Environmental impact of meat production
The environmental impact of meat production varies because of the wide variety of agricultural practices employed around the world. All agriculture practices have been found to have a variety of effects on the environment. Some of the environmental effects that have been associated with meat production are pollution through fossil fuel usage, and water and land consumption. Meat is obtained through a variety of methods, including organic farming, free range farming, intensive livestock production, subsistence agriculture, hunting and fishing. As part of the conclusion to one of the largest international assessments of animal agriculture ever undertaken, the Food and Agriculture Organisation of the United Nations said:
The livestock sector is a major stressor on many ecosystems and on the planet as a whole. Globally it is one of the largest sources of greenhouse gasses and one of the leading causal factors in the loss of biodiversity, while in developed and emerging countries it is perhaps the leading source of water pollution.
Consumption and production trends
Consequences for the environment can result from changes in the amount of production to meet changes in demand for consumption. It has been estimated that global meat consumption may double from 2000 to 2050, mostly as a consequence of increasing world population, but also partly because of increased per capita meat consumption (with much of the per capita consumption increase occurring in the developing world). Global production and consumption of poultry meat have recently been growing at more than 5 percent annually. Trends vary among livestock sectors. For example, global per capita consumption of pork has increased recently (almost entirely due to changes in consumption within China), but global per capita consumption of ruminant meats has been declining.
Grazing and land use
In comparison with grazing, intensive livestock production requires large quantities of harvested feed. The growing of cereals for feed in turn requires substantial areas of land. However, where grain is fed, less feed is required for meat production. This is due not only to the higher concentration of metabolizable energy in grain than in roughages, but also to the higher ratio of net energy of gain to net energy of maintenance where metabolizable energy intake is higher. A pound of beef (live weight) requires about seven pounds of feed, compared to more than three pounds for a pound of pork and less than two pounds for a pound of chicken. However, assumptions about feed quality are implicit in such generalizations. For example, production of a pound of beef cattle live weight may require between 4 and 5 pounds of feed high in protein and metabolizable energy content, or more than 20 pounds of feed of much lower quality.
Free-range animal production requires land for grazing, which in some places has led to land use change. According to the Food and Agriculture Organization (FAO), "Ranching-induced deforestation is one of the main causes of loss of some unique plant and animal species in the tropical rainforests of Central and South America as well as carbon release in the atmosphere." This for example has implications for meat consumption in Europe, which imports significant amounts of feed from Brazil.
Raising animals for human consumption accounts for approximately 40% of the total amount of agricultural output in industrialized countries. Grazing occupies 26% of the earth's ice-free terrestrial surface, and feed crop production uses about one third of all arable land.
Land quality decline is sometimes associated with overgrazing. Rangeland health classification reflects soil and site stability, hydrologic function and biotic integrity. By the end of 2002, the US Bureau of Land Management had evaluated rangeland health on 7,437 grazing allotments (i.e. 35 percent of its grazing allotments or 36 percent of the land area contained in its grazing allotments) and found that 16 percent of these failed to meet rangeland health standards due to existing grazing practices or levels of grazing use. This led the BLM to infer that a similar percentage would be obtained when such evaluations were completed. Soil erosion associated with overgrazing is an important issue in many dry regions of the world. However, on US farmland, much less soil erosion is associated with pastureland used for livestock grazing than with land used for production of crops. Sheet and rill erosion is within estimated soil loss tolerance on 95.1 percent, and wind erosion is within estimated soil loss tolerance on 99.4 percent of US pastureland inventoried by the US Natural Resources Conservation Service.
Environmental effects of grazing can be positive or negative, depending on the quality of management,  and grazing can have different effects on different soils  and different plant communities. Grazing sometimes reduces, but sometimes increases biodiversity of grassland ecosystems. A study comparing virgin grasslands under some grazed and nongrazed management systems in the US indicated somewhat lower soil organic carbon but higher soil nitrogen content with grazing. In contrast, at the High Plains Grasslands Research Station in Wyoming, the top 30 cm of soil contained more organic carbon as well as more nitrogen on grazed pastures than on grasslands where livestock were excluded. Similarly, on previously eroded soil in the Piedmont region of the US, pasture establishment with well-managed grazing of livestock resulted in high rates of both carbon and nitrogen sequestration relative to results obtained where grass was grown without grazing. Such increases in carbon and nitrogen sequestration can help mitigate greenhouse gas emission effects. In some cases, ecosystem productivity may be increased due to grazing effects on nutrient cycling.
Virtual water use for livestock production includes water used in producing feed. However, virtual water use data, such as those shown in the table, are often unrelated to environmental impacts of water use. For example, in a high-rainfall area, if similar soil infiltration capacity is maintained across different land uses, mm of groundwater recharge and hence sustainability of water use tends to be about the same for food crop production, meat-yielding livestock production, and saddle horse production, although virtual water use per kg of food produced may be several hundred L, several thousand L, and an infinite number of L, respectively. In contrast, in some low-rainfall areas, some livestock production is more sustainable than food crop production, from a water use standpoint, despite higher virtual water use per kg of food produced. This is because unirrigated land in many water-short areas may support grassland ecosystems in perpetuity, and thus may be able to support well-managed, extensive production of grazing cattle or sheep with a sustainable level of water use, even where large-scale production of more water-demanding food crops would be unsustainable in the long run due to inadequate surface water supplies and inadequate groundwater recharge to sustain a high level of withdrawn water use for irrigation. Such considerations are important on much rangeland in western North America and elsewhere that can support cow-calf operations, backgrounding of stocker cattle, and sheep flocks. In the US, withdrawn surface water and groundwater use for crop irrigation exceeds that for livestock by about a ratio of 60:1.
Also, the high virtual water use figures associated with meat production do not necessarily imply reduction of water use if food crops are produced, instead of livestock. For example, some grazing lands are unsuitable for food crop production, so that evapotranspirational water use would continue on land vacated by livestock, while additional water would be needed for crops to provide substituting food from lands elsewhere, and additional water would also be needed to produce substitutes for the non-food products of livestock. (In the US, Land Capability Classes V, VI and VII contain soils unsuited for cultivation, much of which is suitable for grazing. Of non-federal land in the US, about 43 percent is classed as unsuitable for cultivation.)
Irrigation accounts for about 37 percent of US withdrawn freshwater use, and groundwater provides about 42 percent of US irrigation water. Irrigation water applied in production of livestock feed and forage has been estimated to account for about 9 percent of withdrawn freshwater use in the United States. Groundwater depletion is a concern in some areas because of sustainability issues (and in some cases, land subsidence and/or saltwater intrusion). A particularly important North American example where depletion is occurring involves the High Plains (Ogallala) Aquifer, which underlies about 174,000 square miles in parts of eight states, and supplies 30 percent of the groundwater withdrawn for irrigation in the US. Some irrigated livestock feed production is not hydrologically sustainable in the long run because of aquifer depletion. However, rainfed agriculture, which cannot deplete its water source, produces much of the livestock feed in North America. Corn (maize) is of particular interest, accounting for about 91.8 percent of the grain fed to US livestock and poultry in 2010.:table 1–75 About 14 percent of US corn-for grain land is irrigated, accounting for about 17 percent of US corn-for-grain production, and about 13 percent of US irrigation water use, but only about 40 percent of US corn grain is fed to US livestock and poultry.:table 1–38 Together, these figures indicate that most production of grain used for US livestock and poultry feed does not deplete water resources and that irrigated production of grain for livestock feed accounts for a small fraction of US irrigation water use. However, where production relies on irrigation from groundwater reserves, water table monitoring is appropriate to provide timely warning if groundwater depletion occurs.
Effects on aquatic ecosystems
In the Western United States many stream and riparian habitats have been negatively affected by livestock grazing. This has resulted in increased phosphates, nitrates, decreased dissolved oxygen, increased temperature, turbidity, and eutrophication events, and reduced species diversity. Livestock management options for riparian protection include salt and mineral placement, limiting seasonal access, use of alternative water sources, provision of "hardened" stream crossings, herding, and fencing. In the Eastern United States waste release from pork farms have also been shown to cause large-scale eutrophication of bodies of water, including the Mississippi River and Atlantic Ocean (Palmquist, et al., 1997). However, in North Carolina, where Palmquist's study was done, measures have since been taken to reduce risk of accidental discharges from manure lagoons; also, since then there is evidence of improved environmental management in US hog production. Implementation of manure and wastewater management planning can help assure low risk of problematic discharge into aquatic systems. (See Animal Waste section, below.)
Energy consumption and greenhouse gas emissions
At a global scale, the FAO has recently estimated that livestock (including poultry) accounts for about 14.5 percent of anthropogenic greenhouse gas emissions estimated as 100-year CO2 equivalents. (A previous, widely cited FAO report had estimated 18 percent. FAO's new report provides information on this difference in the supplementary material of its latter report, stating: "All manure emissions were accounted for in [the previous report], but only emissions related to manure management and manure application on feed crops or pasture are accounted for in this report" and "The [previous report's] assessment includes GHG emissions related to the production of feed (including pasture) fed to all animal species [...], whereas this report only accounts for feed materials fed to the studied species.") Because this emission percentage includes contributions associated with livestock used for production of draft power, eggs, wool and dairy products, the percentage attributable to meat production alone is significantly lower. The indirect effects contributing to this percentage include emissions associated with production of feed consumed by livestock and carbon dioxide emission from deforestation in Central and South America, attributed to livestock production. Mitigation options for reducing methane emission from ruminant enteric fermentation include genetic selection, immunization, rumen defaunation, diet modification and grazing management, among others. The principal mitigation strategies identified for reduction of agricultural nitrous oxide emission are avoiding over-application of nitrogen fertilizers and adopting suitable manure management practices. Mitigation strategies for reducing carbon dioxide emissions in the livestock sector include adopting more efficient production practices to reduce agricultural pressure for deforestation (notably in Latin America), reducing fossil fuel consumption, and increasing carbon sequestration in soils.
At a national level, livestock represents up to half of New Zealand's greenhouse gas emissions. Livestock sources (including enteric fermentation and manure) account for about 3.1 percent of US anthropogenic greenhouse gas emissions expressed as carbon dioxide equivalents, according to US EPA figures compiled using UNFCCC methodologies. Not all forms of meat and animal–based foods affect the environment equally. One study estimates that red meats are 150% more greenhouse gas intensive than chicken or fish. According to another research group, the ranking of some food products in relation to greenhouse gas emissions is lamb (#1), beef (#2), cheese (#3), and pork (#4). However, such ranking may not be broadly representative. Among sheep production systems, for example, there are very large differences in both energy use and prolificacy; both factors strongly influence emissions per kg of lamb production.
Because a large fraction of GHG emissions attributed to livestock production involves methane (from enteric fermentation and manure management), care is appropriate in considering contributions of these emissions to global warming. Relative to carbon dioxide, atmospheric methane has a 100-year global warming potential of 25 (including indirect effects on ozone and stratospheric water vapor). Methane emission from livestock and other anthropogenic sources has contributed substantially to past warming; however, it is of much less significance for current and recent warming. This is because there has been relatively little increase in atmospheric methane concentration in recent years, with total methane sources, estimated at 582 Tg per year, being nearly balanced by methane sinks, estimated at 581 Tg per year. In a tabulation by the IPCC, estimates of methane emission from global livestock range from 80 to 115 Tg per year.
Data of a USDA study indicate that about 0.9 percent of energy use in the United States is accounted for by raising food-producing livestock and poultry. In this context, energy use includes energy from fossil, nuclear, hydroelectric, biomass, geothermal, technological solar, and wind sources. (It excludes solar energy captured by photosynthesis, used in hay drying, etc.) The estimated energy use in agricultural production includes embodied energy in purchased inputs.
Intensification and other changes in the livestock industries influence energy use, emissions and other environmental effects of meat production. For example, in the US beef production system, practices prevailing in 2007 are estimated to have involved 8.6 percent less fossil fuel use, 16.3 percent less greenhouse gas emissions, 12.1 percent less water use and 33.0 percent less land use, per unit mass of beef produced, than in 1977. These figures are based on analysis taking into account feed production, feedlot practices, forage-based cow-calf operations, backgrounding before cattle enter a feedlot, and production of culled dairy cows.
Unless well managed, manure and other substances from livestock operations may cause water contamination. Concerns about such problems are particularly acute in the case of CAFOs (concentrated animal feeding operations). In the US, a permit for a CAFO requires implementation of a plan for management of manure nutrients, contaminants, wastewater, etc., as applicable, to meet requirements under the Clean Water Act. There are over 12,000 regulated CAFOs in the US. Environmental performance of the US livestock industry can be compared with several other industries. The US EPA (Environmental Protection Agency) has published 5-year and 1-year data for 32 industries on their ratios of enforcement orders to inspections, a measure of non-compliance with environmental regulations: principally, those under Clean Water Act and Clean Air Act. For the livestock industry, inspections focused primarily on CAFOs. Of the 31 other industries, 4 (including crop production) had a better 5-year environmental record than the livestock industry, 2 had a similar record, and 25 had a worse record in this respect. For the most recent year of the five-year compilation, livestock production and dry cleaning had the best environmental records of the 32 industries, each with an enforcement order/inspection ratio of 0.01. For crop production, the ratio was 0.02. Of the 32 industries, oil and gas extraction and the livestock industry had the lowest percentages of facilities with violations.  Regulatory enforcement for CAFOs is a priority of the EPA, and EPA enforcement in fiscal 2010 included 21 actions against CAFOs for such violations as failure to obtain a permit, failure to operate according to the terms of a permit, and contamination of water.
With good management, manure has environmental benefits. Manure deposited on pastures by grazing animals themselves is applied efficiently for maintaining soil fertility. Animal manures are also commonly collected from barns and concentrated feeding areas for efficient re-use of many nutrients in crop production, sometimes after composting. For many areas with high livestock density, manure application substantially replaces application of synthetic fertilizers on surrounding cropland. Manure was spread as a fertilizer on about 15.8 million acres of US cropland in 2006. Manure is also spread on forage-producing land that is grazed, rather than cropped. Altogether, in 2007, manure was applied on about 22.1 million acres in the United States. Substitution of animal manure for synthetic fertilizer has important implications for energy use and greenhouse gas emissions, considering that between about 43 and 88 MJ (i.e. between about 10 and 21 Mcal) of fossil fuel energy are used per kg of N in production of synthetic nitrogenous fertilizers.
Manure can also have environmental benefit as a renewable energy source, in digester systems yielding biogas for heating and/or electricity generation. Manure biogas operations can be found in Asia, Europe, North America, and elsewhere. The US EPA estimates that as of July 2010, 157 manure digester systems for biogas energy were in operation on commercial-scale US livestock facilities. System cost is substantial, relative to US energy values, which may be a deterrent to more widespread use, although additional factors, such as odor control and carbon credits, may improve benefit /cost ratios.
Effects on wildlife
Grazing (especially, overgrazing) may detrimentally affect certain wildlife species, e.g. by altering cover and food supplies. However, habitat modification by livestock grazing can also benefit some wildlife species. For example, in North America, various studies have found that grazing sometimes improves habitat for elk, blacktailed prairie dogs, sage grouse, mule deer, and numerous other species. A survey of refuge managers on 123 National Wildlife Refuges in the US tallied 86 species of wildlife considered positively affected and 82 considered negatively affected by refuge cattle grazing or haying. Such mixed effects suggest that wildlife diversity may be enhanced and maintained by grazing livestock in some places while excluding livestock in some places. The kind of grazing system employed (e.g. rest-rotation, deferred grazing, HILF grazing) is often important in achieving grazing benefits for particular wildlife species.
Beneficial environmental effects
Among other environmental benefits of meat production is conversion of materials that might otherwise be wasted, to produce high-protein food. For example, Elferink et al. state that "Currently, 70 % of the feedstock used in the Dutch feed industry originates from the food processing industry." US examples of "waste" conversion with regard to grain include feeding livestock the distillers grains (with solubles) remaining from ethanol production. For the marketing year 2009/2010, dried distillers grains used as livestock feed (and residual) in the US amounted to 25.0 million metric tons. Much soy meal used as livestock feed is produced from material left after extraction of the soybean oil used in foods and in production of biodiesel, soaps and industrial fatty acids. Similarly, canola meal for livestock feed is produced from material left after oil extraction (for food and biodiesel) from canola seed. Examples with regard to roughages include straw from barley and wheat crops (feedable especially to large-ruminant breeding stock when on maintenance diets), and corn stover. Also, small-ruminant flocks in North America (and elsewhere) are sometimes used on fields for removal of various crop residues inedible by humans, converting them to food.
There are environmental benefits of meat-producing small ruminants for control of specific invasive or noxious weeds (such as spotted knapweed, tansy ragwort, leafy spurge, yellow starthistle, tall larkspur, etc.) on rangeland. Small ruminants are also useful for vegetation management in forest plantations, and for clearing brush on rights-of-way. These represent food-producing alternatives to herbicide use.
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