J. de Wit1, H. van Keulen2,3, 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 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.
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
Nitrogen |
Phosphorus |
|
Relative contribution to crop nutrient requirements (%) |
6-10 |
40-59 |
Fertilizer equivalents (FE) (kg/ha) |
16-18 |
7.9-11.7 |
Economic value of FE (US$/ha) |
5.9-10.2 |
1.8-2.6 |
Fossil energy requirements for FE (MJ/ha) |
1 131-1 943 |
27-39 |
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
Intake |
Retention |
Excretion |
% Nmin1 |
Animal |
N |
P |
N |
P |
N |
P |
(kg) |
Dairy cow2 |
163.7 |
22.6 |
34.1 |
5.9 |
129.6 |
16.7 |
69 |
Dairy cow3 |
39.1 |
6.7 |
3.2 |
0.6 |
35.8 |
6.1 |
50 |
Sow2 |
46 |
11 |
14 |
3 |
32 |
8 |
73 |
Sow3 |
18.3 |
5.4 |
3.2 |
0.7 |
15.1 |
4.7 |
64 |
Growing pig2 |
20 |
3.85 |
6 |
1.3 |
14 |
2.5 |
78 |
Growing pig3 |
9.8 |
2.9 |
2.7 |
0.6 |
7.1 |
2.3 |
59 |
Layer hen2 |
1.23 |
0.26 |
0.36 |
0.04 |
0.87 |
0.22 |
82 |
Layer hen3 |
0.55 |
0.15 |
0.05 |
0.006 |
0.50 |
0.14 |
70 |
Broiler2 |
1.09 |
0.17 |
0.45 |
0.075 |
0.64 |
0.10 |
83 |
Broiler3 |
0.41 |
0.11 |
0.13 |
0.018 |
0.28 |
0.09 |
60 |
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.
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.
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
Nitrogen |
Phosphorus |
|
Total excretion (kg) |
6.9*109 |
1.9*109 |
Nutrient losses (kg) |
2 800*106 |
32*106 |
Economic opportunity cost (US$) |
1 000*109 |
14*106 |
Fossil energy opportunity cost (MJ) |
140*109 |
0.15*109 |
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.
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 NOX. 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).
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.
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
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:
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:
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.
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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