emissions have been estimated on a national basis for many animal-based agricultural products, such as milk [17,18], beef [14,19], pork [20,21] and poultry [22,23] but many estimates are at a much smaller scale. For example, Beauchemin et al. [24] used Holos, a whole-farm GHG emissions model, developed by Agriculture and Agri-Food Canada to estimate the GHG emissions associated with a small operation typical of western Canadian beef production. Other tools such as DairyGHG [25] and the Cool Farm Tool [26] have been used to calculate the GHG emissions of animal-based agricultural products at the farm scale.

Cattle herd structure and management can have a substantial impact on the carbon footprint. In most developed, beef producing countries, the beef industry usually consists of cow-calf, stocker, feedlot and dairy systems, with an exchange of stock occurring between each of these systems (Figure 1). Although this structure is common in most nations, the management and proportion of cattle in the various categories varies considerably from country to country. For example, there is a greater proportion of dairy cows to beef cows in the EU as compared to the U.S. Feeding strategies can also have a substantial impact on the carbon footprint. For example, grain-fed feedlot cattle versus grass-fed cattle generally have a significantly different carbon footprint because of the time required to reach a certain weight. In grass-fed cattle, the quality of pasture can also affect productivity and the carbon footprint.

Although many carbon footprint estimates are available in the literature, it is often difficult to compare results between studies (Table 1). There are many reasons for this, including a lack of consistency in terms of the boundaries chosen for the studies. For instance, in most regions, the beef and dairy sectors are linked, as surplus dairy calves are fattened for beef production and culled dairy cows also provide beef. However, the allocation of the environmental burdens between the beef and dairy sector for many of these cases is inconsistently performed. Casey and Holden [19] have shown that the allocation of emissions from dairy cows and dairy calves that enter in the beef production system greatly reduced GHG emissions from beef production in Ireland from 13.0 to 9.8 kg CO₂e per kg LW. The degree of reduction is a function of the number of calves sourced from the dairy sector and the number of culled cows to the total number of slaughtered animals. Comparison of the carbon footprint between studies is difficult for other reasons, including an inconsistent inclusion of sources of emissions. For instance, calculations rarely include GHG emissions associated with capital goods such as the manufacturing of farm machinery [14]. Further, the impact of land use change and land management change on soil carbon is frequently not considered in the carbon footprint, but if it is, there are significant challenges to determine the appropriate amortization period and spatial extent associated with the emissions from land use change [9]. As a result, most of the carbon footprint estimates associated with beef production reported in the literature are limited in some way, making comparison between studies challenging.

The live weight carbon footprint is only one aspect of the carbon footprint of cattle. From the farm gate, cattle must be transported to the slaughterhouse and be slaughtered; two activities which result in additional GHG emissions. The emissions of these gases were estimated in Canada using the (Cafoo)² calculator which was initially developed for estimating the carbon footprint of Canadian dairy products [43] but which has now been adapted to calculate the carbon footprint of Canadian meat products. The transportation emissions were estimated assuming that the cattle are transported from the farm or feedlot to the slaughterhouse in a standard four-axle cattle transportation trailer. A vehicle of this size can accommodate approximately 43 heifers or steers [44] for an average payload of 26.5 tonnes. GHG emissions for a vehicle of this type were estimated using the SimaPro life LCA software as 0.123 kg CO₂e per tonne km assuming an empty return trip. An average one-way trip from farm to slaughterhouse of 311 km was assumed for short haul [45] and 1081 km for long haul [46]. We assumed that 15% of the cattle are hauled over a long distance (representing Canadian interprovincial exchange of stock) and that on average, weight loss of 4% and 8% occurred during short and long haul, respectively [44]. We then estimated the GHG emissions to bring the cattle to the slaughterhouse to be 53.5 g CO₂e per kg shrunk LW (SLW), or 51 g CO₂e per kg farm LW.

In addition to meat, the slaughtering process provides valuable co-products and non-marketable residues managed with slaughterhouse wastes and wastewater [47]. Table 3 presents the average mass balance within the Canadian beef industry context in 2006, considering a dressing percentage (CW/SLW) of 60% and a cutting yield of 63 kg primal cuts meat per 100 kg Carcass Weight (CW). Bones and fat from cutting, and some dressing residues are intended for rendering, while hide and offal are marketed. Wastes are the non-recovered blood, paunch content, gut, feet, and account for around 21% of the shrunk live weight. The energy required to slaughter cattle has been estimated from the disaggregation of national energy use breakdown for the whole Canadian meat manufacturing sector [48]. Available U.S. statistics [49] permitted the estimation of the national share of the red meat slaughtering sector for electricity and fossil fuels use. The data were further allocated to meat types (cattle, calves, pork, and sheep and lamb) and then to Canadian Provinces according to Canadian slaughter and weight statistics. The following national average energy use was derived for the slaughtering of 3549 thousand cattle in 2006: 152,279.1 MWh electricity, 1,443.1 TJ natural gas, 5.5 TJ heavy fuel oil, 27.8 TJ middle distillates, and 6.7 TJ propane. The GHG emissions from energy use were found to be 69 g CO₂e per kg SLW using the LCA software. Additionally, GHG emissions have also been calculated for water use, assuming an average rate of 2.7 L per kg SLW, i.e., 1.6 m³ per head [50,51] and from wastewater treatment, assuming an average 20% volume loss from water input, and Canadian beef slaughterhouse wastewater characteristics [52]. Wastewater GHG emissions were estimated at 54 g CO₂e per kg SLW. Other GHG emissions related to refrigerant use and fugitive losses and due to chemical cleaners input have not been included due to lack of available data. As well, no impact or credit from the use or the sale of biogas recovered from anaerobic wastewater treatment, nor from sludge valorization has been accounted for. Therefore, the GHG emissions associated with transportation to the slaughterhouse and slaughtering operations were 0.18 kg CO₂e per kg SLW (0.17 kg CO₂e per kg LW at farm), or about 2% of on-farm GHG emissions. Although we have excluded any other emissions associated with transportation (e.g., from cow-calf ranch to feedlot), this would only have a minor impact on the emissions intensity.

The livestock industries in many countries have managed to reduce their beef cattle carbon footprints substantially during the last 30 years through the adoption of improved management practices and by so doing they have reduced the carbon footprint of beef, and cattle by-products proportionally. Whereas some of the reductions in the carbon footprints have resulted from low roughage diets, grain-finished feedlot cattle rely partly on land that is capable of growing feed grains. Conversely, grass-finished cattle typically exploit areas with little or no suitability for annual crops. Beef production, therefore, competes with other possible uses for higher quality land when it is grain-finished. For example, the diversion of grains intended for feedlot cattle to non-ruminants (hogs or poultry) would result in a lower GHG emission intensity on a unit of protein basis [61]. Both the grassland and grain producing land could be used for a combined system of grass-finished beef and non-ruminants. Furthermore, this system could produce animal protein (and cow hides) with a lower carbon footprint than using all of that land for grain-finished beef. Including this type of livestock interaction in the carbon footprint assessment would give this assessment a better land use perspective.

In recent years, the changes in soil carbon due to land management have begun to be considered in carbon footprint analysis, but there are still many countries for which this information is unavailable or too uncertain to be used. Long-term measurement and modeling has enabled Canada to include soil carbon change as a function of management practices in its national inventory of greenhouse gas emissions and gives us confidence to include these estimates in the carbon footprint of beef cattle. However, uncertainty in soil carbon change remains, and has been estimated at ±20% at the national scale [37], likely with greater uncertainty at smaller spatial scales. Soil carbon sequestration cannot continue indefinitely, and as agricultural soils in Canada begin to approach equilibrium with respect to soil carbon, the changing rate of carbon storage should be reflected in the carbon footprint of beef cattle. Further, sequestered carbon is not necessarily permanent, and ongoing monitoring of management practices and carbon stocks is necessary to determine if reversal of stored carbon has occurred, which should also be reflected in any carbon footprint estimate of beef cattle. In other countries, changes in soil carbon stocks could provide opportunities to keep reducing the GHG emissions associated with cattle production but in some regions it could result in a larger carbon footprint because of deforestation and land degradation.

Most of the carbon footprint estimates in the literature are dominated by European, North American, South American and Australian estimates. No estimates are available for countries such as India, China and Africa that have large stocks of cattle. The cattle from these countries are likely to have larger carbon footprints because of their relatively low productivity. The objective of this review was to provide an estimate of the carbon footprint of beef cattle for some of the main beef exporting nations as well as to investigate the allocation of GHGs to other cattle by-products. Towards this objective, estimates of the carbon footprint of beef cattle for some of the main beef exporting countries were examined. It has been difficult to compare results from the literature due to the different assumptions used in the calculations. We have observed a wide range of carbon footprint values (8 to 22 kg CO₂e per kg of LW at the farm gate with a few larger values) depending on the type of farming system, the location, the type of management practices, the boundaries of the studies, as well as the resource use considered. It appears that on average, the carbon footprint of beef cattle from Canada, the U.S., the EU-27 and Southern Australia have similar carbon footprints with large regional differences in most of these countries. The carbon footprint of cattle grazing under extensive conditions, such as in Northern Australia and Brazil, are likely to be considerably larger.

We have shown that the type of allocation influence the results. For conditions in Canada, using mass allocation we estimated that at the exit gate of the slaughterhouse, the carbon footprint of Canadian cattle was 12.9 kg CO₂e per kg of product in 2006. However, based on an economic allocation very different intensity values were obtained for the various by-products and these carbon footprint values vary substantially from year to year. Finally, it is important to remember that the carbon footprint of cattle is only one aspect associated with beef production. The impact on ecological goods and services and biodiversity are also very important to the environmental sustainability of this product.