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Pork Production

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Pork production is an important component of American agriculture and an important part of the American diet and way of life. Fewer than100,000 farms were producing pork in 2000 with production is concentrated in the Corn Belt states and in North Carolina.

Modern pork production is mostly done in enclosed buildings to protect animals from the weather, from predators and from the spread of diseases. While larger operations enabled farmers to significantly increase the efficiency of production using less labor, it resulted in environmental challenges with larger amounts of manure concentrated in a small area.

This module will look at pork production as it has evolved over the past 300 years in the U.S., at the economic value of pork to the U.S. and American agriculture and at typical production and manure handling systems in use today.

Background of Pork Production in U.S.

Food Guide Pyramid

Source: U.S. Dept. of Agriculture/Dept. of Health and Human Services

Wild boars domesticated in N. Europe c.1500 B.C., are believed to be the ancestor of modern domesticated hogs, along with a genetic input from smaller Asian species domesticated in China around 3000 B.C. Pork, the meat from swine, was widely consumed throughout the ancient world and the Roman Empire. Pigs were not indigenous to the Americas, but came from Europe and the Orient. Columbus brought hogs on his second voyage to the Americas in 1493. Polynesians may have brought pigs from the Orient to the Hawaiian Islands even earlier.

For much of the 19th and 20th centuries, pork was the preferred meat in the U.S. Hogs were valued not just for their meat but for the lard, which was used for everything from cooking and lamp oil to baking and making candles and soap. As Americans became more health conscious, they lost much of their appetite for animal fats, switching to more healthy vegetable oils. Production began to focus on the pigs’ ability to efficiently convert feed into protein, which resulted in a much leaner type of pig being produced.

There has also been a significant change in how and where hogs are produced in the U.S. over the past 50 years. Low consumer prices, and therefore low producer prices, have resulted in larger, more efficient operations, with many smaller farms no longer able to produce pigs profitably.

Number of Hog Operations, USA

Source: USDA - NASS

U.S. Hog Operations Percent of Operations and Inventory, 2000

Source: USDA - NASS

Cattle, Broilers, Hogs, Turkeys Value of Production, 1945-1999

Source: USDA - NASS

In 1997, sales of all animals in the U.S. totaled over $75 billion. Currently, most of the swine in the United States are produced in North Carolina and the Midwestern and plains states, including Nebraska, Iowa, Minnesota, Missouri, Indiana and Illinois. Worldwide, China is by far the largest producer of pork, producing nearly four times as much as the U.S.

There are many breeds of swine, such as Hampshire, Duroc, Poland, China, Landrace, etc., but most farms use crossbreeds to try to gain the best traits of each breed.

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Products from Pork

Pork is the most widely eaten meat in the world.

Pork is the most widely consumed meat in the world. People eat many different pork products, such as bacon, sausage, pork chops and ham. A 250-pound market hog yields about 150 pounds of pork.

Several valuable products or by-products, in addition to meat, come from swine. These include insulin for the regulation of diabetes; valves for human heart surgery; suede for shoes and clothing; and gelatin for many food and non-food uses. Swine by-products are also important parts of such products as water filters, insulation, rubber, antifreeze, certain plastics, floor waxes, crayons, chalk, adhesives and fertilizer.

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Pork Production Phases

The phases of pork production that take place on the farm to produce hogs ready for market are called: breeding-gestation, farrowing, nursery and grow-finish.


Apply Downward Pressure on Her Back
Holding Her Ears Erect

Swine production can be logically separated into a number of phases, beginning with the sow being bred. Historically, this has been done by placing a number of sows in a pen with one or more boars. In confinement buildings, boars are often rotated between sow pens to make sure that all sows are bred while they were in heat. Sows in enclosed shelters come into estrous, 3 until 5 days after their pigs are weaned. The estrous period, or standing heat, is the period when the sow can be bred. Estrous only lasts a short time, so it is critical that the sow is bred at this time. During estrous, the sow shows outward signs of being willing to accept the boar, such as standing still when the producer applies downward pressure on her back or holding her ears erect. If the sow is not bred during this period, she normally returns to estrous about 21 days later. These two periods are known as "first heat breeding" and "second heat breeding". The non-pregnant sow is considered "unproductive" during this 3-week period, since she still must be fed and housed. Most modern operations have sows bred only on first heat. Sows that fail to breed during this estrous are often sent to market and replaced in the sow herd by gilts, or young females that are removed from the grow-finish group of pigs. After breeding, the sow "gestates" her "litter" for 113 to 116 days before the pigs are born or "farrowed." A good way to remember gestation length for swine is that it is approximately "3 months, 3 weeks and 3 days".

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Farrowing Crate

Protecting baby pigs in a farrowing crate
Source: Purdue University

Straw Bedding on Solid Floors

Pen farrowing
Source: USDA

Just before giving birth, called farrowing, sows are normally moved into a "farrowing room." Sows typically farrow from eight to twelve piglets, which as a group are called a litter. Most confinement operations place the sow in a temperature-controlled environment and usually in a farrowing pen or crate which restricts her movement to protect her baby pigs. The baby pigs spend most of their time in a "creep area on one or both sides of the crate where they have ready access to their mother, but are protected from crushing when she lies down. A few farrowing operations in the U.S. use larger pens and provide deep straw bedding on solid floors. While this is a more natural process for the sow, it involves more labor and often results in higher crushing losses.

An average sow will raise three to five litters of pigs in her lifetime. Sows may be culled and sent to market, because of age, health problems, failure to conceive, or if they are able to raise only a low number of pigs per litter.

Pigs are born with eight needle-sharp teeth and curly tails. The tips of the teeth are clipped at birth to prevent injury to the sow's utter and other piglets and the tail is shortened to prevent tail biting. Piglets weigh about three pounds at birth and are weaned from the sow at anywhere from five days to four weeks, with most operations weaning pigs at two to three weeks.

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Nursery Pigs

Nursery Pigs

Nursery pigs should have ready access to water and feed

Source: Purdue University

After weaning, pigs are normally placed in a "nursery" where they are kept in a temperature-controlled environment, usually on slotted floors. The floors in a nursery are usually constructed from plastic or plastic covered steel instead of concrete to provide additional comfort for the small pigs. Pigs are normally given around three square feet of space each and provided with ready access to water and feed. Nursery pens are sometimes elevated, with their slotted floor above the room floor level 8 to 12 inches. This is done to minimize the possibility of cold floor drafts chilling the young pigs. Immediately after weaning, the temperature in the nursery may be as much as 85 degrees, and then dropped gradually to about 70 degrees as the pigs grow. Pigs are normally removed from the nursery at about 6 to 10 weeks of age and placed in a "grow-finishing" building. Nursery rooms are almost always heated with furnaces and ventilated with mechanical fans, controlled by a thermostat, in order to keep the pigs warm and dry throughout the year.

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This phase is where pigs are fed as much as they wish to eat until they reach market weight of 250 to 275 pounds and provided around 8 sq. ft. of space per pig. Marketing normally occurs at five to six months of age, depending on genetics and any disease problems encountered. Some gilts are returned from the grow-finish phase to the sow herd for breeding purposes, to replace older sows that are culled.

Animals in a Grow-Finish Operation

Grow-finish pigs on concrete slotted floor
Source: Purdue University

Large Sidewall Vents Opened

Naturally ventilated (grow-finish building)

Large Ventilation Fans

Mechanical ventilation in grow-finish building Source: Purdue University

Animals in a grow-finish operation are larger and produce a great deal of body heat. Ventilation to keep the animals cool is usually more of a concern than providing heat in winter. Animals at this age grow best at around 60-70 degrees. In winter, they are protected from winter winds in a moderately well insulated building. Enough ventilation must be provided to remove moisture and to provide fresh air for the animals. In summer, large sidewall vents are opened or large ventilation fans are operated to keep the animals comfortable. This is referred to, respectively, as naturally ventilated (air change due to the wind) or mechanically ventilated (where air is drawn into the buildings through vents due to a negative pressure created with wall fans that exhaust inside air.

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Production Systems

Pasture Systems

Gestation sows on pasture

Before the 1960s, most pork in the U.S. was raised in outside lots or on pasture systems. With the development of slotted floors and liquid manure handling equipment, it became possible for producers to more easily care for larger numbers of animals, and to do so protected from the weather. Enclosed buildings overcame most weather problems, predators and minimized the potential pollution from outside lot runoff. It also made it practical to farrow sows twice a year, rather than once. This was the beginning of intensive production schedules on relatively small areas as found throughout the world today. 

Enclosed Buildings

Enclosed housing for swine

In a continuous flow barn, animals of many different stages of development may be housed in close proximity to one another and the facilities are never empty. Advantages are that space is efficiently used, because pigs can be moved to larger pens as they grow, and new arrivals replace them in the smaller pens. Continuous systems also simple to plan; if the producer wants to wean two litters each week, two sows must be bred each week.

Disadvantages are that different ages of animals (with different degrees of disease resistance) are housed together, facilitating disease spread, stress levels can be heightened with changing social groups, adequate cleaning and disinfecting are not feasible, and higher levels of antibiotics and other medications are normally required to control disease.

Most swine today are raised in “all-in, all-out” (AIAO) systems, where each room or building is completely emptied and sanitized between groups of pigs. Each new group of pigs enters a freshly disinfected environment, and stays there for this phase of their life. The facility has a separate room or building for each group of pigs weaned, with extra space if needed to allow workers time to clean the room before the next group of pigs. AIAO animals in each room are of a uniform age and size and are isolated to the extent possible to decrease the possibility of diseases spreading from older animal groups to younger ones.

The primary advantages are that disease spread can be better contained, animals are less stressed because they remain with the same age and social group throughout their development, and complete cleaning and disinfecting between groups is possible. The disadvantage is that space is less efficiently allocated, and that more space may be needed to allow rooms to be empty for cleaning between groups.

Until around 1990, swine production systems were usually housed on a single site, because of labor savings and convenience. Health concerns have since caused many swine operations to house the various production phases at different sites to further minimize contact between pigs of different ages. This is either a two-site or a three-site system. A two-site system has breeding and gestation at one site and farrowing/nursery and grow finish pigs at a separate site, while a three site also places the nursery at a separate site.

In the last few years, some producers have constructed “wean to finish” barns where pigs go immediately after weaning, and stay until market. This combines the nursery and grow-finish phases of production. These barns provide substantially more space per pig than is needed initially, but provide the advantage of only moving pigs once during their lifetime. This reduces stress on the animals and saves labor since buildings are not cleaned until the hogs are marketed.

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Swine have a digestive system similar to humans and different from ruminants such as cattle and sheep, which can eat forages or grasses. Pigs are fed a diet that is primarily ground corn to supply heat and energy and soybean meal to provide protein. Vitamins and minerals are also added in their feed. Rations are closely tailored to optimize health and growth at each stage in their life. Many producers even modify the ration based on the pig’s gender.

The ration is normally changed to provide more energy and less protein as the pig grows. The goal is to optimize feed utilization for different stages of growth. Since nutritional needs are different for male and female grow-finish pigs, larger operations may even modify the ration, based on gender. Recent studies indicate that ration modifications that can reduce the amount of nitrogen and phosphorous excreted in the manure, while maintaining optimum pig growth and health. It takes nearly 1000 pounds of feed to raise a hog to market weight. This same pig drinks about one-and-a-half to two gallons of water a day over its six-month life.

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Common Manure Handling Systems

Solid Manure

Solid Manure Collected from Solid Floor Shelters

Schematic of solid manure systems
Source: Purdue University

surface application of solid manure

Surface application of solid manure
Source: Purdue University

Swine manure was historically handled as a solid, either deposited directly by grazing animals, or collected in bedding placed on solid shelter floors to absorb the urine. Pastured animals spread the manure over the land as they grazed. Manure deposited on solid floors is typically stored where it falls, with more bedding added as needed to maintain a dry floor. Liquid drains away from the manure dropped on an outside lot and must be collected in a storage, leaving the solid manure behind. The manure composts in place somewhat and is removed every few months. Fertilizer value is recovered by spreading on cropland to complete the nutrient cycle. Solid manure is normally surface applied, but in some cases may be incorporated into the soil with a farm tillage operation shortly after spreading. Composting is another option for solid manure management.

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Lot Runoff

Outside Lots

Options for handling manure from open lot
Source: Purdue University

Clean Runoff

Options for handling manure from open lot holding pond for runoff from outside lot
Source: Davis-Purdue Farms

Manure is typically scraped from outside lots every week or two and stacked until it can be hauled to cropland. It is important to keep an outside lot relatively free of manure to control odor and so that rainfall runoff stays mostly free of manure. This facilitates storage of relatively clean runoff for irrigation onto cropland. It is even possible to divert runoff from small operations directly to pasture or to a vegetated filter strip where it can infiltrate. It must be prevented from entering waterways. Clean upslope water and roof water should be diverted away from the open lot to minimize the amount of wastewater that must be handled as a manure.

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Liquid Manure

Slotted Floor

Grow-finish pigs on slotted floor
Source: Purdue University

Most swine manure is handled as a liquid. Manure typically falls through a slotted floor (with the size of slot depending on the size and age of animal) into either a gutter or a concrete storage pit. Storage pits provides from 3-12 months storage of the manure. This pit may either be located directly under the slotted floor and may be from 4' to 10' deep. In some operations, the manure falls into a shallow pit or gutter which is periodically pumped, flushed or drained to a large outside storage. The outside storage may either be constructed in the earth or a commercial steel or concrete storage purchased and erected onsite . Storage size is dictated by regulatory agencies in most states and are usually sized large enough to hold at least six month’s accumulation in Midwestern states. This avoids the need to apply manure during the crop growing season and when weather conditions are unsuitable – such as on frozen ground or when the soil is wet enough that heavy application vehicles could compact and damage the soil for crop production.

A Sheepsfoot Roller is Used to Compact Soil for an Earthen Storage Structure
Commercial Steel or Concrete Storage Purchased and Erected Onsite

Fabricated steel manure storage


Agitate Liquid Manure Thoroughly

Liquid manure pit agitation
Source: Purdue University

Liquid Manure Applied to Exposed Soil Surface

Surface applied liquid manure
Source: Purdue University

Liquid manure from storage is normally agitated thoroughly to make the manure nutrient content between loads more uniform and hauled to the field for application in large tanker wagons or trucks. Liquid manure is either applied to the soil surface or is incorporated during or shortly after application to control loss of volatile ammonia and release of odors.  Incorporation is very effective at controlling runoff of manure nutrients and reducing odor from land application if done during or within a few hours after application.  One method is a soil injector, where liquid manure is “injected” directly into the soil to a depth of 6 to 9” as the tanker passes over the field.  This immediate contact between the manure and soil is highly effective at controlling odor.

Liquid Manure Injected Directly into the Soil

Injecting liquid manure
Source: Purdue University

In remote areas, liquid manure may be pumped to the land application site and then irrigated onto cropland.  Spray irrigating liquid manure is a very efficient method of land application, in terms of speed and labor, but odor emissions can be significant; therefore, it is not feasible to use this method in populated areas.

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Aerial view of Lagoon

Two-stage lagoon system
Source: Purdue University

Lagoons are different from liquid manure storage because they are operated to encourage anaerobic digestion of organic material while it is being stored.  This reduces odor when the treated manure is land applied. A properly designed and operated treatment lagoon is much larger and more expensive than a liquid manure storage with the same storage time, and the organic solids are much less concentrated in the liquid.

Gravity Drain, Flat Bottom Gutter

Gravity drain, flat bottom gutter
Source: Purdue University

Reverse Hairpin Flat-Bottom Gutter

Reverse hairpin, flat-bottom gutter
Source: Purdue University

Pit Recharge System

Recirculating lagoon water to flush manure from a gutter under a slotted floor
Source: Purdue University

In the Midwest, an equal part of relatively clean dilution water must be added for each part manure.  Furthermore, manure must be added slowly and uniformly to the lagoon, to avoid an upset (and subsequent release of odors) to the biological treatment system.  One common method of doing this is to utilize shallow pits or gutters under slotted floors and drain or flush manure to the lagoon on a frequent basis, usually every three days to three weeks. This is done by simply pulling a plug in the bottom of the pit, called gravity drain, use of a scraper system running in the underfloor gutter, through a process called a "hairpen" gutter or by recirculating a volume of relatively clean effluent from the lagoon to flush manure out of the building and into the lagoon. Recirculation involves either a flushing action that takes place several times a day or a "pit recharge" system that works basically like a toilet that is flushed every few days.

Treatment Volume

Single cell waste treatment lagoon
Source: Purdue University

A portion of the lagoon contents or "minimum design volume" must be left in the lagoon after its contents are pumped to the land to provide a large number of microbial organisms to treat the new manure entering the system.  In spite of proper operation, there is an “over turning” of the lagoon contents that occurs in the fall of the year for a couple of weeks, as ambient temperature drops and cools the top layer of liquid in the lagoon.  As its density increases, it “overturns” or drops to the bottom of the lagoon, forcing the bottom layer, containing partially digested manure solids, to the top. This phenomenon results in higher odor levels for a week or two around the lagoon. Multiple Lagoons in series normally emit fewer odors than single cell lagoons.

Spray Irrigation

Irrigation of lagoon effluent onto cropland
Source: Purdue University

Lagoon contents are normally applied to cropland by spray irrigation systems.  If the lagoon is properly designed and operated, spray irrigation should not release much odor because most of the organic solids should have been biologically degraded.  In a well-operated lagoon, typical effluent should have only about 20% as much nitrogen (N) and about 30% to 40% as much phosphorous (P) and potassium (K) as the raw manure, because of treatment and sedimentation of solids to the bottom of the lagoon. Note that the P and K "lost" actually accumulate in the sludge and must be utilized properly when removed. These solids, or sludge, must be removed every few years and the operation should plan to handle them as a part of their nutrient management plan.  Because this material is more concentrated, it may be practical to haul the sludge off site to more distant cropland that can better utilize the nutrients contained in the sludge.  Because of the nuisance potential of this partially stabilized material, it should be incorporated as a liquid manure if possible.

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Potential Environmental Impacts of Animal Feeding Operations

(Adapted in part from Livestock and Poultry Environmental Stewardship Curriculum, MidWest Plan Service; and Proposed US EPA Confined Feeding Rule.)

USEPA's 1998 National Water Quality Inventory indicates that agricultural operations, including animal feeding operations (AFOs), are a significant source of water pollution in the U.S. States estimate that agriculture contributes in part to the impairment of at least 170,750 river miles, 2,417,801 lake acres, and 1,827 estuary square miles (Table 1). Agriculture was reported to be the most common pollutant of rivers and streams.

However, one should not overlook the many positive environmental benefits of agriculture. For example, agricultural practices that conserve soil and increase productivity while improving soil quality also increase the amount of carbon-rich organic matter in soils, thereby providing a global depository for carbon dioxide drawn from the atmosphere by growing plants. The same farming practices that promote soil conservation also decrease the amount of carbon dioxide accumulating in the atmosphere and threatening global warming.

Other benefits compared to urban or industrial land use include greatly reduced storm runoff, groundwater recharge and water purification as infiltrating surface water filters through plant residue, roots and several feet of soil to reach groundwater.

In many watersheds, animal manures represent a significant portion of the total fertilizer nutrients added. In a few counties, with heavy concentrations of livestock and poultry, nutrients from confined animals exceed the uptake potential of non-legume harvested cropland and hayland. USDA estimates that recoverable manure nitrogen exceeds crop system needs in 266 of 3,141 counties in the U.S. (8%) and that recoverable manure phosphorus exceeds crop system needs in 485 counties (15%). It should be pointed out that while legumes are able to produce their own nitrogen, they will use applied nitrogen instead if it is available. The USDA analysis does not consider actual manure management practices used or transport of applied nutrients outside the county; however, it is a useful indicator of excess nutrients on a broad scale. Whole-farm nutrient balance is a very useful tool to identify potential areas of excess.

Air emissions from Animal Feeding Operations (AFO) can be odorous. Furthermore, volatilized ammonia can be redeposited on the earth and contribute to eutrophication of surface waters.

Animal manures are a valuable fertilizer and soil conditioner, if applied under proper conditions at crop nutrient requirements. Potential sources of manure pollution include open feedlots, pastures, treatment lagoons, manure stockpiles or storage, and land application fields. Oxygen-demanding substances, ammonia, nutrients (particularly nitrogen and phosphorus), solids, pathogens, and odorous compounds are the pollutants most commonly associated with manure. Manure is also a potential source of salts and trace metals, and to a lesser extent, antibiotics, pesticides and hormones. This problem has been magnified as poultry and livestock production has become more concentrated. AFO pollutants can impact surface water, groundwater, air, and soil. In surface water, manure's oxygen demand and ammonia content can result in fish kills and reduced biodiversity. Solids can increase turbidity and smother benthic organisms. Nitrogen and phosphorus can contribute to eutrophication and associated algae blooms which can produce negative aesthetic impacts and increase drinking water treatment costs. Turbidity from the blooms can reduce penetration of sunlight in the water column and thereby limit growth of seagrass beds and other submerged aquatic vegetation, which serve as critical habitat for fish, crabs, and other aquatic organisms. Decay of the algae (as well as night-time algal respiration) can lead to depressed oxygen levels, which can result in fish kills and reduced biodiversity. Eutrophication is also a factor in blooms of toxic algae and other toxic estuarine microorganisms, such as Pfiesteria piscicida. These organisms can impact human health as well as animal health. Human and animal health can also be impacted by pathogens and nitrogen in animal manure. Nitrogen is easily transformed into the nitrate form and if transported to drinking water sources can result in potentially fatal health risks to infants. Trace elements in manure may also present human and ecological risks. Salts can contribute to salinization and disruption of the ecosystem. Antibiotics, pesticides, and hormones may have low-level, long-term ecosystem effects.

In ground water, pathogens and nitrates from manure can impact human health via drinking water. Nitrate contamination is more prevalent in ground waters than surface waters. According to the U.S. EPA, nitrate is the most widespread agricultural contaminant in drinking water wells, and nearly 2% of our population (1.5 million people) is exposed to elevated nitrate levels from drinking water wells.

Table 1. Summary of U.S. Water Quality Impairment Survey
Total Quantity in US Amount of Waters Surveyed Quantity Impaired by All Sources Quantity Impaired by Agriculture
3,662,255 miles
23% of total
840,402 miles
36% of surveyed
248,028 miles
59% of impaired
170,750 miles
Lakes, Ponds, and Reservoirs
41,600,000 acres
42% of total
17,400,000 acres
39% of surveyed
6,541,060 acres
31% of impaired
2,417,801 acres
90,500 square miles
32% of total
28,889 square miles
38% of surveyed
11,025 square miles
15% of impaired
1,827 square miles
Reference: National Water Quality Inventory: 1998 Report to Congress (EPA, 2000a). AFOs are a subset of the agriculture category. Summaries of impairment by other sources are not presented here.

Table 2 lists the leading pollutants impairing surface water quality in the U.S. Agricultural production is a potential source of most of these.

Table 2. Five Leading Pollutants Causing Water Quality Impairment in the U.S.
(Percent of incidence of each pollutant is shown in parentheses. For example, siltation is listed as a cause of impairment in 38% of impaired river miles.)
Rank Rivers Lakes Estuaries
1 Siltation (38%) Nutrients (44%) Pathogens (47%)
2 Pathogens (36%) Metals (27%) Oxygen-Depleting Substances (42%)
3 Nutrients (29%) Siltation (15%) Metals (23%)
4 Oxygen-Depleting Substances (23%) Oxygen-Depleting Substances (14%) Nutrients (23%)
5 Metals (21%) Suspended Solids (10%) Thermal Modifications (18%)


List of Contaminants in Animal Manure:


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Oxygen-Demanding Substances

When discharged to surface water, biodegradable material is decomposed by aquatic bacteria and other microorganisms. During this process, dissolved oxygen is consumed, reducing the amount available for aquatic animals. Severe depressions in dissolved oxygen levels can result in fish kills. There are numerous examples nationwide of fish kills resulting from manure discharges and runoff from various types of AFOs.

Manure may be deposited directly into surface waters by grazing animals. Manually-collected manure may also be introduced into surface waters. This is typically via storage structure failure, overflow, operator error, etc.

Manure can also enter surface waters via runoff if it is over-applied or misapplied to land. For example, manure application to saturated or frozen soils may result in a discharge to surface waters. Factors that promote runoff to surface waters are steep land slope, high rainfall, low soil porosity, and proximity to surface waters. Incorporation of the manure into the soil decreases runoff.

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The Nitrogen Cycle

Nitrogen (N) is an essential nutrient required by all living organisms. It is ubiquitous in the environment, accounting for 78 percent of the atmosphere as elemental nitrogen (N2). This form of nitrogen is inert and does not impact environmental quality since it is not bioavailable to most organisms and therefore has no fertilizer value. Nitrogen can form other compounds, however, which are bioavailable, mobile, and potentially harmful to the environment. The nitrogen cycle shows the various forms of nitrogen and the processes by which they are transformed and lost to the environment.

Nitrogen in manure is primarily in the form of organic nitrogen and ammonia nitrogen compounds. In its organic form, nitrogen is unavailable to plants. However, organic nitrogen can be transformed into ammonium (NH4+) and nitrate (NO3-) forms, via microbial processes which are bioavailable and have fertilizer value. These forms can also produce negative environmental impacts when they are transported in the environment.


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"Ammonia-nitrogen" includes the ionized form (ammonium, NH4+) and the un-ionized form (ammonia, NH3). Ammonium is produced when microorganisms break down organic nitrogen products such as urea and proteins in manure. This decomposition occurs in both aerobic and anaerobic environments. In solution, ammonium is in chemical equilibrium with ammonia.

Ammonia exerts a direct biochemical oxygen demand (BOD) on the receiving water since dissolved oxygen is consumed as ammonia is oxidized. Moderate depressions of dissolved oxygen are associated with reduced species diversity, while more severe depressions can produce fish kills.

Additionally, ammonia can lead to eutrophication, or nutrient over-enrichment, of surface waters. While nutrients are necessary for a healthy ecosystem, the overabundance of nutrients (particularly nitrogen and phosphorus) can lead to nuisance algae blooms.

Pfiesteria often lives as a nontoxic predatory animal, becoming toxic in response to fish excretions or secretions (NCSU, 1998). While nutrient-enriched conditions are not required for toxic outbreaks to occur, excessive nutrient loadings can help create an environment rich in microbial prey and organic matter that Pfiesteria uses as a food supply. By increasing the concentration of Pfiesteria, nutrient loads increase the likelihood of a toxic outbreak (Citizens Pfiesteria Action Commission, 1997).

The degree of ammonia volatilization is dependent on the manure management system. For example, losses are greater when manure remains on the land surface rather than being incorporated into the soil, and are particularly high when the manure is spray irrigated onto land. Environmental conditions also affect the extent of volatilization. For example, losses are greater at higher pH levels, warmer temperatures and drier conditions, and in soils with low cation exchange capacity, such as sands. Losses are decreased by the presence of growing plants. (Follett, 1995)

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Nitrifying bacteria can oxidize ammonium to nitrite (NO2-) and then to nitrate (NO3-). Nitrite is toxic to most fish and other aquatic species, but it typically does not accumulate in the environment because it is rapidly transformed to nitrate in an aerobic environment. Alternatively, nitrite (and nitrate) can undergo bacterial denitrification in an anoxic environment. In denitrification, nitrate is converted to nitrite, and then further converted to gaseous forms of nitrogen - elemental nitrogen (N2), nitrous oxide (N2O), nitric oxide (NO), and/or other nitrogen oxide (NOx) compounds. Nitrification occurs readily in the aerobic environments of receiving streams and dry soils while denitrification can be significant in anoxic bottom waters and saturated soils.

Nitrate is a useful form of nitrogen because it is biologically available to plants and is therefore a valuable fertilizer. However, excessive levels of nitrate in drinking water can produce negative health impacts on infant humans and animals. Nitrate poisoning affects infants by reducing the oxygen-carrying capacity of the blood. The resulting oxygen starvation can be fatal. Nitrate poisoning, or methemoglobinemia, is commonly referred to as "blue baby syndrome" because the lack of oxygen can cause the skin to appear bluish in color. To protect human health, EPA has set a drinking water Maximum Contaminant Level (MCL) of 10 mg/l for nitrate-nitrogen. Once a water source is contaminated, the costs of protecting consumers from nitrate exposure can be significant. Nitrate is not removed by conventional drinking water treatment processes; its removal requires additional, relatively expensive treatment units.

Nitrogen in livestock manure is almost always in the organic, ammonia or ammonium form but may become oxidized to nitrate after being diluted. It can reach surface waters via direct discharge of animal wastes. Lagoon leachate and land-applied manure can also contribute nitrogen to surface and ground waters. Nitrate is water soluble and moves freely through most soils. Nitrate contributions to surface water from agriculture are primarily from groundwater connections and other subsurface flows rather than overland runoff (Follett, 1995).

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The Phosphorus Cycle

Animal wastes contain both organic and inorganic forms of phosphorus (P). As with nitrogen, the organic form must mineralize to the inorganic form to become available to plants. This occurs as the manure ages and the organic P hydrolyzes to inorganic forms. The phosphorus cycle is much simpler than the nitrogen cycle because phosphorus lacks an atmospheric connection and is less subject to biological transformation.

Phosphorus is of concern in surface waters because it can lead to eutrophication. Phosphorus is also a concern because phosphate levels greater than 1.0 mg/l may interfere with coagulation in drinking water treatment plants (Bartenhagen et al., 1994). A number of research studies are currently underway to decrease the amount of P in livestock manure, primarily through enzymes and animal ration modifications that make phosphorous in the feed more available (and usable) by the animal. This means that less phosphorus must be fed to ensure an adequate amount for the animal and, as a result, less phosphorous is excreted in the manure.

Phosphorus predominantly reaches surface waters via direct discharge and runoff from land application of fertilizers and animal manure. Once in receiving waters, the phosphorus can become available to aquatic plants. Land-applied phosphorus is much less mobile than nitrogen since the mineralized form (inorganic Phosphate) is easily adsorbed to soil particles. For this reason, most agricultural phosphorus control measures have focused on soil erosion control to limit transport of particulate phosphorus. However, soils do not have infinite phosphate adsorption capacity and with long-term over-application, inorganic phosphates can eventually enter waterways even if soil erosion is controlled.

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Both manure and animal carcasses contain pathogens (disease-causing organisms) which can impact human health, other livestock, aquatic life, and wildlife when introduced into the environment. Several pathogenic organisms found in manure can infect humans.

Table 1. Some Diseases and Parasites Transmittable to Humans from Animal Manure
Disease Responsible Organism Symptoms
Anthrax Bacillus anthracis Skin sores, fever, chills, lethargy, headache, nausea, vomiting, shortness of breath, cough, nose/throat congestion, pneumonia, joint stiffness, joint pain
Brucellosis Brucella abortus, Brucella melitensis, Brucella suis Weakness, lethargy, fever, chills, sweating, headache
Colibaciliosis Escherichia coli (some serotypes) Diarrhea, abdominal gas
Coliform mastitis-metritis Escherichia coli (some serotypes) Diarrhea, abdominal gas
Erysipelas Erysipelothrix rhusiopathiae Skin inflammation, rash, facial swelling, fever, chills, sweating, joint stiffness, muscle aches, headache, nausea, vomiting
Leptospirosis Leptospira Pomona Abdominal pain, muscle pain, vomiting, fever
Listeriosis Listeria monocytogenes Fever, fatigue, nausea, vomiting, diarrhea
Salmonellosis Salmonella species Abdominal pain, diarrhea, nausea, chills, fever, headache
Tetanus Clostridium tetani Violent muscle spasms, “lockjaw” spasms of jaw muscles, difficulty breathing
Tuberculosis Mycobacterium tuberculosis, Mycobacterium avium Cough, fatigue, fever, pain in chest, back, and/or kidneys
Q fever Coxiella burneti Fever, headache, muscle pains, joint pain, dry cough, chest pain, abdominal pain, jaundice
Foot and Mouth Virus Rash, sore throat, fever
Hog Cholera Virus  
New Castle Virus  
Psittacosis Virus Pneumonia
Coccidioidycosis Coccidioides immitus Cough, chest pain, fever, chills, sweating, headache, muscle stiffness, joint stiffness, rash wheezing
Histoplasmosis Histoplasma capsulatum Fever, chills, muscle ache, muscle stiffness, cough, rash, joint pain, join stiffness
Ringworm Various microsporum and trichophyton Itching, rash
Balantidiasis Balatidium coli  
Coccidiosis Eimeria species Diarrhea, abdominal gas
Cryptosporidiosis Cryptosporidium species Watery diarrhea, dehydration, weakness, abdominal cramping
Giardiasis Giardia lamblia Diarrhea, abdominal pain, abdominal gas, nausea, vomiting, headache, fever
Toxoplasmosis Toxoplasma species Headache, lethargy, seizures, reduced cognitive function
Ascariasis Ascaris lumbricoides Worms in stool or vomit, fever, cough, abdominal pain, bloody sputum, wheezing, skin rash, shortness of breath
Sarcocystiasis Sarcosystis species Fever, diarrhea, abdominal pain
References: USDA, 1992 (for diseases and responsible organisms). Symptom descriptions were obtained from various medical and public health service Internet websites. Pathogens in animal manure are a potential source of disease in humans and other animals. This list represents a sampling of diseases that may be transmittable to humans.

The treatment of public water supplies reduces the risk of infection via drinking water. However, protecting source water is the best way to ensure safe drinking water. Cryptosporidium parvum, a protozoan that can produce gastrointestinal illness, is a concern, since it is resistant to conventional treatment. Healthy people typically recover relatively quickly from such illnesses. However, they can be fatal in people with weakened immune systems such as the elderly and small children.

Runoff from fields where manure has been applied can be a source of pathogen contamination, particularly if a rainfall event occurs soon after application. The natural filtering and adsorption action of soils typically strands microorganisms in land-applied manure near the soil surface (Crane et al., 1980). This protects underlying groundwater, but increases the likelihood of runoff losses to surface waters. Depending on soil type and operating conditions, however, subsurface flows can be a mechanism for pathogen transport.

Soil type, manure application rate, and soil pH are dominating factors in bacteria survival (Dazzo et al., 1973; Ellis and McCalla, 1976; Morrison and Martin, 1977; Van Donsel et al., 1967). Experiments on land-applied poultry manure have indicated that the population of fecal organisms decreases rapidly as the manure is heated, dried, or exposed to sunlight on the soil surface (Crane et al., 1980).

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Antibiotics, Pesticides, and Hormones

Antibiotics, pesticides, and hormones are organic compounds which are used in animal feeding operations and may pose risks if they enter the environment. For example, chronic toxicity may result from low-level discharges of antibiotics and pesticides. Estrogen hormones have been implicated in the reduction in sperm counts among Western men (Sharpe and Skakkebaek, 1993) and reproductive disorders in a variety of wildlife (Colburn et al., 1993). Other sources of antibiotics and hormones include municipal waste waters, septic tank leachate, and runoff from land-applied sewage sludge. Sources of pesticides include crop runoff and urban runoff.

Little information is available regarding the concentrations of these compounds in animal wastes, or their fate/transport behavior and bioavailability in waste-amended soils. These compounds may reach surface waters via runoff from land-application sites.

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Airborne Emissions from Animal Production Systems

With the trend toward larger, more concentrated production operations, odors and other airborne emissions are rapidly becoming an important issue for agricultural producers.

Whether there is a direct impact of airborne emissions from animal operations on human health is still being debated. There are anecdotal reports about health problems and quality-of-life factors for those living near animal facilities have been documented.

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Source of Airborne Emissions

Liquid Manure Injected Directly into the Soil

Odor emissions from animal production systems originate from three primary sources: manure storage facilities, animal housing, and land application of manure.

In an odor study in a United Kingdom county (Hardwick 1985), 50% of all odor complaints were traced back to land application of manure, about 20% were from manure storage facilities, and another 25% were from animal production buildings. Other sources include feed production, processing centers, and silage storage. With the increased use of manure injection for land application, and longer manure storage times, there may be a higher percentage of complaints in the future associated with manure storage facilities and animal buildings and less from land application.

Animal wastes include manure (feces and urine), spilled feed and water, bedding materials (i.e., straw, sunflower hulls, wood shaving), wash water, and other wastes. This highly organic mixture includes carbohydrates, fats, proteins, and other nutrients that are readily degradable by microorganisms under a wide variety of suitable environments. Moisture content and temperature also affect the rate of microbial decomposition.

A large number of volatile compounds have been identified as byproducts of animal waste decomposition. O'Neill and Phillips (1992) compiled a list of 168 different gas compounds identified in swine and poultry wastes. Some of the gases (ammonia, methane, and carbon dioxide) also have implications for global warming and acid rain issues. It has been estimated that one third of the methane produced each year comes from industrial sources, one third from natural sources, and one third from agriculture (primarily animals and manure storage units). Although animals produce more carbon dioxide than methane, methane has as much as 15 times more impact on the greenhouse effect than carbon dioxide.

Dust, pathogens, and flies are from animal operations also airborne emission concerns. Dust, a combination of manure solids, dander, feathers, hair, and feed, is very difficult to eliminate from animal production units. It is typically more of a problem in buildings that have solid floors and use bedding as opposed to slotted floors and liquid manure. Concentrations inside animal buildings and near outdoor feedlots have been measured in a few studies; however, dust emission rates from animal production are mostly unknown.

Pathogens are another airborne emission concern. Although pathogens are present in buildings and manure storage units, they typically do not survive aerosolization well, but some may be transported by dust particles.

Flies are an additional concern from certain types of poultry and livestock operations. The housefly completes a cycle from egg to adult in 6 to 7 days when temperatures are 80 to 90°F. Females can produce 600 to 800 eggs, larvae can survive burial at depths up to 4 feet, and adults can fly up to 20 miles. Large populations of flies can be produced relatively quickly if the correct environment is provided. Flies tend to proliferate in moist animal production areas with low animal traffic.

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Emission Movement or Dispersion

The movement or dispersion of airborne emissions from animal production facilities is difficult to predict and is affected by many factors including topography, prevailing winds, and building orientation. Prevailing winds must be considered to minimize odor transport to close or sensitive neighbors. A number of dispersion models have been developed to Airborne Emission Regulations.

Most states and local units of government deal with agricultural air quality issues through zoning or land use ordinances. Setback distances may be required for a given size operation or for land application of manure. A few states (for example, Minnesota) have an ambient gas concentration (H2S for Minnesota) standard at the property line. Gas and odor standards are difficult to enforce since on-site measurements of gases and especially odor are hard to do with any high degree of accuracy. Producers should be aware of odor- or dust-related emissions regulations applicable to their livestock operation.

Source: Lesson 40 of the LPES: Adapted from Livestock and Poultry Environmental Stewardship curriculum, lesson authored by Larry Jacobson, University of Minnesota; Jeff Lorimor, Iowa State University; Jose Bicudo, University of Kentucky; and David Schmidt, University of Minnesota, courtesy of MidWest Plan Service, Iowa State University, Ames, Iowa 50011-3080, Copyright (c) 2001.

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Environmental Impacts of Animal Feeding Operations Study Questions

Identify the definition that best fits the following terms:

  1. What are some of the positive environmental benefits of production agriculture?

    Improved soil quality
    Carbon repository
    Reduced storm runoff
    Ground water recharge
    All of the above


  2. USDA estimates that manure phosphorous exceeds crop needs in what percent of U.S. countries?

  3. What is the common name for the class of production agricultural plants that do not need commercial nitrogen fertilizer?
    N Converters

  4. If biodegradable organic matter is added to a stream, fish kills most often result from:
    Lack of oxygen
    Lack of visibility
    Build up of sludge deposits

  5. Most manure spills that enter streams result from:
    Pastured animals with stream access
    Rupture of manure storage
    Improper land application of manure
    Damage to manure transfer or irrigation pipes
    Feedlot runoff from animal area
    All of the above

  6. Nitrogen gas (N2) account for ___________ % of the atmosphere?

  7. Nitrogen in manure can take many chemical forms. Which of the following is NOT included?
    Nitrogen gas
    Organic nitrogen
    Catatonic nitrogen

  8. Which nutrient in runoff from agricultural land has been blamed for the hypoxia problem in the Gulf of Mexico?
    Soil erosion

  9. Which of the forms of nitrogen are volatile?

  10. High nitrate levels in drinking water can lead to the following serious condition in infants:

    "Green baby syndrome"


  11. The most important method of reducing phosphorous entering streams is:
    Placing riprap along edge of stream
    Preventing soil erosion
    Improving field drainage

  12. Which of the following is NOT a significant source of air emissions around a livestock or animal production operation?
    Feed processing
    Land application of manure
    Animal production lots and buildings
    Manure Storage

Score in Percentage:

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Comprehensive Nutrient Management Planning

Recently, the concept of Comprehensive Nutrient Management Planning (CNMP) was introduced by the U. S. Environmental Protection Agency (EPA) and U.S. Department of Agriculture’s (USDA’s) Natural Resources Conservation Service (NRCS). It is anticipated that the CNMP will serve as a cornerstone of environmental plans assembled by animal feeding operations to address federal and state regulations. EPA and NRCS guidelines for CNMP are given in Table 1.

Table 1. Summary of Issues addressed by a CNMP as initially definied by EPA's Guidance
Planning components of CNMP Issues addressed
A manure handling and storage plan
  1. Diversion of clean water
  2. Prevention of leakage storage plan
  3. Adequate storage
  4. Manure treatment
  5. Management of mortality
Land application plan
  1. Proper nutrient application rates to achieve a crop nutrient balance
  2. Selection of timing and application methods to limit risk of runoff
Site management plan Soil conservation practices that minimize movement of soil and manure components to surface and groundwater
Record keeping Manure production, utilization, and export to off-farm users
Other utilization options Alternative safe manure utilization strategies such as sale of manure, treatment technologies, or energy generation
Feed management plan Alternative feed programs to minimize the nutrients in manure
Reference: USDA/EPA Unified National Strategy for Animal Feeding Operations (PDF) (34 pp, 404K)

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Pork Production Study Questions

Identify the definition that best fits the following terms:

  1. Boar
    A female pig
    An adult male hog, normally used for breeding
    A tedious individual
    A type of feeding system

  2. Gilt
    An older female swine that failed to conceive
    A non-innocent individual
    A male hog
    A female pig that has not yet farrowed

  3. Farrowing
    Conservation process whereby land lies dormant
    Feeding program for grow-finish hogs
    Period when pigs are born to the sow
    Practice of moving pigs from sow to sow to create more uniform litters

  4. Production Schedule
    Feeding program for pigs, from birth to market
    Marketing program for a swine operation
    Schedule by which sows are bred, farrowed, pigs are weaned, moved from production phase to production phase and eventually marketed

  5. Mating
    Sometimes follows dating
    Matching baby pigs at birth to their mothers
    Process by which pigs are matched by weight to determine pen mates
    Inseminating sows or gilts at estrous via exposure to a boar (or artificial insemination)

  6. Pen
    Something to write with
    Confined area where swine or other animals are kept
    Farmstead area
    Prison facilities

  7. Shoat
    A large male swine
    An immature swine
    A type of knife used for castration
    A type of portable shelter used when swine are kept when placed on pasture land

  8. Slotted floors
    A type of flooring used under stacked hay, to facilitate drying
    Flooring that has been grooved to improve footing for animals
    A type of porous floor that allows waste to drain away, keeping animals dry and comfortable

  9. Barrow
    The container located above the wheel in a wheelbarrow
    A male swine who has been castrated to reduce aggressiveness and improve the flavor of the meat
    An excavated pit from which soil or gravel is removed for a construction project

  10. Estrus

    An indication that the sow is ready to be bred
    A type of grain fed to grow-finish swine
    The process of removing pigs from the sow
    An important religious holiday


  11. Number of tons of pork produced in the U.S. each year

  12. Leading hog producing state, in terms of market animals
    North Carolina

  13. Hoop structure
    A popular game in the U.S. during the 1950s
    A type of portable, low cost shelter used for gestation and grow finish swine. Limited mainly to smaller operations
    A general purpose building used mainly as a utility building for farm shops, feed or machinery storage.
    A sound making device used to herd animals

  14. Land application
    Legal forms that much be completed when applying for financing to purchase a farm
    The application of manure to cropland at a rate that matches the nutrients needed by the plants
    Application of irrigation water to a farm field

  15. Whole farm nutrient balance
    A type of pork cut from the loin area
    A method of balancing the nutrients in a ration for swine
    Balancing nutrient inputs to the production unit with the use and removal of those nutrients from the operation

Score in Percentage:

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