Animal manure: asset or liability?

J. de Wit¹, H. van Keulen^2,3, H.G. van der Meer³ and A.J. Nell⁴

The authors can be contacted as follows: ¹ Munsel 90, 5283 TP, Boxtel; ² Department of Animal Production Systems, Agricultural University, PO Box 338, 6700 AH Wageningen; ³ Research Institute for Agrobiology and Soil Fertility, PO Box 14, 6700 AA Wageningen; ⁴ International Agricultural Centre, PO Box 88, 6700 AB Wageningen, the Netherlands.
Acknowledgements. The authors are grateful to J.F.F.P. Bos, P.J. Brandjes, J.C.M. Jansen and P.T. Westra, co-authors of the reports on which this article is based, and to the overall coordinators of the "Livestock and Environment Study", C. de Haan, H. Steinfeld and H. Blackburn, for their critical comments on the study.

LE FUMIER: ATOUT OU CONTRAINTE?

Le fumier peut apporter une contribution utile � la production agricole, mais �galement constituer un probl�me du point de vue de l'environnement lorsqu'il est surabondant ou remplac� par des engrais artificiels �conomiquement plus avantageux. Le pr�sent article fait le point des informations concernant les d�jections et les probl�mes qu'elles posent au niveau de l'environnement, en analysant les diff�rentes solutions permettant d'att�nuer cet impact. Une application �quilibr�e de suppl�ments au niveau des parcelles, des exploitations et des �levages, est recommand�e pour assurer l'analyse de la gestion des nutriments. Il est d�montr� qu'un bilan phosphorique (P) donne une indication pr�cise des probl�mes d'environnement potentiels. Avec un bilan phosphorique �gal � z�ro, ces probl�mes seront, pour la plupart, probablement limit�s ou insignifiants, m�me si les syst�mes de r�colte et de stockage du fumier et les �poques de fumure constituent, eux aussi, des param�tres importants. L'�mission de m�thane, les odeurs et les �missions d'ammoniac sont aussi � prendre en consid�ration. Pour �tre r�alisables et efficaces, les solutions techniques et politiques doivent �tre sp�cifiquement adapt�es aux situations locales.

ESTIERCOL ANIMAL: �UN BIEN O UNA CARGA?

El esti�rcol animal puede contribuir de manera considerable a la producci�n agr�cola, pero tambi�n puede convertirse en un problema para el medio ambiente si lo hay en exceso o si lo sustituyen fertilizantes artificiales competitivos desde el punto de vista econ�mico. Se presenta informaci�n sobre la excreci�n de nutrientes y los problemas del medio ambiente y se examinan y eval�an diversas opciones para atenuar �stos. Se defiende la aplicaci�n equilibrada de suplementos en los sistemas de parcelas, fincas y ganado como mecanismo �til en el an�lisis de la ordenaci�n de los nutrientes. Se argumenta que el equilibrio de f�sforo (P) es un indicador preciso de posibles problemas para el medio ambiente. Si se mantiene un equilibrio de P cero, probablemente la mayor�a de los problemas ecol�gicos ser�n peque�os o insignificantes, aunque tambi�n son par�metros importantes los sistemas de recolecci�n y almacenamiento del esti�rcol y el momento de su aplicaci�n. Son excepciones la emisi�n de metano, el olor y la emisi�n de amon�aco. Las soluciones de orden t�cnico y normativo deben ajustarse a cada situaci�n local si se quiere que sean viables y eficaces.

Animal manure is a controversial product. In many extensive mixed farming systems, animals are appreciated as a source of manure. However, this is not the case when alternative cheap chemical fertilizers are available, nor in intensive landless livestock production systems, where its disposal often presents environmental problems. In this article the various issues are discussed, with particular emphasis given to the possible negative impacts. The main objective is to provide guidelines to mitigate these effects and to stimulate consideration of the advantages to be gained from its correct use.

1
Estimated value of pig and ruminant manure in mixed irrigated production systems in (sub)humid Asia¹
Valeur estimative du fumier produit par les porcs et les ruminants dans des syst�mes irrigu�s de production mixte dans les r�gions (sub)humides d'Asie
Valor estimado del esti�rcol de cerdo y de rumiantes en sistemas mixtos de producci�n de regad�o en el Asia (sub)h�meda

¹ Based on average ruminant and pig densities in mixed irrigated production systems in Asia, rice production of 4 tonnes/ha^-1/year^-1.
Source: Jansen and de Wit, 1995.

2
Annual nutrient excretions by different animals
Excr�tions annuelles par animal
Excreciones anuales de nutrientes de distintos animales

¹ % Nmin = the percentage of N-excretion in mineral form, assumed equivalent to urine N excretion. As mineral N is susceptible to volatilization, this percentage is often lower in manure applied on the land.
² Highly productive situations.
³ Less productive situations.
Note: Owing to the variation in intake and nutrient content of the feeds, these values represent examples, not averages, for highly and less productive situations. Often reasonably accurate estimates for different local conditions can be made by deducting estimated retention from intake, using standard values for feeding values and retention per kg of product, and educated guesses for feed intake and feed composition.
Source: Brandjes et al., 1995.

ANIMAL MANURE: A VALUABLE FERTILIZER

In many developing countries, manure is often considered as important as milk, meat or draught power. Romney, Thorne and Thomas (1994) quote a study in Zimbabwe which recorded that farmers reduced grazing time by keeping cattle penned longer in order to collect more manure, even though this meant a reduced feed intake and thus adversely affected production. The importance of animal manure in keeping arable land fertile is illustrated in Table 1. In Southeast Asia it provides an important percentage of the phosphorus (P) required for intensive rice production. Its contribution to the nitrogen (N) requirement of crops is, however, rather low, owing to the lack of adequate provision for urine collection, uncovered storage and the exposure of the manure to the atmosphere after application. Its benefits as a crop nutrient, in terms of economic value and as a substitute for artificial fertilizer derived from fossil energy, are also given in Table 1.
Since a considerable proportion of the excreted N is present in organic form (Table 2), it becomes available to crops only gradually. This may be a disadvantage, as some of the nutrients mineralize outside the cropping season and may leach, particularly N and K. However, during periods of high rainfall, this "slow release" characteristic of animal manure may result in lower nutrient losses when compared with artificial fertilizers. In flooded fields, on the other hand, the N efficiency may be higher owing to associated effects such as enhanced biological fixation and the provision of plant nutrients other than N and P (Wolf and van Keulen, 1989). Moreover, organic matter also improves soil structure and increases water holding and cation exchange capacity. The average composition of animal manure derived from various species is given in Table 3. The main contribution of manure is through the transfer of plant nutrients from grazing to crop areas. Hence, its proper management may result in a substantial contribution to the crop nutrient supply. However, continuous translocation of plant nutrients from grazing areas to cultivated land may deplete soil fertility in the former.

ANIMAL MANURE: A BULKY PROBLEM

Table 3 illustrates the relatively low and variable nutrients of animal manure which, when combined with the rather unpredictable N value, may render it an unreliable source for nutrients, particularly when the grazing pressure is high. Where cheap artificial fertilizer is available and high crop yields are sought, the volume of animal manure produced may present problems. Although manure surpluses do exist at a regional level, specifically in parts of Western Europe but also near large cities in developing countries, most occur at the local level because of the large size of modern livestock enterprises. This is particularly so in the United States where some large feedlots market well over 50 000 heads annually and consequently produce more manure than 9 000 ha of high productive crops would require to maintain the P equilibrium (de Wit, Westra and Nell, 1995).
Because of the low agricultural value of manure, many farmers are indifferent to its proper management. This may result in the dumping of manure on land or discharging it directly into surface water. This may not only have serious environmental consequences but it is also a waste of valuable resources which subsequently need to be replaced (Table 4).

3
Average nutrient content in various types of animal manure
Teneur moyenne en nutriments par type de fumier
Contenido medio de nutrientes en diversos tipos de esti�rcol animal

	DM	N-tot	P	K	Ca	Mg	Na	Mn	Source
in kg/1 000 kg manure
Cattle - FYM	710-775	16.0-38.0	3.8-11.9	3.7-6.9	1.2-6.5	1.6-3.0		0.05-0.11	3
Cattle - slurry	40-100	2.4-5.1	0.4-0.9	2-5.3		0.2-0.7			1
Cattle - slurry	95	4.4	0.8	4.6	1.5	0.6	0.7		2
Cattle - FYM	215	5.5	1.7	2.9	2.9	0.9	0.7		2
Cattle - urine	25	4.0	0.09	6.6	0.07	0.12	0.7		2
Pigs - slurry	30-88	3-6.8	0.9-1.8	1.7-3.7		0.5-0.7			1
Growing pigs - slurry	75	6.5	3.9	6.8	3.5	1.5	1.0		2
Sows - slurry	55	3.6	3.6	3.6	4.6	1.2	0.6		2
Sows - FYM	230	7.5	3.9	2.9	6.4	1.5	0.7		2
Sows - urine	10	2.0	0.9	2.5	0.6	0.1	0.1		2
Poultry - manure	230-630	12.5-51.0	4.6-10.0	4.9-11.0		1.7-2.1			2
Laying hens - slurry	145	10.6	3.5	5.1	12.3	1.2	0.8		2
Broilers - FYM	580	26.0	10.5	17.8	14.7	3.6	3.0		2
Broilers - FYM	777-864	22.4-35.2	7.7-13.0	14.8-20.7	11.2-16.3	2.6-3.1	3.1-5.1	0.085-0.21	3

Note: For slurry, nutrient content is correlated to the dry matter (DM) content, but variation remains high as feed characteristics and, particularly for N, manure management is highly variable; for farmyard manure variation is even higher because of the variable characteristics as well as relative amounts of bedding.
Sources: 1: Fleming and Mordenti (1993), range of means of various publications; 2: IKC (1993); 3: FAO (1980), recalculated means of various publications.

4
Estimated worldwide annual nitrogen and phosphorus emissions from landless monogastric systems before manure application, and their opportunity costs
Emissions mondiales annuelles estimatives de N et de P des syst�mes d'�levage hors sol d'animaux monogastriques avant la fumure et leurs co�ts d'opportunit�
Emisiones anuales estimadas de N y P de los sistemas de monog�stricos sin tierras en todo el mundo antes de la aplicaci�n de esti�rcol y sus costos de sustituci�n

Note: Landless monogastric production systems consist of the highly intensive pig and poultry industry. Losses are due to runoff, discharge and volatilization.
Source: Bos and de Wit, 1995.

ENVIRONMENTAL PROBLEMS

The environmental problems associated with animal manure relate mainly to the emission of N and C into the air and the leaching of N and P to ground and surface water, while the accumulation of heavy metals in the soil may induce levels in crops that exceed human health standards.
Nitrogen (N) exists in many chemical forms, both in organic (mainly protein) and inorganic compounds. The latter is very mobile and can easily be emitted into the environment. In all but extreme cases, the inorganic fraction is equivalent to the urine N and larger than the organic fraction (Table 2). Part of the inorganic N may be emitted as ammonia (NH₃), much of which will be deposited in the vicinity of the source (about 30 percent within a radius of 5 km). It has the potential to cause eutrophy, acidification and subsequent toxic effects on the ecosystem (mainly by release of aluminium). The scale of this impact will depend on the capacity of the ecosystem to utilize N. Ammonia deposition is particularly harmful in ecosystems that are susceptible to eutrophication or acidification. The use of fossil energy adds to these eutrophying and acidifying effects, mainly through the emission of SO₂ and NO_X. In Europe, it is estimated that NH₃ contributes about 24 percent to the total emission of potentially acidifying compounds, while in defined small areas with high animal densities, such as the Netherlands, NH₃ may become a major contributor to the total deposit of potentially acidifying compounds (Heij and Schneider, 1995). In closed stables, the concentration of NH₃ sometimes reaches levels which, when combined with dust particles, may have toxic effects on animals and humans. Moreover, when combined with volatile organic compounds, NH₃ may produce odours and become a major nuisance near residential areas. Under aerobic conditions, inorganic N is transformed into nitrate which, if not absorbed by the crops, will leach to groundwater, especially in areas of high rainfall. Water becomes unsuitable for drinking if nitrate levels are too high, while high N concentrations in surface water will lead to eutrophication. This will result in the excessive growth of algae, causing oxygen shortage and, hence, high fish mortality. Under anaerobic conditions, nitrate is transformed into harmless N₂ (denitrification). However, harmful by-products are formed during this process, particularly nitrous oxide (N₂O), which contributes to the breakdown of the ozone layer and global warming. Current knowledge, however, is insufficient to quantify these effects. Phosphorus (P) is barely mobile in soil and easily transformed into insoluble compounds. However, continuous overfertilization will saturate the soil. Subsequent P leaching has similar effects to nitrate leaching, but its eutrophying potential is much higher. Manure runoff, prominent in open livestock systems, and direct discharge of manure into open water may cause serious pollution of surface water, both by biodegradation of organic matter and by the eutrophic effects of N and P. It is estimated that livestock contribute about 3 percent to global warming through the production of methane and manure decomposition accounts for approximately 25 percent of this total (Safley et al., 1992). In some parts of the world, the long-term implication of high levels of manure have resulted in heavy metal accumulation in the soil, with negative effects on both animal and human health. Generally, more than 90 percent of ingested heavy metals are excreted in the faeces. Most problems relate to the occurrence of copper (Cu), zinc (Zn) and cadmium (Cd) and are exacerbated when livestock are fed high-concentrate diets, since these heavy metals are often added to the feed because of their positive effects on animal production. Identification of threshold values for each of the indicators for pollution is highly complicated and subject to uncertainties in measurement and differences in objectives. Thus, threshold values are likely to be location-specific and rather arbitrary. For example, the maximum value of nitrate concentration in drinking water is 50 mg/litre. Given a precipitation surplus of 300 mm/year, less than 34 kg of N per hectare may leach annually. However, translation into a maximum N surplus is complicated and location-specific, as the amounts of N emitted via denitrification and ammonia volatilization vary widely. Furthermore, there is no consensus on the basic objective that all water should be multifunctional. The social cost of these environmental problems may be enormous, depending on the opportunity value of the resources affected. Removing nitrate from drinking-water, for example, costs nearly US$10/kg (van der Meer and Wedin, 1989).

BASIC CONCEPTS FOR MANURE MANAGEMENT

The advantages and disadvantages of using manure are closely interrelated; gaseous emissions at an early stage after application inevitably reduce the later positive effects on soils and crops. This is shown in the Figure, where atmospheric emissions are given at the top and emissions reaching surface water and groundwater are given at the bottom. The proportions of N, P, K, etc. taken up by the crop determine the agricultural value of the manure and depend on its original nutrient content and the losses incurred in transit from animal to crop. Manure management should be considered poor when the import of nutrients and organic matter (left in the Figure) is much higher than the export (right in the Figure). However, a lower import than export of nutrients is still no guarantee of good manure management. The annual average nutrient export and loss from agricultural land in sub-Saharan Africa is estimated to be 22 kg N and 2.5 kg P per hectare higher than import (Stoorvogel, Smaling and Jansen, 1993), but in many instances manure management could be improved. Application of nutrient balances at plot, farm and livestock system level is a powerful tool in nutrient management analysis (Stoorvogel, Smaling and Jansen, 1993; van der Meer and van der Putten, 1995; van Keulen, van der Meer and de Boer, 1996), and is more accurate than a fixed threshold for livestock density, since nutrient excretion (Table 2), potential crop production and relevant soil and climatic conditions vary significantly throughout the world.
The numbers in the Figure indicate those leverage points where manure management decisions can influence the magnitude of nutrient emissions. The numbers are in order of the likely impact of the respective measures on the total nutrient balance for intensive livestock production systems. In extensive systems, points 2 and 4 are of major importance in improving the efficiency of manure utilization.

• (1a) Dosage. The dosage of P applied through manuring should not exceed the amount of P removed by the crops, since this may result in P leaching. Overdosing with N is a major cause of NO₃ leaching. Because of the N:P ratio in manure, NO₃ leaching as a result of manure application is usually not a problem if a zero P balance is maintained. When mineral fertilizers are also used, their optimal dose should be determined after calculating the N, P and K fertilizer equivalents of the applied animal manure, also taking into account the residual effects of previous applications. When the fertilizing effect of manure is thus calculated, the risk of NO₃ and P leaching is reduced. The risk of heavy metal accumulation in the soil, except for Cu in pig manure, is also low if a zero P balance is maintained.
• (1b) Timing of manure application. The extent of NO₃ leaching, K leaching (on sandy soils only) and surface runoff is influenced by the time lapse between manure application and growing period of the crop. Autumn application of manure in regions with a high winter rainfall can result in almost a complete loss of N before spring planting. The application of manure should be synchronized as closely as possible with the period of nutrient demand by the crop.
• (2) Manure collection and storage. In many livestock systems urine is seldom collected, subsequently high losses of nutrients are incurred through leaching and runoff, particularly of N and K. This results in the pollution of surface and groundwater. Covering manure storage areas and protecting them from rain reduces leaching losses considerably. Disposal or leakage of liquid manure should be prevented, as should the direct contact of manure with the underlying soil, except on some hardly permeable soils.
• (3) Method of application. NH₃ emission following application to the land may involve all of the inorganic N. NH₃ volatilization is highest when manure is thinly spread and left exposed to the air. Substantial reductions (40 to 95 percent) in NH₃ emission can be achieved by minimizing this exposure through direct incorporation into the soil, dilution, watering-in and the use of special machinery for slurry application.
• (4) Storage method and gaseous emissions. When manure is burnt, most of the carbon content and all N and S will be lost; other nutrients may be returned to the land as ash. When urine is collected, under conditions prevailing in the Netherlands, NH₃ emission from manure in stables and in storage can vary from 5 to 35 percent of the total N excreted during a storage period of six months. Reducing NH₃ losses from manure held in livestock buildings and in storage depends on the measures taken to decrease the NH₃ concentration in the manure and on minimizing its exposure to the air, for example by providing cover. The addition of bedding also reduces NH₃ losses, both during storage and application, since it temporarily combines with and immobilizes mineral N (ca. 7.5 kg N/tonne straw).
• (5) Feeding. In intensive production systems with an overall nutrient surplus, improved nutrient balances can be realized by adjusting the animal diet and/or improved feed conversion. Adjusting the feeding ration may be done by using feeds with a high nutrient digestibility; using phytase to increase P digestibility; eliminating antinutritional factors; and controlling feeding to meet the animals' requirements (e.g. phase feeding, addition of synthetic amino acids, reduced rumen-degradable protein surplus). These approaches may significantly increase feeding costs.

Possible nutrient losses from manure between the time of excretion and crop uptake
Possible perte des nutriments du fumier entre sa production et l'absorption radiculaire
Procesos de p�rdida de nutrientes del abono org�nico entre la deyecci�n y la absorci�n radicular

OPTIONS FOR IMPROVEMENT

Apart from the general management options described above, there are a few alternative technological options that may be of practical use in other specific situations.
Recycling manure as animal feed is limited, partly owing to widespread reluctance to use manure as feed and also probably out of consideration for the human health hazards. Moreover, most types of manure have low nutritive value as ruminant feed, with the exception of poultry manure. In less intensive livestock production systems, where utilization of low-quality feeds is common, high collection and/or opportunity costs (manure as fertilizer or fuel) restrict the use of manure for feed. Alternatively, integrated fish production seems to offer better opportunities, although the effective exploitation of high fish yields requires high management capabilities (NRC, 1981). Manure may also be exploited as an energy source through biogas production. However, the efficient man-agement of biogas slurry is complex and can only be profitable in countries where fuel prices are relatively high (e.g. US$0.19/m³ biogas) (Ten Have and Chiappini, 1993). Other methods of manure management and treatment include the discharge of effluent to surface water and the production of dry fertilizer which can be transported over long distances. These are associated with high, often prohibitive, costs and may not produce acceptable environmental results. The main exception is the fast drying of poultry manure, which does reduce NH₃ emission (Ten Have and Chiappini, 1993). There are many policy options available to reduce the negative environmental impacts of using animal manure, some of which are already being implemented, for example:

• a ban on discharging manure in surface water and on dumping; a spatial redistribution, often in relation to distance between residential areas and livestock units to minimize undesirable odours (this option may result in high livestock densities in specific areas, making manure treatment imperative); technical protocols for the storage and application of manure; the introduction of a permit system for control over livestock holdings.

Most of these regulations require efficient im-plementation through highly qualified agencies. Where enforcement of legislation is too problematic, alternative economic policies can be adopted, for example the removal of (indirect) subsidies or the introduction of levies on:

• artificial fertilizer to increase the value of animal manure; concentrate and/or fossil energy to discourage high livestock densities.

A major disadvantage of these indirect economic policies, however, is that tradeoffs to other sectors will occur, and strong opposition may develop. Thus, the feasibility of implementing the different options is largely dependent on various socio-economic conditions, ranging from civil servant salaries to the relative political and economic powers of the sectors affected. For example, the removal of barley subsidies, which are a major macroeconomic burden and ineffective in controlling rangeland degradation, in western Asian and North African countries, will probably result in a sharp reduction in sheep fattening, thereby reducing the negative environmental impact of inefficient manure collection and storage by some large sheep fattening enterprises. However, these subsidies are not easily removed, since this would also affect bread prices and perhaps provoke social unrest. The effectiveness of the different policies can be discussed in relation to specific situations only. For example, the introduction of a compulsory nutrient accounting system, as proposed in several Western European countries, combined with low permissable manure surpluses (1 to 10 kg P₂O₅ per hectare/year, Brandjes et al., 1995) may result in even higher manure surpluses in situations where both arable farmers and intensive livestock farmers are present within relatively small distances from each other. Arable farmers are likely to prefer artificial fertilizers instead of animal manure to attain the low permitted surplus at higher levels of crop production. This may result in an increase in unusable waste, as intensive livestock farmers would eliminate feeds with a low nutrient digestibility, such as crop residues and agro-industrial by-products.

CONCLUSIONS

Animal manure is a liability where high livestock density is combined with the use of cheap artificial fertilizers for intensive crop production. In most other situations, animal manure is an asset, particularly if artificial fertilizers are unavailable or expensive.
Threshold values of pollution indicators are not easily identified, but the P balance is an accurate indicator for most environmental problems, with a wider applicability than livestock density. Methane emission is an exception, since the problem of global warming is not related to local concentrations of livestock or manure. Odour and ammonia emissions may be problematic when large livestock units are located near residential areas or natural reserves. For nitrate leaching and the maintenance of P equilibrium, the timing of manure application is of some importance. Various technical options to reduce nutrient losses from manure are available. Most of these will enhance its value as fertilizer. Technologies for improved manure collection and storage may help significantly in situations where no surplus exists. A number of legislative and economic policy options are available. As a general rule, intensive landless livestock production should be discouraged, even though it may seem economically attractive at first. Since the feasibility and effectiveness of both technical and policy options are strongly dependent on variable natural and socio-economic conditions, they should be realistic and not overly theoretical if they are to succeed.

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