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Chapter 21

Greenhouse Gas Emission Sources from Beef and Dairy Production Systems in the United States Kimberly R. Stackhouse, Sara E. Place, Michelle S. Calvo, Qian Wang, and Frank M. Mitloehner* Department of Animal Science, University of California, Davis, 2151 Meyer Hall, One Shields Avenue, Davis, CA 95616 *E-mail: [email protected]

Contribution of beef and dairy cattle production in the United States (U.S.) to anthropogenic climate change is a growing public policy concern. In recent years, research in this area has begun to quantify greenhouse gas (GHG) emission sources from beef and dairy cattle production systems and develop initial mitigation strategies to reduce GHG emissions without compromising animal productivity. Developing an in-depth and accurate understanding of GHG emission sources from these cattle systems is challenging because of the variability across farms, ranches, and feedlots, and the variation between animal types on individual operations. Emission sources can be distinguished into two main categories: enteric fermentation and manure. However, to fully quantify all GHG emission sources from the beef and dairy production systems, a Life Cycle Assessment (LCA) should be completed that includes all emission sources from cradle to fork. Included in this assessment would be GHG sources associated with animal management, emissions during processing, and emissions associated with transportation of the finished product to the consumer. This chapter focuses on GHG emissions from the dairy and beef production systems derived from enteric fermentation and manure.

© 2011 American Chemical Society Guo et al.; Understanding Greenhouse Gas Emissions from Agricultural Management ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Introduction Cattle production is considered an important anthropogenic source of greenhouse gases (GHG) contributing to climate change and has received considerable political and public attention in recent years. Two kinds of GHG emission sources have been identified from cattle production systems: direct and indirect emissions. Direct emissions refer to gases produced by the animal, including enteric fermentation and that are emitted from excreted feces and urine (1). Indirect emissions refer to gases that occur during the production of feed, including manure soil application, carbon dioxide (CO2) during the production of fertilizer and other inputs used to grow feed crops, CO2 from energy consumption during feed crop harvesting, and CO2 from transportation of cattle, meat, and milk. Indirect and direct emissions are interlinked and dependent on the efficiency of the cattle production system and the length of the animal’s productive life. Therefore, reporting emissions ‘per unit of productive output’ (e.g., kg of carcass weight for beef cattle and kg of milk for dairy cattle) rather than ‘per head’ allows for the proper accounting of emissions and development of the most effective mitigation techniques. Developing an in-depth and accurate understanding of both the direct and indirect GHG emission sources from a whole system such as the dairy or beef industry requires a complete GHG Life Cycle Assessment (LCA). A LCA provides a comprehensive analysis of the GHG emission sources from the entire production system as supported with data from field monitoring. A LCA of the beef and dairy production systems includes all GHG emission sources from ‘cradle to fork’, which includes GHG from the entire supply chain. This in-depth analysis is beyond the scope of this chapter, and such an analysis has yet to be extensively evaluated due to the lack of comprehensive information that fully account for all GHG sources and potential sinks from beef and dairy production systems. Therefore, this chapter will focus primarily on direct GHG emission sources from dairy and beef cattle production.

Overview of the U.S. Beef and Dairy Industries In 2009, the U.S. beef and dairy industries supplied 11.8 billion kg of beef and 85.9 billion kg of milk to human consumption (2). In 2010, there were a total of 100.8 million cattle, with approximately 90 million beef cattle (Continental and British Beef Breeds) and 10 million dairy cattle (predominantly Holstein and Jersey breeds) (3). The cattle industries span the continental U.S. and range from large to small scale operations. This makes the industries diverse and complex, and GHG emissions difficult to quantify. In general, management of the beef industry is more extensive and more segmented than the dairy industry, which increases the variability of environmental emissions, such as GHGs. U.S. beef cattle production is most often comprised of a three-phase system with specialized producers in each sector: 1) cow-calf production, 2) stocker cattle production, and 3) finishing cattle production. In the cow-calf sector, cows are maintained extensively on pasture or rangeland and give birth to calves, which are weaned and sold at 6-8 months of age. In the 408 Guo et al.; Understanding Greenhouse Gas Emissions from Agricultural Management ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


stocker sector, additional body weight is added to the weaned calf by grazing pasture and rangeland. The calf exits the stocker system at approximately 350 kg at 9-14 months of age. In both the cow-calf and stocker sectors, the majority of the pasture and rangeland used for grazing is not usable for row-crop production (e.g., corn, vegetables, soybean, etc.). The most intensive sector in beef cattle production is the finishing phase. This is the final phase and involves housing animals in a confined feedlot, where animals are fed a corn-rich concentrate diet to a harvest weight of approximately 550 kg at 14-20 months of age. In the feedlots, manure is managed and is typically removed from the pens every 4-6 months. This differs from pastures, where manure is un-managed and deposited directly on rangeland and pasture. Unlike the three-phased beef cattle operations, dairy operations are more typically integrated on a single farm that houses calves, growing heifers, lactating cows, and dry cows. Animals are typically fed and maintained in confinement and manure is removed from housing areas on a daily basis. Typical manure management involves scraping corrals and piling into dry storage piles and/or scraping or flushing of manure from concrete floored freestall barns into liquid storage. Calves are fed a milk or milk replacer and grain supplement and are housed individually in calf hutches from birth until weaning (approximately 2 months of age). Growing heifers are housed in groups, fed a forage based total mixed ration (TMR), and are managed to calve at 2 years of age. Between lactation periods, dry cows (cows not lactating) are fed a high forage TMR for a 60 day period, which allows them to recuperate for the next lactation cycle. Lactating cows have high nutrient requirements; therefore, a properly balanced TMR is provided to reach maximum milk production levels. While many dairy operations still manage every aspect of production (from calf raising to lactation), there is a growing number of producers that specialize in producing heifers and young calves, especially for dairies with large herd sizes. Manure management is an important component of large and small confined animal feeding operations in the U.S.. Manure that is managed properly can be a valuable resource for producers, because it contains nutrients that can be utilized for crop fertilization and to enhance soil quality (4). As facility size, herd size, and industry type (dairy versus beef, for instance) vary so will manure management strategies and manure holding structures that contain waste (animal manure, wastewater, contaminated runoff, and mixtures of manure with bedding (5)). Manure storage structures can comprise of areas or portions of buildings designated specifically for manure storage or treatment, whereas more sophisticated manure storage structures can consist of lagoons, pits, ponds, or tanks (5). After storage and treatment, manure and wastewater can be applied to pasture or crop fields as fertilizer. In addition, manure can also be managed using anaerobic digester systems or aerobic composting, to potentially reduce environmental impact, control odors, and capture biogas to be used as a source of energy (6).

409 Guo et al.; Understanding Greenhouse Gas Emissions from Agricultural Management ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

Greenhouse Gas Emissions Sources from Beef and Dairy Production Systems


The three main GHG known to emanate from beef and dairy production are methane (CH4), nitrous oxide (N2O), and carbon dioxide (CO2). These gases are predominately emitted from enteric fermentation, animal respiration, and through microbial decomposition of manure. The following sections will detail the processes responsible for the production of these gasses.

Methane Methane is the most well understood source of GHGs from beef and dairy production, as it was originally studied in significant depth to improve the nutritional use of feedstuffs in the rumen environment. Methane from cattle production is considered to be the most important source of anthropogenic CH4 emissions (7). Many factors affect CH4 emissions from cattle including feed intake, animal size, diet, growth rate, milk production, and energy consumption (1, 8). In beef and dairy production systems, methane is primarily produced from the microbial digestive processes of ruminant livestock species. This microbial fermentation is referred to as enteric fermentation, whereby CH4 is produced as a by-product and expelled through eructation (1, 7, 9). As a ruminant, the bovine promotes digestion of cellulose and hemicellulose through hydrolysis of polysaccharides by bacteria and protozoa. This is then followed by microbial fermentation, generating hydrogen (H2), CO2 and, CH4 as a byproduct (10). Production of CH4 in ruminants is considered a loss of energy (1, 8). Cattle typically lose 2-12% of their ingested energy as eructated CH4 (8). Additionally, enteric CH4 emissions from cattle show diurnal variation, maintaining higher concentrations during the day than at night, with the highest emissions occurring after feeding and during rumination events (1, 11, 12). Feed energy intake is the primary factor driving CH4 emissions and is thought to be one of the best predictors of enteric gas production (8, 13, 14). Feed intake differs between dairy and beef cattle and contributes to observed differences in CH4 emissions. In addition to emissions from enteric fermentation, CH4 is also produced during the decomposition of manure (1, 9). Manure storage areas are the second largest contributors to CH4 emissions on a commercial dairy (15). However, the relative contributions of stored manure to total CH4 emitted from a dairy or feedlot is relatively (8-20%) small when compared to enteric fermentation values. Methane is emitted under anaerobic conditions during manure breakdown in systems such as: storage ponds, tanks, and pits where microorganisms are able to ferment organic materials (10, 15–17). Methane emissions from manure storage areas are variable depending on management of the manure. Emissions can be affected by fecal matter excreted, physical form of the manure, excretal form of the waste, environmental conditions, and time elapsed before decomposition (16). There are also minor amounts emitted from barn floors or feedlots and following field application (18). Additionally, soil can actually absorb methane, but total amounts seems to be very small under typical conditions (19). 410 Guo et al.; Understanding Greenhouse Gas Emissions from Agricultural Management ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Nitrous Oxide Nitrous oxide is a highly potent GHG; therefore, reducing N2O emissions from U.S. beef and dairy production systems can have a significant impact on reducing overall GHG emissions from cattle production (20). Approximately 10% of the N2O produced by agriculture originates from manure management on livestock facilities (21). The main sources of N2O from cattle production systems are manure management (removal and storage of manure), chemical nitrogen (N) fertilizer, and animal manure applied to cropland and pasture Nitrous oxide is a gaseous intermediate that is produced during the incomplete microbial processes of nitrification and denitrification. Nitrification is the aerobic microbial oxidation of ammonium (NH4+) to nitrate (NO3-), and denitrification is the anaerobic microbial reduction of NO3- to nitrogen gas (N2) (22). Denitrification in manure storage areas and after incorporation of manure into the soil is considered the largest contributor of N2O (23). Denitrification also occurs in slurry that contains NO3- from oxidizing NH4+ and in soils containing NO3- from chemical fertilizers or land application of manure (10). Ruminant animals are considered to contribute a small amount of N2O emission from enteric fermentation (9, 10, 12). It is suspected that N2O is also produced in the rumen as a byproduct of dissimilatory NO3- reduction to NH4+ (9). However, there is a scarcity of literature on enteric N2O emissions from dairy cows (1). Animal treading and trampling on pastures could result in N2O emissions, since animal treading reduces soil aeration, which leads to the anaerobic conditions conducive to denitrification (16, 24). Carbon Dioxide In most GHG evaluations, CO2 emitted from the respiration of cattle, microorganisms in manure, and human consumers is not considered a net contributor to climate change because the animals consume plants that fix CO2 during photosynthesis (25). The CO2 fixed during feed production is thus returned to the atmosphere by this respiration. Nevertheless, CO2 emitted by modern cattle production systems from fossil fuel combustion for on-farm use, electricity generation, transportation, processing, and refrigeration has been considered a net source of GHGs in several LCAs (7, 26, 27) and whole-farm emissions models (28). However, the following discussion of direct sources of GHG from beef and dairy production will focus solely on the animal and its waste. Therefore, CO2 mitigation from fossil fuel use will not be discussed.

Beef and Dairy Production Systems Effect on Greenhouse Gas Emissions Greenhouse gas sources and their relative contributions from beef and dairy production systems are largely dependent on management. Variation in management techniques and producer objectives can result in different emission rates within cattle production systems. For example, a lactating beef cow is managed to produce adequate milk to wean a live calf; whereas, a lactating 411 Guo et al.; Understanding Greenhouse Gas Emissions from Agricultural Management ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

dairy cow is managed to maximize high quality milk production for human consumption. Although both cows are lactating, their metabolic rates and physiological needs differ, and thus, so do their emissions. The following sections will highlight these disparities in GHG emissions from beef and dairy production by focusing on extensive and intensive management systems.


Extensive Cattle Production Extensive production refers to cattle that are raised on rangeland or pasture, and consume (through grazing) a high forage diet. Extensive cattle systems are unique because the animals graze non-arable land and convert this forage into a protein rich human food source. In the U.S. beef production system, this includes both the cow-calf and stocker operations. However, in some cases, steers are also finished on grass and those animals would also fall into this category. Ruminants are known for their ability to digest fibrous plant material into usable energy. As mentioned previously, plant material is broken down through anaerobic fermentation by ruminant microbes to supply the animal with energy, microbial proteins, and the volatile fatty acids (VFA) acetate, proprionate, and butyrate (29, 30). The primary substrates for methanogensis in the rumen are H2 and CO2. The H2 is primarily produced through the formation of acetate and butyrate, which is favored by intake of roughage based diets (31). Therefore, cattle grazing on extensive rangelands produce more acetate and butyrate, and thus, have a larger H2 pool. This results in the production of higher enteric CH4 levels when compared to cattle on a high grain diet (32–34). Due to the nature of the high forage diets consumed through grazing on extensive operations, enteric derived CH4 emissions from the cow-calf and stocker phases of beef cattle production contribute to a larger portion of total GHG emitted from the system, compared to enteric derived methane from a feedlot animal whom is managed intensively. Methane emissions from beef cattle in a pasture based system ranges from 85.92 to 259.9 g hd-1 d-1 depending on animal type, forage digestibility, and body weight (35–39) which is approximately double the amount of GHG emitted from cattle in feedlots. The large range in CH4 emissions from pasture based beef production systems is indicative of the variation that is present in GHG emissions across different management schemes, forage quality, and cattle types. In extensive systems, cattle waste is unmanaged as it is deposited directly on grazed land. The primary GHG emission source from manure deposited on grazed land is N2O, which results from the N in animal waste (40). Often, the amount of N in the grazed legumes and grasses is higher than the animals’ N (or protein) requirements. Thus, the unused N is primarily excreted as urea in the urine and a portion is emitted into the atmosphere through the denitrification process. Estimates suggest that cattle on pasture emit 0.1 to 0.7% of the N in feces and 0.1 to 3.8% of the N in urine into the atmosphere as N2O (41). There is limited research on N2O emissions from grazing cattle and more research needs to be conducted to improve our understanding of these GHG contributions. Nitrous oxide emissions vary greatly depending on the herbage N content, the soil type and condition, and animal compaction of the soil. Methane is also a source of GHG from unmanaged manure on grazed land; however, due to the aerobic conditions 412 Guo et al.; Understanding Greenhouse Gas Emissions from Agricultural Management ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

that exist on pastures and ranges the overall contribution is less than 1-3 % of total grazed land GHG emissions (34, 40).


Intensive Cattle Production Intensive cattle production refers to animals that are raised in a more confined space, such as a feedlot or dairy, most often with feedstuff imports from outside the farm. Cattle raised intensively receive daily care. Typically, cattle in confined, intensive systems are fed formulated diets that meet their nutritional and physiological needs. In addition, the manure is managed from intense systems. These systems are managed to maximize milk and meat production and to maximize efficiencies. More specifically, feedlots are managed to maximize growth and carcass quality; while, dairies are managed to maximize milk production and milk quality. Significantly more research has been conducted on GHG emission sources from intensive systems than that from extensive systems. The main GHG compounds from intensive systems are CH4 from enteric fermentation and stored manure and N2O from stored and land applied manure. Methane emissions from dairy animals is estimated to range from 3.1 to 8.3 % of gross energy intake (GEI) for dry cows and from 1.7 to 14.9 % of GEI for lactating cows (42). Lactating cows produce approximately 1.3 times more CH4 than non-lactating (dry) cows (12). These emissions vary by stage of lactation and feeding level. Methane emissions from lactating cows range from 238 to 437 g cow-1 d-1 (12, 43, 44). Cattle on high grain diets (corn based), which are typically fed in commercial feedlots, are estimated to lose 3.5 % of GEI as CH4 (45). Methane emissions from feedlot cattle fed high concentrate finishing diets range from 62 to 100 g hd-1 d-1 (33, 46). Manure management is critical for intensive cattle systems, because approximately 22-70 kg of feces and urine is produced daily by beef and dairy animals (47). Manure can either be stored in the dry (solid or semi-solid waste) or liquid form. Dairies typically utilize either solid or liquid management whereas feedlots manage their manure in solid form (47). Manure methane production is found to be highly variable across intensive operations, primarily because of the different diets fed (which affect the composition of manure, i.e. increased fermentable carbohydrates in feces from high-grain-fed animals), the varying environmental conditions (temperature and moisture), and the manure handling systems employed. Methane is the predominant GHG emitted from managed cattle waste, accounting for approximately 81% of the total emissions from manure (40). Nitrous oxide emissions from manure make up for the remaining 19% (40). Managed manure from dairy production systems produces more CH4 emissions than managed manure stores from feedlots (40, 43). While the literature is sparse, one estimate for dairy manure CH4 is estimated at 140 g hd-1 d-1 with N2O emissions at 16 g hd-1 d-1 (43). Nitrous oxide emissions from stored manure depends on the manure storage condition (stacking density, water content, and temperature of the manure) (11, 48, 49). Waste management from feedlots differs slightly from dairies. In the feedlot, cattle are usually housed in drylots for 120-160 days depending on entry 413 Guo et al.; Understanding Greenhouse Gas Emissions from Agricultural Management ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


weight, and manure is scraped from the drylot pens every 4-6 months or after every group rotation of animals. Therefore, a manure pack will develop. Research on manure pack emissions from feedlots show a strong positive correlation between CH4 release and increasing nitrogen content of the manure (50). Additionally, CH4 is released in greater quantities from the manure of housed (grain-fed) animals versus grazed animals (50). Methane and N2O fluxes in feedlot manure packs range from 0.79-1.260 and 0.15-0.16 g hd-1 d-1, respectively (51). Higher CH4 emissions are associated with increased ambient temperatures (52, 53) and manure temperatures (54). Application of manure to soil as fertilizer can result in N2O production. There are three main pathways that manure N will undergo after land application: 1) N is absorbed by the plants which is incorporated into plant protein, 2) N not absorbed by plants can be utilized by microbes to produce N2O released into the atmosphere, and 3) NO3- leaching to the groundwater. In one study, N2O emissions resulting from field application of manure were estimated to be 3.8 g m-3 manure slurry applied (55). It seems that N2O emissions from field application are highest during the first 50 h after slurry application (56). The N2O emissions from land application are driven by carbon and nitrogen substrate availability, temperature, pH, aeration, and moisture. Targeting these environmental drivers are possible avenues for mitigation of N2O from dairies and feedlots. Additionally, manure application methods and soil condition have significant impacts on N2O emissions from manure applied soil.

Life Cycle Assessment Quantifying GHG emission sources from U.S. beef and dairy cattle production systems is complex and challenging due to the variation across farms, ranches, and feedlots. To accurately evaluate the true GHG contribution of these independent cattle systems, an LCA must be used. The LCA tool should be supported with extensive field work to achieve accurate results. Assumptions in the LCA should be specific to cattle type, management strategy, and region. For example, cattle raised in the U.S. should not be compared to cattle raised in a third world country using the same assumptions. Life cycle assessments are most accurate when assumptions are created for systems in specific regions and adjusted to meet those management techniques. Estimation of total global GHG contributions from beef and dairy cattle was attempted by the United Nations, Food and Agriculture Organization in the report, Livestock’s Long Shadow. This report suggested that 18% of total anthropogenic GHGs were caused by livestock, more than global transportation contribution to anthropogenic GHGs (7). This estimated contribution is in stark contrast to the U.S. specific LCA performed by the Environmental Protection Agency (EPA) that suggests livestock contributes less than 3% of total anthropogenic GHGs (57). This demonstrates the need for LCAs that are specific to animal type and climate to fully understand the true sources and contributions of livestock to climate change. Ultimately, every GHG source and mitigation strategy should be evaluated and implemented in conjunction with other environmental (water and soil), animal 414 Guo et al.; Understanding Greenhouse Gas Emissions from Agricultural Management ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

welfare, and economic strategies. Additionally, development of GHG reducing technologies should consider consumer acceptance and product marketability. Taking a holistic approach to GHG emission sources and mitigation techniques is necessary to ensure the continuation of sustainable food production.


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