|Animal Husbandry - Initial Environmental Assessment Series No. 2 (NORAD, 1994)|
|Part I: General account|
|3 Possible environmental impacts|
Manure can pollute air by the emission of smell, ammonia and methane. Besides ammonia and hydrogen sulphide (H2S), the most noticeable smelling substances are some cyclic chemical compounds. Ammonia in large concentrations can do damage to the vegetation. H2S gas developing in a manure cellar can, in cases of carelessness and bad ventilation, cause poisoning that may harm, and at worst kill, animals and humans. Smell of manure is normally more unpleasant than harmful. If wet manure is sprayed over fields, ea. by means of watering equipment, fine drops (particles) can float with the wind and be a nuisance for the surroundings.
Methane (CH4) is a gas that is produced in manure and ferments without the supply of air (biogas), but it is also produced in the rumina of ruminants during anaerobic fermentation. As for monogastrics such as pigs and poultry, methane is virtually not produced in the intestinal canal. Methane is a natural greenhouse gas with an impact on the greenhouse effect that is approximately 8 times stronger than that of carbon dioxide (CO2). Methane contributes about lo per cent to the total global heating caused by greenhouse gases. Approximately 15 per cent of the total amount of methane emitted to the atmosphere is produced by domestic animals (ruminants). The rest of the methane derives from rice fields, earth gas, swamps, land fills and wild ruminants. Some therefore contend that approximately 1.5 per cent of the global heating caused by greenhouse gases can be traced back to livestock.
Careful utilization of livestock manure can be difficult if the areas are limited. Pollution may be the result if the amount of manure in an area exceeds the amount that can be utilized by plants and soil. The growth of aquatic plants and algae will increase when excessive nutrients run into ditches, streams and other surface-water sources. This will cause the oxygen in the water to be consumed, and death of fish will ensue. If the pollution becomes a more or less permanent state, the watercourse may gradually overgrow - eutrophication. Phosphorus is generally the minimum factor for the growth of algae in freshwater, whereas nitrogen is the prime limiting nutrient in seawater. Another consequence of large amounts of manure is that the nitrogen may seep down through layers of soil and finally reach the ground-water. As a consequence the level of nitrates in the ground-water may exceed the danger level (see also booklet 7 Water supply). Flies and other insects propagate quickly in manure and can be a nuisance to the surroundings.
In many developing countries, the problem is too little manure rather than too much. In connection with intensive animal husbandry for industrial production of livestock products, however, problems of too much manure per area unit may arise. With regard to such projects, care should be taken to ensure that the manure is spread over a sufficiently large area. The need of a spreading area (minimum area per animal manure unit) will vary according to climate, soil type, plant growth and the feed contents of nutrients, especially nitrogen (N) and phosphorus (P). One animal manure unit can be reckoned as equivalent to the release of P (or N) of ea. a cow (one standard cow) per year. The number of animals per animal manure unit regarding other domestic animals will depend on the amount they release. The capacity of the animals to bind N and P in the body, and thereby traces of this in the livestock produce, is limited in proportion to the contents in the feed. It is important, therefore, that the feed does not contain much more than what the animals can make use of. As for the ruminants, which need carbohydrates in order to synthesize protein, it is important that one pays attention to the carbohydrate/N balance and tries to make it as optimal as possible. This is different with monogastrics. They have a lesser capacity to break down phytate, which is the compound to which most of the phosphorus in grain and other kinds of feed is bound. Consequently, the contents of phosphorus in the feed must be higher than would have been necessary if the physic phosphorus had been more easily accessible. A preparation has recently been developed containing the enzyme phytase. When it is mixed into the feed, the digestibility of physic phosphorus increases so much that it becomes unnecessary to add unorganic phosphorus to, for example, pig and poultry feed.
Fertilization of fish ponds by means of livestock manure may cause pollution of watercourses (see booklet 5 Aquaculture).
Despite these potential negative impacts, it should be stressed that livestock manure, if applied with care, in most cases will benefit the environment. This is both due to the contents of plant nutrients and to the fact that manure can also be used to produce biogas, thereby contributing to saving forests and other sources of energy.