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Unit 7: Agriculture // Section 6: Livestock: Growing Animals

Many subsistence farmers in traditional societies raise livestock along with their crops, either for their families' use or for sale. But in industrialized nations, animal agriculture has been transformed in much the same way as crop production over the past century. Modern livestock farms are large and specialized and rely heavily on technology inputs. Like major plant crops, meat and dairy products are increasingly produced through a kind of monoculture in which farmers raise one or a few animal strains that have been bred to maximize output—hens that lay more eggs, dairy cows that produce more milk, or pigs that grow quickly and develop lean meat. Producers use technological inputs, such as antibiotics and hormone treatments, to make animals grow larger and more quickly.

To maximize efficiency, large-scale livestock farms confine animals indoors instead of letting them range outside (Fig. 10). Confining animals makes it easier to control the amount and type of feed they receive, administer medications and growth supplements, and artificially inseminate breeding females. But it also generates new management issues. Crowding stresses animals and promotes disease transmission, so many livestock farmers use antibiotics not only to treat sick animals but to prevent illnesses and promote growth. Many of these drugs are identical or similar to antibiotics used in human medicine, so their overuse threatens human health by promoting the development of drug-resistant bacterial strains that can infect humans through the food chain or via direct exposure to farm animals or wastes.

In addition, large farms accumulate massive quantities of animal waste. One cow can produce more than 40 pounds of manure per day. Manure liquefies when it is washed out of barns, so it is too heavy to transport economically over long distances. Many large farms store millions of gallons of manure onsite in tanks or lagoons (which may be lined or unlined, depending on local regulations), until it can be used on neighboring fields.

Confined hog production facility

Figure 10. Confined hog production facility
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Source: United States Geological Survey, Toxic Substances Hydrology Program.

When manure leaks or spills from storage, it sends large pulses of nutrients into local water bodies, causing algal blooms that deplete dissolved oxygen in the water and kill fish when they die and decompose. Nutrient pollution also occurs when manure is applied too heavily to farmland, so that plants cannot take up all of the available nitrogen and phosphate before the manure leaches into nearby rivers and streams. Excess nutrients, mainly from agricultural runoff, are a major cause of "dead zones" in large water bodies such as the Chesapeake Bay and the Gulf of Mexico (for details, see Unit 8, "Water Resources"). Manure also pollutes water with bacteria, hormones, and other chemical residues from animal feed.

Large livestock farms also generate air pollution from manure, dust, and greenhouse gases produced in the digestive systems of cattle and sheep. Many people who live near animal feeding operations complain about smells and suffer physical symptoms such as burning eyes, sore throats, and nausea. A 2003 National Research Council study found that livestock farms produce many air pollutants that are significant hazards at scales ranging from local to global (Table 1). However, the report concluded that more analysis was required to develop accurate measurements of these emissions as a basis for regulations and that the United States lacked standards for quantifying odor, which could be caused by various combinations of hundreds of compounds (footnote 9). (For more details on emissions and health risks, see Unit 6, "Risk, Exposure, and Health"; Unit 11, "Atmospheric Pollution"; and Unit 12, "Earth's Changing Climate.")

Table 1. Potential importance of air emissions from animal feeding operations at different spatial scales. Source: Adapted from National Research Council, Air Emissions From Animal Feeding Operations: Current Knowledge, Future Needs (Washington, DC: National Academies Press, 2003), p. 6.
Emission Global, national, and regional importance Local Importance (property line or nearest dwelling) Primary effects of concern
Ammonia (NH3) Major Minor Acid rain, haze
Nitrous oxide (N2O) Significant Insignificant Global climate change
Nitrogen oxides (NOX) Significant Minor Haze, acid rain, smog
Methane (CH4) Significant Insignificant Global climate change
Volatile organic compounds (VOCs) Insignificant Minor Quality of human life
Hydrogen sulfide (H2S) Insignificant Significant Quality of human life
Particulate matter (PM10) Insignificant Significant Haze
Fine particulate matter (PM2.5) Insignificant Significant Health, haze
Odor Insignificant Major Quality of human life

World demand for meat and dairy products is increasing, driven by population growth and rising incomes in developing countries. Because of this growth and the trend toward raising animals on large-scale farms, the FAO calls livestock farming "one of the top two or three most significant contributors to the most serious environmental problems, at every scale from local to global." According to FAO's estimates, livestock production generates 18 percent of world greenhouse gas emissions (more than the transport sector), accounts for 8 percent of world water use, and is probably the largest sectoral water pollution source (footnote 10).

With global meat and dairy production predicted to roughly double between 2000 and 2050, these environmental impacts will have to be drastically reduced just to keep agricultural pollution from worsening. And as we will see in section 8, "Agriculture and Energy," the fact that humans are eating at higher trophic levels by increasing their meat consumption makes agriculture more energy-intensive than it would be if people relied mainly on plant-based diets.

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