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Pollutants in Animal Manure: Factors of Emission and Strategies for Reduction



W. Windisch
Institute of Nutrition Sciences, Animal Nutrition
University of Technology Munich-Weihenstephan
Hochfeldweg 6, D-85350 Freising

Intensive livestock production is one of the most important bases for supplying the world's population with high-grade, protein-rich foods. It does however also bear an ecological risk by emitting large amounts of manure. By this way, livestock production is one major source of global N emissions and contributes substantially to pollution of the hydrosphere and atmosphere, forest decline, destruction of the stratospheric ozone layer and formation of ozone close to the ground. Phosphate is a further constituent of animal manure with a considerable potential to pollute the environment by eutrophication of surface water. Finally, animal manure may contain considerable amounts of zinc and copper, which may be accumulated in the soil.

Indeed, N, P, Zn and Cu are not pollutants per se. Primarily, they are essential components of the feed as well as of the food, which is produced from the animals' products. Also in manure, these substances serve primarily as essential nutrients to plants. Their polluting potential, however, arises from the large quantities of emissions. This frequently results in the public demand to cut livestock production back to the "natural" level. But excessive emissions of N, P, Zn and Cu are not coupled to animal production itself. They indicate mainly a low efficiency in transforming N, P, Zn and Cu from feed into food. Consequently, the major goal is to reduce the primary emissions of N, P, Zn and Cu from the animals by optimising the efficiency of nutrient transformation within the metabolism. This is mainly a matter of animal nutrition strategies.


Nitrogen is an essential component of protein in feed as well as in food. N emissions from animal husbandry are therefore inevitably coupled with the production of protein-rich foods (meat, milk and egg). In practice, however, the efficiency of N transformation is quite low. On average only about one third of the feed N is transferred into the protein of animal products while the rest is eliminated via excrements (mainly urine). Most of these N quantities remain in the manure and are transferred to the agricultural area. But up to about one fourth may be emitted into the atmosphere directly after excretion and during the storage of manure. Therefore, it seems reasonable to observe the primary N emission of the animal rather than only N in manure.

One major reason for high N-losses is an excessive protein content of the feed. Since the capacity of an animal to grow or to produce milk and eggs is limited, any surplus of dietary protein cannot be utilised by the metabolism and the respective N of the protein has to be eliminated from the body. However, in the course of the production cycle (pregnancy/lactation, start/end of fattening) the animals' protein requirement changes to a considerable extent. In order to guarantee a sufficient protein supply, the farmers chiefly adjust the protein content of the feed to the level of the maximum requirement of the animal. Consequently, the animals receive excessive amounts of protein for most of the time.

Another reason for N-losses especially in monogastric species (e.g. pig, poultry) is the low quality of the feed protein due to deficient contents of essential amino acids (mainly Lys, Thr, Met). In order to secure a sufficient supply of the most limiting essential amino acid, higher quantities of the total protein have to be fed. This generates an additional surplus of non-limiting amino acids whose nitrogen has to be eliminated from the body.

In ruminants, the aspect of protein quality refers mainly to the extent to which utilisable protein reaches the final site of digestion (duodenum, small intestine) after it has been transformed by microbes along the passage through the forestomaches. In this context, considerable N-losses may occur due to a high ruminal degradability of feed protein as well as an due to an excessive ratio between N and energy in the feed.

An additional source of N-emission via excrements is the N, which originates from the inevitable and "non-productive" maintenance metabolism.

Thus, there are 3 general strategies available to minimise N-emissions from livestock production:
1) Applying the official recommendations for protein supply and avoiding dietary protein surplus by adjusting the protein content in the feed to the changing requirement of the animal.

2) Improving the quality of feed protein. This may be achieved in pigs and poultry by adding limiting amino acids in a chemically pure form. In cattle it refers mainly to the use of feed proteins with low ruminal degradability.

3) Increasing the animals' performance in order to "dilute" the indispensable N emission from maintenance turnover among a higher amount of products.

The dominant contribution to reduce the N emissions will arise mainly from the strategy a) and b). Strategy a) may be realised to a large extent by rather simple feeding techniques such as phase feeding (e.g. 3 feed mixes for pigs along the fattening procedure). Also strategy b) may be applied directly to practice, since the relevant amino acids and feed proteins are commercially available. By using such feeding strategies, N emissions from animals may be reduced by 30 to 40 % compared to the present situation (WINDISCH 2000).


The high P-emissions from livestock production reflect mainly the low efficiency of the transformation of phosphorus from feed into animal products. On average about 70 % and more of the P fed to animals is lost by excrements and transfered into manure.

Similarly to the situation in N, one major reason for the high P-losses is an excessive P content in the animals' feed. Since the P content of products (meat, milk, egg) is fixed, any surplus in dietary P cannot be utilised and has to be excreted into the manure. However, the animals' requirement for P changes substantially along the production cycle (pregnancy/lactation, start/end of fattening). Like in N, the farmers tend to adjust the P content of the feed to the level of the maximum requirement. This results in a considerable P excess to the animals during most of the time of the production cycle. Additionally, until recent years there were uncertainties regarding the P-demand especially of dairy cows and beef cattle. But in the meantime new assessment standards and recommendations for cattle permit a more precise supply of phosphorus to the animals.

Another reason for high P emissions is the fact that a large part of the phosphorus is bound in the form of phytate, which is almost indigestible by monogastric animals. This refers in particular to grain and oil seed extracts, which are one of the most important feeds to monogastric livestock. Due to the low availability of native P, the feed has to be supplemented with P from mineral origin. However, during the last years phytate-degrading feed additives of microbial origin (phytase) were developed and brought to a stage suitable for practical application in pig and poultry feeding. In contrast to pig and poultry, phytate P is no problem to ruminants, because the microflora of the forestomachs degrades phytate P into a digestible form.

A further factor in P-emission is the level of the animal performance in relation to the "non-productive" maintenance turnover.

Thus, 3 strategies are available to minimise P-emissions from livestock production. They are in principal the same as in the case of N:
1) Applying the official recommendations for P supply and avoiding P surplus by adjusting the dietary P content to the changing requirement of the animal.

2) Improving the quality (availability) of the P in the feed to pig and poultry by adding phytase.

3) Improving the relation between performance and maintenance by increasing the animals' performance.

Like in N, it is especially strategy a) and b) which provides major contributions to minimise P emissions. The tools to achieve these strategies are rather simple (e.g. phase feeding) and commercially already available (phytase). They have a potential to reduce P emissions by 30 - 50 % compared to the present situation (WINDISCH 2000).

Zinc and copper

Zn and Cu are essential trace elements with various biological functions. However, they may exert also pharmacological effects especially in piglets, such as prevention of diarrhoea and promotion of production performance. These effects require dietary doses of about 50 to 100 times above the requirement, which reflects the (mis)use of the toxic potential of heavy metals rather than the biological function of essential trace elements (WINDISCH et al. 1999, 2000). Pharmacologically effective doses of Zn are prohibited by feed directives and may be applied only under veterinary control, while equivalent doses of Cu may be used legally in practical piglet diets. In total, the pig feeding practice shows an increasing interest in such excessive doses of Zn and Cu. It obviously reflects the search for substitutes to antibiotic feed additives.

The excessive amounts of Zn and Cu are almost completely excreted into the manure. By this way, pure piglet manure supplemented with excessive amount of Zn and Cu may contain these heavy metals in the magnitude of about 15 g Zn and 1 g Cu per kg of dry matter. This would severely exceed the respective limits to sewage sludge. However, piglet manure is usually diluted by manure of other animals. Nevertheless, the excessive use of Zn and Cu in piglet feeding is still visible in the 2fold and 5fold higher mean value for Zn and Cu contents of mixed pig manure compared to the respective contents in cattle manure (LBP 1997).

The average transfer of Zn and Cu via pig manure to the agricultural area ranges at about 0,8 kg Zn and 0,4 kg Cu per hectare and year. It exceeds the withdrawal by plant harvest at about factor 4 (Zn) and up to 20 (Cu) (LBP 1997). In the case of Cu, the mean transfer rates range already above the limit given by the German soil protection directive. Since the mobility of Zn and Cu in the soil is extremely low, these heavy metals are progressively accumulated in areas fertilised with pig manure at rates of about 0,7 kg Zn and 0,4 kg Cu per hectare and year on average (LBP 197). The extent of Cu accumulation is comparable to than on agricultural areas fertilised with sewage sludge.

In total, there is no physiological need to tolerate such high accumulations of Zn and Cu in the manure, because the pharmacological and growth promoting effect of excessive doses of Zn and Cu may be retrieved also by ecologically compatible alternatives (e.g. by organic acids). Furthermore, if Zn and Cu are fed to animals only according to the nutritional recommendations, the respective transfer rates to the agricultural area will decrease to the level of the withdrawal by plant harvest. In this case, the ecological risks of the high contents of Zn and Cu in manure from animal production will completely disappear.

Final conclusions

The excessive emissions of N, P, Zn and Cu via animal manure may be explained largely by excessive supplies (protein, phosphorus), low quality of feed components (protein, phytate) and partially also by the physiologically inadequate use of nutrients (e.g. essential trace elements). Optimising feeding strategies may therefore turn N, P Zn and Cu from pollutants into valuable nutrients and reduce their emissions to a level, which agrees to the requirements of a sustainable animal production. This is also in favour of the animals, because their physiological needs are met the best by an optimised feed.


LBP (Bodenkultur und Pflanzenbau) (1997): Boden-Dauerbobachtungsflächen (BDF). Schriftenreihe der Bayerischen Landesanstalt für Bodenkultur und Pflanzenbau, 5/97 (ISBN 3-9805718-4-X)
Windisch, W., 2000: Contribution of animal nutrition to sustainable livestock production taking phosphorus and nitrogen emissions as an example (Beitrag der Tierernährung für eine nachhaltige Tierproduktion am Beispiel der Emissionen von Phosphor und Stickstoff). 7. Forum Animal Nutrition. BASF. (in press)
Windisch, W., Schwarz, F.J., Gruber, K. and Kirchgessner, M. (1999): Effect of Pharmacological Dietary Doses of Zinc Oxide on Performance and Fecal Characteristics of Weanling Piglets. Agribilol. Res. 51, 277 - 285
Windisch, W., Gotterbarm, G.G. and Roth, F.X. (2000): Effect of potassium diformate (FormiŽ LHS) in combination with high dietary doses of copper on production performance of weanning pigs. Arch. Anim. Nutr. (submitted)

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