Global economic forces have contributed to the industrialization of agriculture (Donham and Thu 1995). In the developed countries, there are trends toward increased specialization, intensity and mechanization. Increased confinement production of livestock has been a result of these trends. Many developing countries have recognized the need to adopt confinement production in an attempt to transform their agriculture from a subsistence to a globally competitive enterprise. As more corporate organizations obtain ownership and control of the industry, fewer, but larger, farms with many employees replace the family farm.

Conceptually, the confinement system applies principles of industrial mass production to livestock production. The concept of confinement production includes raising animals in high densities in structures that are isolated from the outside environment and equipped with mechanical or automated systems for ventilation, waste handling, feeding and watering (Donham, Rubino et al. 1977).

Several European countries have been using confinement systems since the early 1950s. Livestock confinement started to appear in the United States in the late 1950s. Poultry producers were first to use the system. By the early 1960s, the swine industry had also started to adopt this technique, followed more recently by dairy and beef producers.

Accompanying this industrialization, several worker health and social concerns have developed. In most Western countries, farms are getting fewer in number but larger in size. There are fewer family farms (combined labour and management) and more corporate structures (particularly in North America). The result is that there are more hired workers and relatively fewer family members working. Additionally, in North America, more workers are coming from minority and immigrant groups. Therefore, there is a risk of producing a new underclass of workers in some segments of the industry.

A whole new set of occupational hazardous exposures has arisen for the agricultural worker. These can be categorized under four main headings:

1. toxic and asphyxiating gases
2. bioactive aerosols of particulates
3. infectious diseases
4. noise.

Respiratory hazards are also a concern.

Toxic and Asphyxiating Gases

Several toxic and asphyxiating gases resulting from microbial degradation of animal wastes (urine and faeces) may be associated with livestock confinement. Wastes are most commonly stored in liquid form under the building, over a slatted floor or in a tank or lagoon outside the building. This manure storage system is usually anaerobic, leading to the formation of a number of toxic gases (see table 1) (Donham, Yeggy and Dauge 1988). See also the article “Manure and waste handling" in this chapter.

Table 1. Compounds identified in swine confinement building atmospheres

There are four common toxic or asphyxiating gases present in almost every operation where anaerobic digestion of wastes occurs: carbon dioxide (CO₂), ammonia (NH₃), hydrogen sulphide (H₂S) and methane (CH₄). A small amount of carbon monoxide (CO) may also be produced by the decomposing animal wastes, but its main source is heaters used to burn fossil fuels. Typical ambient levels of these gases (as well as particulates) in swine confinement buildings are shown in table 2. Also listed are maximum recommended exposures in swine buildings based on recent research (Donham and Reynolds 1995; Reynolds et al. 1996) and threshold limit values (TLVs) set by the American Conference of Governmental Industrial Hygienists (ACGIH 1994). These TLVs have been adopted as legal limits in many countries.

Table 2. Ambient levels of various gases in swine confinement buildings

It can be seen that in many of the buildings, at least one gas, and often several, exceeds the exposure limits. It should be noted that simultaneous exposure to these toxic substances may be additive or synergistic—the TLV for the mixture may be exceeded even when individual TLVs are not exceeded. Concentrations are often higher in the winter than in the summer, because ventilation is reduced to conserve heat.

These gases have been implicated in several acute conditions in workers. H₂S has been implicated in many sudden animal deaths and several human deaths (Donham and Knapp 1982). Most acute cases have occurred shortly after the manure pit has been agitated or emptied, which may result in a sudden release of a large volume of the acutely toxic H₂S. In other fatal cases, manure pits had recently been emptied, and workers who entered the pit for inspection, repairs or to retrieve a dropped object collapsed without any forewarning. The available post-mortem results of these cases of acute poisoning revealed massive pulmonary oedema as the only notable finding. This lesion, combined with the history, is compatible with hydrogen sulphide intoxication. Rescue attempts by bystanders have often resulted in multiple fatalities. Confinement workers should therefore be informed of the risks involved and advised never to enter a manure storage facility without testing for the presence of toxic gases, being equipped with a respirator with its own oxygen supply, ensuring adequate ventilation and having at least two other workers stand by, attached by a rope to the worker who enters, so they can effect a rescue without endangering themselves. There should be a written confined-space programme.

CO may also be present at acute toxic levels. Abortion problems in swine at an atmospheric concentration of 200 to 400 ppm and subacute symptoms in humans, such as chronic headache and nausea, have been documented in swine confinement systems. The possible effects on the human foetus should also be of concern. The primary source of CO is from improperly functioning hydrocarbon-burning heating units. Heavy accumulation of dust in swine confinement buildings makes it difficult to keep heaters in correct working order. Propane-fuelled radiant heaters are also a common source of lower levels of CO (e.g., 100 to 300 ppm). High-pressure washers powered by an internal combustion engine that may be run inside the building are another source; CO alarms should be installed.

Another acutely dangerous situation occurs when the ventilation system fails. Gas levels may then rapidly build up to critical levels. In this case the major problem is replacement of oxygen by other gases, primarily CO₂ produced from the pit as well as from the respiratory activity of the animals in the building. Lethal conditions could be reached in as few as 7 hours. Regarding the health of the pigs, ventilation failure in warm weather may allow temperature and humidity to increase to lethal levels in 3 hours. Ventilation systems should be monitored.

A fourth potentially acute hazard arises from build-up of CH₄, which is lighter than air and, when emitted from the manure pit, tends to accumulate in the upper portions of the building. There have been several instances of explosions occurring when the CH₄ accumulation was ignited by a pilot light or a worker’s welding torch.

Bioactive Aerosols of Particulates

The sources of dust in confinement buildings are a combination of feed, dander and hair from the swine and dried faecal material (Donham and Scallon 1985). The particulates are about 24% protein and therefore have the potential not only for initiating an inflammatory response to foreign protein but also for initiating an adverse allergic reaction. The majority of particles are smaller than 5 microns, allowing them to be respired into the deep portions of the lungs, where they may produce a greater danger to health. The particulates are laden with microbes (10⁴ to 10⁷/m³ air). These microbes contribute several toxic/inflammatory substances including, among others, endotoxin (the most documented hazard), glucans, histamine and proteases. The recommended maximum concentrations for dusts are listed in table 2. Gases present within the building and bacteria in the atmosphere are adsorbed on the surface of the dust particles. Thus, the inhaled particles have the increased potentially hazardous effect of carrying irritating or toxic gases as well as potentially infectious bacteria into the lungs.

Infectious Diseases

Some 25 zoonotic diseases have been recognized as having occupational significance for agricultural workers. Many of these may be transmitted directly or indirectly from livestock. The crowded conditions prevailing in confinement systems offer a high potential for transmission of zoonotic diseases from livestock to humans. Swine confinement environment may offer a risk for transmission to workers of swine influenza, leptospirosis, Streptococcus suis and salmonella, for example. The poultry confinement environment may offer a risk for ornithosis, histoplasmosis, New Castle disease virus and salmonella. Bovine confinement could offer a risk for Q fever, Trichophyton verrucosum (animal ringworm) and leptospirosis.

Biologicals and antibiotics have also been recognized as potential health hazards. Injectable vaccines and various biologicals are commonly used in veterinary preventive medical programmes in animal confinement. Accidental inoculation of Brucella vaccines and Escherichia coli bacteria has been observed to cause illness in humans.

Antibiotics are commonly used both parenterally and incorporated in animal feed. Since it is recognized that feed is a common component of the dust present in animal confinement buildings, it is assumed that antibiotics are also present in the air. Thus, antibiotic hypersensitivity and antibiotic-resistant infections are potential hazards for the workers.

Noise

Noise levels of 103 dBA have been measured within animal confinement buildings; this is above the TLV, and offers a potential for noise-induced hearing loss (Donham, Yeggy and Dauge 1988).

Respiratory Symptoms of Livestock Confinement Workers

The general respiratory hazards within livestock confinement buildings are similar regardless of the species of livestock. However, swine confinements are associated with adverse health effects in a larger percentage of workers (25 to 70% of active workers), with more severe symptoms than those in poultry or cattle confinements (Rylander et al. 1989). The waste in poultry facilities is usually handled in solid form, and in this instance ammonia seems to be the primary gaseous problem; hydrogen sulphide is not present.

Subacute or chronic respiratory symptoms reported by confinement workers have been observed to be most frequently associated with swine confinement. Surveys of swine confinement workers have revealed that about 75% suffer from adverse acute upper respiratory symptoms. These symptoms can be broken down into three groups:

1. acute or chronic inflammation of the respiratory airways (manifested as bronchitis)
2. acquired occupational (non-allergic) constriction of the airways (asthma)
3. delayed self-limited febrile illness with generalized symptoms (organic dust toxic syndrome (ODTS)).

Symptoms suggestive of chronic inflammation of the upper respiratory system are common; they are seen in about 70% of swine confinement workers. Most commonly, they include tightness of the chest, coughing, wheezing and excess sputum production.

In approximately 5% of workers, symptoms develop after working in the buildings for only a few weeks. The symptoms include chest tightness, wheezing and difficult breathing. Usually these workers are affected so severely that they are forced to seek employment elsewhere. Not enough is known to indicate whether this reaction is an allergic hypersensitivity or a non-allergic hypersensitivity to dust and gas. More typically, symptoms of bronchitis and asthma develop after 5 years of exposure.

Approximately 30% of workers occasionally experience episodes of delayed symptoms. Approximately 4 to 6 hours after working in the building they develop a flu-like illness manifested by fever, headache, malaise, general muscle aches and chest pain. They usually recover from these symptoms in 24 to 72 hours. This syndrome has been recognized as ODTS.

The potential for chronic lung damage certainly seems to be real for these workers. However, this has not been documented so far. It is recommended that certain procedures be followed to prevent chronic exposure as well as acute exposure to the hazardous materials in swine confinement buildings. Table 3 summarizes the medical conditions seen in swine confinement workers.

Table 3. Respiratory diseases associated with swine production

Sources: Donham, Zavala and Merchant 1984; Dosman et al. 1988; Haglind and Rylander 1987; Harries and Cromwell 1982; Heedrick et al. 1991; Holness et al. 1987; Iverson et al. 1988; Jones et al. 1984; Leistikow et al. 1989; Lenhart 1984; Rylander and Essle 1990; Rylander, Peterson and Donham 1990; Turner and Nichols 1995.

Worker Protection

Acute exposure to hydrogen sulphide. Care should always be taken to avoid exposure to H₂S that may be given off when agitating an anaerobic liquid manure storage tank. If the storage is under the building, it is best to stay out of the building when the emptying procedure is going on and for several hours afterwards, until air sampling indicates it is safe. Ventilation should be at the maximum level during this time. A liquid manure storage facility should never be entered without the safety measures mentioned above being followed.

Particulate exposure. Simple management procedures, such as the use of automated feeding equipment designed to eliminate as much feed dust as possible should be used to control particulate exposure. Adding extra fat to feed, frequent power-washing of the building and installing slatted flooring that cleans well are all proven control measures. An oil-misting dust-control system is presently under study and may be available in the future. In addition to good engineering control, a good-quality dust mask should be worn.

Noise. Ear protectors should be provided and worn, particularly when working in the building in order to vaccinate the animals or for other management procedures. A hearing conservation programme should be instituted.

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Animal husbandry—the rearing and use of animals—involves a wide variety of activities, including breeding, feeding, moving animals from one location to another, basic care (e.g., hoof care, cleaning, vaccinations), care for injured animals (either by animal handlers or veterinarians) and activities associated with particular animals (e.g., milking of cows, shearing of sheep, working with draught animals).

Such handling of livestock is associated with a variety of injuries and illnesses among humans. These injuries and illnesses may be due to direct exposure or may be due to environmental contamination from animals. The risk of injury and illness is dependent largely on the type of livestock. The risk of injury also depends on the particulars of animal behaviour (see also the articles in this chapter on specific animals). In addition, persons associated with animal husbandry are often more likely to consume products from the animals. Finally, the specific exposures depend on methods of handling livestock, which have emerged from geographical and social factors that vary across human society.

Hazards and Precautions

Ergonomic Risks

Personnel who work with cattle often have to stand, reach, bend or exert physical effort in sustained or unusual positions. Livestock workers do have an increased risk of joint pain of the back, hips and knees. There are several activities that place the livestock worker at ergonomic risk. For example, assisting with birthing of a large animal may put the farmworker in an unusual and strained position, whereas with a small animal, the worker may be required to work or lie in an inclement environment. Further, the worker may be injured by assisting animals who are ill and whose behaviour cannot be anticipated. More commonly, joint and back pain have to do with a repetitive motion, such as milking, during which the worker may crouch or kneel repeatedly.

Other cumulative trauma diseases are recognized in farmworkers, particularly livestock workers. These may be due to repetitive motion or frequent small injuries.

Solutions to reduce ergonomic risk include intensified educational efforts focused upon appropriate handling of animals, as well as engineering efforts to redesign the work environment and its tasks to accommodate animal and human factors.

Injuries

Animals are commonly recognized as agents of injury in surveys of injuries associated with agriculture. There are several postulated explanations for these observations. Close association between the worker and the animal, which often has unpredictable behaviour, puts the livestock worker at risk. Many livestock have superior size and strength. Injuries are often due to direct trauma from kicking, biting or crushing against a structure and often involve the worker’s lower extremity. The behaviour of workers may also contribute to risk of injury. Workers who penetrate the “flight zone” of livestock or who position themselves in livestock “blind spots” are at increased risk of injury resulting from flight reaction, butting, kicking and crushing.

Figure 1. Panoramic vision of cattle

Women and children are over-represented among injured livestock workers. This may be due to societal factors resulting in women and children doing more of the animal-related work, or it may be due to exaggerated size differences between the animals and worker or, in the case of children, use of handling techniques to which livestock are unaccustomed.

Specific interventions to prevent animal-associated injuries include intense educational efforts, selecting animals that are more compatible with humans, selecting workers who are less likely to agitate animals and engineering approaches that                                                                                                                                 decrease the risk of exposure of humans to animals.

Zoonotic Diseases

Livestock rearing requires close association of workers and animals. Humans may become infected by organisms normally present on animals, which are rarely human pathogens. In addition, the tissues and behaviour associated with infected animals may expose workers who would experience few, if any, exposures if they were working with healthy livestock.

The relevant zoonotic diseases include numerous viruses, bacteria, mycobacteria, fungi and parasites (see table 1). Many zoonotic diseases, such as anthrax, tinea capitis or orf, are associated with skin contamination. In addition, contamination resulting from exposure to a diseased animal is a risk factor for rabies and tularaemia. Because livestock workers often are more likely to ingest under-treated animal products, such workers are at risk of diseases such as Campylobacter, cryptosporidiosis, salmonellosis, trichinosis or tuberculosis.

Table 1. Zoonotic diseases of livestock handlers

The control of zoonotic diseases must focus on the route and source of exposure. Elimination of the source and/or interruption of the route are essential to disease control. For example, there must be proper disposal of the carcasses of diseased animals. Often, the human disease can be prevented by eliminating the disease in animals. Additionally, there should be adequate processing of animal products or tissues before use in the human food chain.

Some zoonotic diseases are treated in the livestock worker with antibiotics. However, routine prophylactic antibiotic usage on livestock may cause emergence of resistant organisms of general public health concern.

Blacksmithing

Blacksmithing (farrier work) involves primarily musculoskeletal and environmental injury. The manipulation of metal to be used in animal care, such as for horseshoes, does demand heavy work requiring substantial muscle activity to prepare the metal and position animal legs or feet. Furthermore, applying the created product, such as a horseshoe, to the animal in farrier work is an additional source of injury (see figure 2).

Figure 2. Blacksmith shoeing a horse in Switzerland

Often, the heat required to bend metal involves exposure to noxious gases. A recognized syndrome, metal fume fever, has a clinical picture similar to pulmonary infection and results from inhalation of fumes of nickel, magnesium, copper or other metals.

Adverse health effects associated with blacksmithing can be alleviated by working with adequate respiratory protection. Such respiratory devices include respirators or powered air-purifying respirators with cartridges and pre-filters capable of filtering acid gas/organic vapours and metal fumes. If the farrier work occurs in a fixed location, local exhaust ventilation should be installed for the forge. Engineering controls, which place distance or barricades between the animal and the worker, will reduce the risk of injury.

Animal Allergies

All animals possess antigens which are non-human and could therefore serve as potential allergens. In addition, livestock are often hosts for mites. Since there are a large number of potential animal allergies, recognition of a specific allergen requires careful and thorough disease and occupational histories. Even with such data, recognition of a specific allergen may be difficult.

The clinical expression of animal allergies may include an anaphylaxis-type picture, with hives, swelling, nasal discharge and asthma. In some patients, itching and nasal discharge may be the only symptoms.

Controlling exposure to animal allergies is a formidable task. Improved practices in animal husbandry and changes in livestock facility ventilation systems may make it less likely that the livestock handler will be exposed. However, there may be little that can be done, other than desensitization, to prevent the formation of specific allergens. In general, desensitizing a worker can be performed only if the specific allergen is adequately characterized.

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Understanding what influences animal behaviour can help make for a safer work environment. Genetics and learned responses (operant conditioning) influence the way an animal behaves. Certain breeds of bulls are generally more docile than others (genetic influence). An animal that has balked or refused to enter an area, and is successful at not doing so, will likely refuse to do so the next time. On repeated tries it will get more agitated and dangerous. Animals respond to the way in which they are treated, and draw upon past experiences when reacting to a situation. Animals that are chased, slapped, kicked, hit, yelled at, frightened and so on, will naturally have a sense of fear when a human is near. Thus, it is important to do everything possible to make movement of animals successful on the first attempt and as free of stress as possible for the animal.

Domesticated animals living under fairly uniform conditions develop habits which are based on doing the same thing each day at a specific time. Confining bulls in a paddock and feeding them allows them to get used to humans and can be utilized with bull-confinement mating systems. Habits are also caused by regular changes in environmental conditions, such as temperature or humidity fluctuations when daylight turns to darkness. Animals are most active at the time of greatest change, which is at dawn or dusk, and least active either in the middle of the day or the middle of the night. This factor can be used to advantage in the movement or working of animals.

Like animals in the wild, domesticated animals can protect territories. During feeding, this can appear as aggressive behaviour. Studies have shown that feed distributed in large, unpredictable patches eliminates territorial behaviour in livestock. When feed is distributed uniformly or in predictable patterns, it may result in fighting by animals to secure the feed and exclude others. Territorial protection may also occur when a bull is permitted to remain with the herd. The bull may view the herd and the range they cover as his territory, which means he will defend it against perceived and real threats, such as humans, dogs and other animals. Introducing a new or strange bull of breeding age into the herd almost always results in fighting to establish the dominant male.

Bulls, due to having their eyes on the side of their head, have panoramic vision and very little depth perception. This means they can see about 270° around them, leaving a blind spot directly behind them and right in front of their noses (see figure 1). Sudden or unexpected movements from behind can “spook” the animal because it cannot determine the proximity or seriousness of the perceived threat. This can cause a “flight or fight” response in the animal. Because cattle have poor depth perception, they can also be easily frightened by shadows and movements outside of working or holding areas. Shadows falling within the working area may appear as a hole to the animal, which can cause it to balk. Cattle are colour blind, but do perceive colours as different shades of black and white.

Many animals are sensitive to noise (compared with humans), especially at high frequencies. Loud, abrupt noises, such as metal gates clanging shut, head chutes latching and/or humans yelling can cause stress in the animals.

Figure 1. Panoramic vision of cattle

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The importance of the management of waste has increased as the intensity of agricultural production on farms has increased. Waste from livestock production is dominated by manure, but also includes bedding and litter, wasted feed and water and soil. Table 1 lists some relevant characteristics of manure; human waste is included both for comparison and because it too must be treated on a farm. The high organic content of manure provides an excellent growth medium for bacteria. The metabolic activity of bacteria will consume oxygen and maintain bulk-stored manure in an anaerobic state. Anaerobic metabolic activity can produce a number of well-known toxic gaseous by-products, including carbon dioxide, methane, hydrogen sulphide and ammonia.

Table 1. Physical properties of manure as excreted per day per 1,000 lb of animal weight, excluding moisture.

Source: USDA 1992.

Management Processes

The management of manure involves its collection, one or more transfer operations, storage or/and optional treatment and eventually utilization. The moisture content of manure as listed in table 1 determines its consistency. Wastes of different consistencies require different management techniques and therefore can present different health and safety hazards (USDA 1992). The reduced volume of solid or low-moisture manure generally permits lower equipment costs and energy requirements, but handling systems are not easily automated. The collection, transfer and any optional treatments of liquid waste are more easily automated and require less daily attention. Storage of manure becomes increasingly mandatory as the seasonal variability of the local crops increases; the storage method must be sized to meet the production rate and utilization schedule while preventing environmental damage, especially from water runoff. Options for utilization include use as plant nutrients, mulch, animal feed, bedding or a source to produce energy.

Manure Production

Dairy cows are typically raised on pastures, except when in holding areas for pre- and post-milking and during seasonal extremes. Water use for cleaning in milking operations can vary from 5 to 10 gallons per day per cow, where flushing of wastes is not practised, to 150 gallons per day per cow where it is. Therefore, the method used for cleaning has a strong influence on the method chosen for manure transport, storage and utilization. Because the management of beef cattle requires less water, beef manure is more often handled as a solid or semi-solid. Composting is a common storage and treatment method for such dry wastes. The local precipitation pattern also strongly influences the preferred waste management scheme. Excessively dry feedlots are apt to produce a downwind dust and odour problem.

The major problems for swine raised on traditional pastures are the control of runoff and soil erosion due to the gregarious nature of pigs. One alternative is the construction of semi-enclosed pig buildings with paved lots, which also facilitates the separation of solid and liquid wastes; solids require some manual transfer operations but liquids can be handled by gravity flow. Waste-handling systems for fully enclosed production buildings are designed to collect and store waste automatically in a largely liquid form. Livestock playing with their watering facilities can increase the volumes of swine waste. Manure storage is generally in anaerobic pits or lagoons.

Poultry facilities are generally divided into those for meat (turkeys and broilers) and egg (layers) production. The former are raised directly on prepared litter, which maintains the manure in a relatively dry state (25 to 35% moisture); the only transfer operation is mechanical removal, generally only once per year, and transport directly to the field. Layers are housed in stacked cages without litter; their manure can either be allowed to collect in deep stacks for infrequent mechanical removal or be automatically flushed or scraped in a liquid form much like swine manure.

The consistency of waste from most other animals, like sheep, goats and horses, is largely solid; the major exception is veal calves, because of their liquid diet. Waste from horses contains a high fraction of bedding and may contain internal parasites, which limits its utilization on pasture land. Waste from small animals, rodents and birds may contain disease organisms that can be transmitted to humans. However, studies have shown that faecal bacteria do not survive on forage (Bell, Wilson and Dew 1976).

Storage Hazards

Storage facilities for solid wastes must still control water runoff and leaching into surface and ground water. Thus, they should be paved pads or pits (that may be seasonal ponds) or covered enclosures.

Liquid and slurry storage is basically limited to ponds, lagoons, pits or tanks either below or above ground. Long-term storage is coincident with onsite treatment, usually by anaerobic digestion. Anaerobic digestion will reduce the volatile solids indicated in table 1, which also reduces odours emanating from eventual utilization. Unguarded below-surface holding facilities can lead to injuries or fatalities from accidental entry and falls (Knoblauch et al. 1996).

The transfer of liquid manure presents a highly variable hazard from mercaptans produced by anaerobic digestion. Mercaptans (sulphur-containing gases) have been shown to be major contributors to the odour of manure and are all quite toxic (Banwart and Brenner 1975). Perhaps the most dangerous of the effects from H₂S shown in table 2 is its insidious capacity to paralyze the sense of smell in the 50- to 100-ppm range, removing the sensory capacity to detect higher, rapidly toxic levels. Liquid storage for as short as 1 week is enough to initiate the anaerobic production of toxic mercaptans. Major differences in long-term manure gas generation rates are thought to be due to uncontrolled variations in the chemical and physical differences within the stored manure, such as temperature, pH, ammonia and organic loading (Donham, Yeggy and Dauge 1985).

Table 2. Some important toxicologic benchmarks for hydrogen sulphide (H₂S)

TLV-TWA = Threshold limit values–Time weighted average; STEL = Short-term exposure level; IDLH = Immediately dangerous to life and health.

The normally slow release of these gases during storage is greatly increased if the slurry is agitated to resuspend the sludge that accumulates at the bottom. H₂S concentrations of 300 ppm have been reported (Panti and Clark 1991), and 1,500 ppm has been measured during the agitation of liquid manure. The rates of gas release during agitation are much too large to be controlled by ventilation. It is most important to realize that natural anaerobic digestion is uncontrolled and therefore highly variable. The frequency of serious and fatal over-exposures can be predicted statistically but not at any individual site or time. A survey of dairy farmers in Switzerland reported a frequency of about one manure gas accident per 1,000 person-years (Knoblauch et al. 1996). Safety precautions are necessary each time agitation is planned to avoid the unusually hazardous event. If the operator does not agitate, sludge will build up until it may have to be removed mechanically. Such sludge should be left to dry before someone physically enters an enclosed pit. There should be a written confined-space programme.

Rarely used alternatives to anaerobic ponds include an aerobic pond, a facultative pond (one using bacteria that can grow under both aerobic and anaerobic conditions), drying (dewatering), composting or an anaerobic digester for biogas (USDA 1992). Aerobic conditions can be created either by keeping the liquid depth no more than 60 to 150 cm or by mechanical aeration. Natural aeration takes more space; mechanical aeration is more costly, as are the circulating pumps of a facultative pond. Composting may be conducted in windrows (rows of manure which must be turned every 2 to 10 days), a static but aerated pile or a specially constructed vessel. The high nitrogen content of manure must be reduced by mixing a high carbon amendment that will support the thermophilic microbial growth necessary for composting to control odours and remove pathogens. Composting is an economical method of treating small carcasses, if local ordinances permit. See also the article “Waste disposal operations” elsewhere in this Encyclopaedia. If a rendering or disposal plant is not available, other options include incineration or burial. Their prompt treatment is important to control herd or flock disease. Swine and poultry wastes are particularly amenable to methane production, but this utilization technique is not widely adopted.

Thick crusts can form on top of liquid manure and appear solid. A worker may walk on this crust and break through and drown. Workers can also slip and fall into liquid manure and drown. It is important to keep rescue equipment near the liquid manure storage site and avoid working alone. Some manure gases, such as methane, are explosive, and “no smoking” signs should be posted in or around the manure storage building (Deere & Co. 1994).

Application Hazards

Transfer and utilization of dry manure can be by hand or with mechanical aids like a front-end loader, skid-steer loader and manure spreader, each of which presents a safety hazard. Manure is spread onto land as fertilizer. Manure spreaders are generally pulled behind a tractor and powered by a power-take-off (PTO) from the tractor. They are classified into one of four types: box-type with rear beaters, flail, V-tank with side discharge and closed tank. The first two are used to apply solid manure; the V-tank spreader is used to apply liquid, slurry or solid manure; and the closed tank spreader is used to apply liquid manure. The spreaders throw the manure over large areas either to the rear or sides. Hazards include the machinery, falling objects, dust and aerosols. Several safety procedures are listed in table 3.

Table 3. Some safety procedures related to manure spreaders

1. Only one person should operate the machine to avoid inadvertent activation by another person.

2. Keep workers clear of active power-take offs (PTOs), beaters, augers and expellers.

3. Maintain all guards and shields.

4. Keep persons clear of rear and sides of the spreader, which can project heavy objects mixed into the manure as far as 30 m.

5. Avoid dangerous unplugging operations by preventing spreader plugging:

• Keep stones, boards and other objects out of the spreader.

• In freezing weather, make sure flails and chains on flail-type spreaders are loose and unfrozen before operation.

• Keep chains and beaters on beater-type spreaders in good operating order by replacing stretched chains and avoiding dropping loads of frozen manure onto the spreader chains.

• Never get into an operating spreader to clean it.

• Maintain the unloading auger and discharge expeller on V-tank spreaders so they operate freely.

• In cold weather, clean the spreader insides so wet manure will not freeze the moving parts.

6. Use good tractor and PTO safety practices.

7. Make sure the relief valve on closed-tank spreaders is operative to avoid excessive pressures.

8. When unhooking the spreader from the tractor, make sure the jack that holds the weight of the spreader tongue is secure and locked to prevent the spreader from falling.

9. When the spreader is creating airborne dust or aerosols, use respiratory protection.

Source: Deere & Co. 1994.

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