Modern animal production is increasingly regarded as a source of solid, liquid and gaseous emissions which can be both a nuisance and environmentally harmful. Solid and liquid manure and waste water contain nitrogen and phophorus which are the most important plant nutrients, but are harmful when applied to agricultural land in excess amounts thereby leading to pollution of ground water by nitrates, surface water with phosphorous (causing eutrophication) and soil with heavy metals such as zinc and copper which are used as growth promoters in the feed stuff. A third group of potentially hazardous effluents are drug residues, such as antibiotics, which may be present in the excreta of farm animals after medical treatment and which are passed to the environment during grazing or spreading of animal manure where they may conceivably contribute to the formation of antibiotic resistance in certain strains of bacteria. The same risk arises when sludge and waste water from sewage plants containing residues of antibiotics and other drugs from human consumption are discharged as fertiliser in the soil and water body.

The most important aerial pollutants are odours, gases, dust, micro-organisms and endotoxins, also called bioaerosols, which are emitted by way of the exhaust air into the environment from buildings and during manure storage, handling and disposal as well as grazing. More than 130 different gaseous compounds have been identified in the air of animal houses, which are a major source of these pollutants. Aerial pollutants can give cause for concern for several reasons. Firstly, there is strong epidemiological evidence that the health of farmers working in animal houses may be harmed by regular occupational exposure to air pollutants. Secondly, an animal's respiratory health may be compromised by these pollutants. In some herds, half of all slaughter pigs may show signs of pneumonia, pleuritis or other respiratory disease. In broilers, about 30% of the birds which are rejected at meat inspection show lung lesions. The third reason for concern is that aerial pollutants from livestock contribute to soil acidification (ammonia, NH3) and global warming (eg. methane, CH4, nitrous oxide, N2O). For example, animal production emits about 750,000 t of NH3 per year in Germany. About 20% of global methane production originates from ruminants. Animal production systems which use straw release distinct higher amounts of nitrous oxide than those employing liquid manure systems. Fourthly, particulate emissions, such as dust and microorganisms, from livestock buildings may be a source of complaint from people living in the vicinity of livestock farms. The travel distance of viable bacteria from animal houses via the air is presently estimated at 200 to 300 m downwind; Mycoplasma species may travel about 400 m. From epidemiological modelling and studies, we know that the virus causing Mouth and Foot Disease can be transmitted over more than 75 km while in an airborne state. Very little is known about the distribution characteristics of bioaerosols, such as dust particles, endotoxins, fungi and their spores, in the air surrounding animal houses. Dispersion models for these pollutants are lacking.

Table 1 summarizes our present knowledge of the impact of emissions from livestock farming on farm livestock and man and the distance over which the emissions may have effects. Odours are relevant closer to animal houses only. Ammonia can act directly on needles and leaves of trees close to sources where high amounts are released. It also causes damage in the far environment by overfertilizing soils and water and contributes to the decay of forests (via acid rain). Indoors, ammonia is an irritant for the respiratory tract of man and animal. Hydrogen sulphide is noticed as a prominent odourous compound outside animal houses. Occasionally indoors it can be fatal to animals and man at very high concentrations after the release of high amounts, e.g. when old liquid manure is agitated. Methane and nitrous oxide contribute to the greenhouse effect, but do not cause significant problems indoors. Little is known about the fate of dust, microorganisms and endotoxins outside livestock buildings, although there is some concern that these compounds may cause a nuisance to the population living in the vicinity of animal enterprises, particularly in areas with high animal densities. Nitrate and its product nitrite can cause pollution of ground and drinking water. The effects are local and the impact on human health is low. Together with phosphate both nutrients can enhance eutrophication of surface waters. Zinc and copper, which are increasingly used as growth promoters instead of antibiotics in animal feed, are accumulating eg. in pig liver and locally in soils and plants that then cause health problems in grazing sheep. Not much is known about the fate of veterinary drugs such as antibiotics in the environment which are excreted with the faeces. There is some concern that they may contribute to the development of drug resistance in bacteria.

Conclusions

Recommendations

1. Adequate and efficient feeding regimes are required with minimal wastage of nitrogen and phosphorous and limited use of growth promoters.
2. The development of low emission production systems should be encouraged including mitigation techniques, eg. biofilters, bioscrubbers, covered manure pits and shallow manure application.
3. The administration of drugs has to be restricted to the treatment of diseases only. The fate of the drugs in the environment has to be investigated.
4. There is an urgent need to establish safe distances between farms and to residential areas to prevent transmission of harmful substances. This should become an essential part of local and regional planning.
5. Environmental standards for animal production should be established and applied to all European countries. 6. An environmental risk analysis is required to compare different production systems and different regions in the world.
7. For the realization of these aims the cooperation of farmers, agricultural engineers, veterinarians and governmental agencies is necessary.

Acknowledgements:
This conference was funded by Deutsche Bundesstiftung Umwelt.
Much of the work described was funded by the Commission of the European Union under Project No PL900703.
Supplementary funding was also received from the Ministry of Agriculture, Fisheries and Food of Lower Saxony, Germany.

Table 1: Environmental impact from livestock sources