Greenhouse Gas Emissions from Animal Production Systems

Donald E. Johnson

Department of Animal Sciences, Colorado State University, Fort Collins, Colorado, U.S.A. Kristen A. Johnson

Department of Animal Sciences, Washington State University, Pullman, Washington, U.S.A.

INTRODUCTION

The burning of fossil fuels and allied human activities of recent centuries result in increasing concentrations of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) in the earth's atmosphere. These major gases and other miscellaneous gases and particles act like a greenhouse, blocking portions of the long-wave radiation of sunlight energy back out to space. The amount of blockage, and thus warming, varies with the gas. The warming potential of CH4 and N2O are 23 and 296 times stronger than CO2 per kilogram of gas added to the atmosphere. Radiative forcing has caused a global warming of an estimated 0.6°C with the potential for much more warming and a cacophony of related climate changes with future greenhouse gas (GHG) releases over the coming decades as described by the International Panel on Climate Change.[1] Globally, the major anthropogenic GHG is CO2 (~55%) from fossil-fuel combustion in autos, power plants, etc. with CH4 (from landfills, livestock, rice fields, etc.) comprising ~18% and N2O (from agricultural soils, manure storage and use) ~6% of the warming equivalent. In the U.S., the portion arising from CO2 is considerably higher, ^85%, although an estimated 12% is currently being offset by carbon sequestration through land-use changes and forestry practices. Several other gases contribute to warming including chlorofluorocarbons, ozone, and aerosols. There are also several natural sources of GHGs. Natural sources include wetlands, termites, oceans, and leakage of hydrate deposits for CH4, as well as bacterial reactions in soils and oceans, and atmospheric reactions as sources of N2O.[2]

CONTRIBUTIONS OF ANIMALS AND THEIR PRODUCTION SYSTEMS

During the production and processing of crops to feedstuffs, through animal digestion and metabolism, and during the handling and disposition of manure, a portion of each of the C and N elements are converted to the second and third most important GHGs (CH4 and N2O). Additionally, the production, processing, and handling of animals, feedstuffs, and manure consume fossil fuel and emit the primary GHG, CO2. An analysis of whole-system emissions by typical beef and dairy production systems around the U.S.A. (Table 1) indicates that each U.S. beef cow along with associated replacements, stocker, and finishing animals produce ^5500 kg of CO2 equivalent annually. Methane and N2O are the primary sources, 47% and 42%. A dairy cow, along with heifers reared to replace her, will produce from 9000 to 11,000 kg CO2 equivalent annually depending on milk production per cow and dominance of anaerobic lagoon use for manure disposal. Fossil-fuel CO2 emissions are a relatively small portion of GHGs from beef operations, but make a substantial contribution to dairy system emissions.

As a relative perspective, animal system emissions can be compared to other economic sector emissions in the U.S.A. As one example, the beef and dairy sector emission estimates are 128 and 45 Tg CO2 equivalent,[4] while passenger vehicles, cars, and light trucks produce an estimated 1000 Tg annually by U.S. Environmental Protection Agency estimates.

Livestock CH4 Emissions

The livestock contribution to the global CH4 budget results from emissions from enteric fermentation of feedstuffs and anaerobic manure decomposition. Enteric (gastrointestinal) CH4 results from microbial fermentation of dietary components, primarily carbohydrate, to volatile fatty acids, hydrogen, and subsequently CH4 within the gut of the animal. Ruminants are the greatest contributors of enteric CH4 because of their unique gastro-intestinal structure that promotes the fermentation of cellulosic diet components in the ruminoreticulum and also because of their large world population, body size, and appetites. These animals produce 96% of global enteric CH4 (Table 2).

Table 1 Source strengths of GHG emissions from representative U.S. beef and dairy production systems (annual CO2 equivalent per 100 cow herd, with replacements, plus offspring through stocker and feedlot for beef herds), mean ±SEM

Beef cattle Dairy cattle

Table 1 Source strengths of GHG emissions from representative U.S. beef and dairy production systems (annual CO2 equivalent per 100 cow herd, with replacements, plus offspring through stocker and feedlot for beef herds), mean ±SEM

Beef cattle Dairy cattle

Enteric CH4

255

i

3.5

46

375

i

36

32

Manure CH4

7

i

0.6

1

177

i

77

15

N2O

232

i

12

42

314

i

40

27

CO2

60

i

2.4

11

290

i

39

25

Total CO2 equivalent:

Ton per herd

554

i

15

1156

i

183

kg/producta

15

i

0.4

1.3

i

0.06

aProduct is the live weight sold of culls plus finished cattle from beef herd or milk sold from dairies (with CO2 Equivalent of CH4 x 23, N2O x 296). (From Ref.[3].)

aProduct is the live weight sold of culls plus finished cattle from beef herd or milk sold from dairies (with CO2 Equivalent of CH4 x 23, N2O x 296). (From Ref.[3].)

Horses, pigs, and most other nonruminants also produce CH4 from hindgut fermentation of feed residues, but this contribution is small (Table 2). Methane production by microbes in the gut is a mechanism by which the ruminal ecosystem disposes of excess reductive hydrogen enhancing continued metabolism of dietary substrates by other bacteria. Methanogenic bacteria reside in several niches within the ruminal microbial community and derive energy as they produce CH4. There is also an active population of methanogenic bacteria existing in the ruminant hindgut. The majority of CH4 gas is eructated (belched) or absorbed through the bloodstream and exhaled.[6]

Question: Are CH4 emissions from cattle today, any greater than that from the vast herds of bison in the U.S.A. during the last century? The anecdotal accounts of bison populations suggest similar amounts of CH4 to that of cattle in the U.S.A. today. However, the bison herds were essentially gone before the start of the more than doubling of atmospheric CH4 during the last century. Also, the global population of cattle/ ruminants[7] approximately parallels the rise in CH4. Thus, domesticated cattle or ruminants worldwide contributed to, but did not by themselves cause, the global rise in CH4.

The amount of CH4 produced by a ruminant is primarily dependent on diet dry-matter consumption. The chemical composition of the diet can have marked effects on consumption and in some cases on the CH4 yield per unit of diet. When the predominant fermented ingredients in the diet are cellulose or hemicellulose, CH4 production is the predominant hydrogen-disposal method. With fermentable starch, propionate's role as a hydrogen sink increases and CH4 yield decreases. In fact, most of the mitigation strategies available enhance propionate and other hydrogen sinks. When starch is the primary fermentable substrate, the CH4 yield can fall dramatically. Intermediate diets, such as those fed to dairy cows, result in the greatest amount of CH4 produced.

Emissions of CH4 from manure, particularly in the U.S.A., are more prominently from nonruminants owing to widespread use of anaerobic lagoons for manure management. Emissions from lagoons are both temperature and wind-speed dependent and are not only a result of the degradative processes occurring during the lagoon storage period, but can also arise from field surface application of the liquids. Composting manure solids reduces CH4 emissions, but increases N2O and ammonia emissions. Other agricultural sources

Table 2 Global sources

of GHG emissions from livestock

Species

Enteric CH4(Tg/yr)a

Manure CH4(Tg/Yr)a

Manure N2O (Gg/yr)b

Cattle and buffalo

66

6

190

Sheep, other ruminants

11

0.7

3

Swine, poultry

1

7.1

10

Horses

2

2

Total

80

13.8

205

of CH4 include rice fields and biomass burning, estimated to approximately equal enteric and manure sources. Energy production (coal, oil, and gas) and waste sectors (e.g., landfills) are estimated to produce CH4 emissions about equal to those from agriculture as do emissions from natural sources (wetlands, termites, etc.). Overall CH4 entry into the atmosphere is estimated to total 598 Tg/yr, while CH4 sinks including the hydroxyl ion and soil oxidation total ^576 Tg/yr.[8] This imbalance has resulted in a greater than twofold increase in atmospheric concentrations to current levels of 1750 ppb.

Emissions of N2O from Livestock Systems

Livestock systems produce N2O during manure storage and disposition and crop and pasture fertilization, and from volatilized and leached nitrogen. Nitrous oxide is formed when microbes in soil or manure oxidize ammonia or organic sources of N to nitrate (nitrification) under aerobic conditions or during reduction of nitrate under anaerobic conditions (denitrification). The major loses from livestock manure storage or accumulation facilities is from fecal and urine deposited by grazing animals or in dry-lot facilities where 2% of N is commonly given off as N2O-N. All nitrogen amendments to soils as manure-N, synthetic fertilizer-N, legume fixed-N, or N in crop residues are subject to the same microbial processes and release N2O. Variability in soil conditions lead to highly variable and episodic processes but average 1.25% of N2O-N emissions. Additionally, volatilized or leached N that eventually returns to soils or bodies of water adds to N2O emissions. Simulations of U.S. dairy operations[9] show these emissions to arise in approximately equal amounts from manure handling or disposal and from legume or synthetic-N fertilization of crops and forages.

Carbon Dioxide: Energy/fuel Use in Animal Production Systems

The third source of GHGs from livestock operations is CO2 from fossil fuel use in feed and fertilizer production, irrigation, transport, and processing. Life-cycle analyses also require estimation of CO2 from embodied energy use for equipment and building production. Although these costs will vary widely around the world, approximately 11% and 26% of the total CO2 equivalent from U.S. beef and dairy systems, respectively, is produced from fuel consumption.[3]

Question: Does CO2 exhaled by animals metabolizing feed nutrients add to the GHG burden? No, metabolic CO2 has been removed from the atmosphere during photosynthesis, thus contributing no net increase.

CONCLUSIONS

Animal production systems do contribute to GHG emissions; however, the opportunity exists to both enhance animal production systems and reduce GHG emissions simultaneously.

The first principle of consideration when evaluating methods to reduce livestock GHG emissions is that management or mitigation interventions need to examine effects on the whole of the production system. Many trade-offs occur, e.g., changing manure-handling systems can reduce CH4 but increase N2O, or forage treatment to enhance digestibility may require fuel energy inputs, and offset any advantage. The second principle is that options that enhance feed or animal production efficiency will usually reduce GHG per unit of product. Emissions of all GHGs are rather closely related to the amount of feedstuff going into the production system; thus, reduced feed inputs reduce GHG outputs.

Livestock systems have potential to increased soil-C storage to higher plateau levels partially offsetting emissions for a period of years. Intensive grazing practices in eastern U.S. locations, for example, stored C in pastures to offset from 12% to 18% of total emissions from beef cattle systems.[10] Silvopastoral production systems add another dimension and can offset even larger fractions of emissions as indicated by estimates for Costa Rican cattle operations.[11]

With the value of CO2 increasing to >EU 20/T, methods to capture, decrease, or offset GHG emissions will likely become economically important and a potential source of revenue by agricultural industries. Opportunities include adoption of methods to decrease lagoon CH4, production of renewable fuel, implementing management to decrease dry-lot N2O, enhancing soil C-sequestration, etc.

REFERENCES

1. IPCC. Intergovernmental Panel on Climate Change, Third assessment report, 2004; (http:// www.ipcc.ch/pub.htm).

2. EIA/DOE. Emissions of Greenhouse Gases in the United States in 2003; 2004; (www.eia.doe.gov).

3. Johnson, D.E.; Phetteplace, H.W.; Seidl, A.F. Methane, nitrous oxide and carbon dioxide emissions from ruminant livestock production systems. 44 52; In Greenhouse Gases and Animal Agriculture, Proceedings of the 1st International Conference on Greenhouse Gases and Animal Agriculture, Obihiro, Japan, November 7-11, 2001; Takahashi, J., Young, B., Eds. Elsevier Science (www.elsevier.com), ISBN 0444510125, 372 pp.

4. USDA. U.S. Agriculture and forestry greenhouse gas inventory: 1990 2001. Tech. Bul. 2004, 1907.

5. Johnson, D.E.; Ward, G.M.; Ramsey, J.J. Livestock methane: current emissions and mitigation potential. In Nutrient Management of Food Animals to Enhance and Protect the Environment; Kornegay, E.T., Ed.; CRC Press, 1996; 219 234.

6. Murray, R.M.; Bryant, A.M.; Leng, R.A. Measurement of methane production in sheep. In Tracer Studies on Non-Protein Nitrogen for Ruminants II; International Atomic Energy Agency: Vienna, Austria, 1975.

7. FAO. Agricultural production, live animals, 2004; (www.faostat.org).

8. IPCC. In Climate Change 2001: The Scientific Basis; Cambridge University Press: Cambridge, U.K., 2001; 89 90.

9. Johnson, D.E.; Phetteplace, H.W.; Seidl, A.F.; Davis, J.G.; Stanton, T.L.; Wailes, W.R. Estimates of gaseous and phosphorus emissions from cattle operations. Part I: Dairy Cattle. Animal Sci. Res. Rep. Colo. State Univ. 2003, 45 48.

10. Conant, R.T. Potential soil carbon sequestration in overgrazed grassland ecosystems. Global Biogeochem. Cycles 2002, 16 (4), 90/1 90/9.

11. Johnson, D.E.; Phetteplace, H.W.; Seidl, A.F. Report to USEPA. Silvopastoral livestock greenhouse gas emissions. Colo. State Univ. Ft. Collins, CO, 2003, 80523.

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