4 Discussion and implications of manure management

4.1 Technical aspects
4.2 Economic aspects
4.3 Manure legislation
4.4 Assessment of the impact of manure

4.1 Technical aspects

Proper manure management aims at reducing the negative effects and stimulating the positive effects of manure. Based on the environmental impact assessments of animal manure presented in the preceding chapter, conclusions can be drawn about manure management decisions on animal feeding, dose of animal manure, adjustment of nutrient supply with additional mineral fertilizers, control of emissions from animal housing and manure storage, timing of manure application and method of application.

a. Feeding: In intensive systems with an overall nutrient surplus in animal manure, adjustment of the diet to reduce the nutrient excretion of the animals is a way to better balance nutrient import and export.

Measures to reduce P excretion are based on changes in the feed composition toward ingredients with a higher P digestibility and by the addition of phytase to the feed to increase the ability of animals to digest P.

Though the cost of compound feed might be reduced because of the growing application of phase feeding, adjustments in feed rations will inevitably result in higher costs for additional farm infrastructure (silo's), increased management needs, transport costs, etc. (Wijnands and Amadei, 1993). The maximum practical nutrient efficiency of animals will remain fairly low (in most cases below 50%), although major improvements are possible in highly productive situations where currently luxury consumption of nutrients is common.

b. Dose: Apart from dose of animal manure applied to the soil, no more nutrients should be added other than exported in crops plus an environmentally acceptable nutrient overdose. Soil fertility should be maintained by keeping in balance, the macronutrients N, P and K, but also for the mesonutrients Ca, Mg, S and trace elements such as Cu, Zn, Pb and Mn. It is, therefore, important to know the composition of the applied manure. When the manure dose is adjusted to meet the P removal by crops, N and K supply through in manure will be less than or equal to that removal. For some mesonutrients or micronutrients, oversupply is unlikely but still possible. Copper from pig manure, however, may remain problematic.

c. Adjustment of mineral fertilizer dose: Farmers using both animal manure and mineral fertilizers, should estimate the fertilizing value of the manure and reduce the dose of mineral fertilizers accordingly. Provided that nutrient composition of the manure and working coefficients are known, farmers can estimate the nutrient supply to their crops from a certain dose of manure. If an additional supply is needed for optimal crop growth, they can top up the supply by means of mineral fertilizers.

d. Emission reduction before application: In stables and manure storage, contact between manure and soil should be prevented and exposure to the air minimized. Simply covering manure in rainy weather will reduce leaching losses substantially. The urine should never be allowed to drain into the soil or surface water. Exposure to the air in the stables can be minimized by thorough cleaning. If ample water is used, cleaning will be easier and an additional advantage is the dilution effect lowering the vapour pressure of NH₃ in the manure. The disadvantage is the increase in volume of manure to be transported to the field.

e. Timing of manuring: Manure should be applied at the time when the crop needs nutrients. Autumn application of manure in temperate climates with a winter precipitation surplus may lead to almost complete loss of the mineral N from the manure. Moreover, on sandy soils a considerable part of the K is also leached. The best time of application is shortly before planting the crop. Slurry injection on grassland, and direct application to a growing crop are efficient ways of manuring.

f. Method of application: Injection, "sod-manuring" and direct working-in are methods to prevent or minimize exposure of manure to the air when volatilization of NH₃ is much reduced. Also surface runoff is minimized. Dilution of the manure and raining-in reduces NH₃ volatilization by reducing NH₃ vapour pressure in the slurry and increasing the rate of infiltration into the soil.

4.2 Economic aspects

Global economic evaluation of the environmental effects of manure is extremely complicated due to the large variability in relevant conditions, such as:

- concentration of the production and dispersion rates of harmful compounds (e.g. NH₃ emission is not easily translated into deposition rates because of climatic interference);

- production of similar compounds by other sources (e.g. SO₂ by cars and industry which is also causing "acid rain", pollution by cities and industry of the same water bodies into which manure is discharged);

- the types of natural resources affected, their alternate use and the valuation of these alternate uses (e.g. where is the ammonia deposited and what is society’s, view of these areas? Is the water body, at risk of pollution, used for drinking water, recreation or agriculture?).

The case of ammonia emission is illustrative here. If deposited on agricultural land it can be valued as N fertilizer, free of cost. If, on the other hand ammonia is deposited on urban, recreational or nature reserve areas, negative effects become prominent. However, valuation of these negative effects varies widely.

In many countries with abundant natural areas, the negative effect of ammonia deposition on some areas may be considered to be hardly problematic, though if rare species are at risk, high values may be given because of reduced biodiversity. Contrarily, ammonia deposited in the Netherlands on the few small areas intended for nature development, is viewed as highly problematic, partly because of loss of recreation space but, probably even more importantly because of emotional values attached to these small "natural" areas in a highly industrialized and small country. Such different values might be assessed, for instance using "Willingness to Pay" concepts, but they will remain location- or even group-specific as large differences in valuation of the same location often exist among different social groups (e.g. livestock farmers and nature fans).

A partial economic evaluation is possible at higher aggregation levels. These are focused on foregone benefits and costs of achieving environmental quality standards, as there are sizeable conceptual and methodological problems with assessing damage functions of the pollution level (Winpenny, 1991; Wijnands and Amadei, 1993). First, for water quality aspects the costs of removing polluting compounds can be assessed. These costs vary according to the technology used, local prices for energy, labour, etc. Exploitation costs for removing N and P were estimated at 20-37 ECU (i.e. ca. US$ 27-50) per kg nutrient emission from point source pollution (like manure directly discharged into surface water). These values refer to Eastern European countries; they are based on 8% discount rate and are dependent on the level of nutrient reduction aimed at (Haskoning, 1994).

Information on costs of liquid manure treatment is still scarce. The few available estimates on total operational costs of purification plants vary between 5 and 15 ECU (i.e. 6.75 and 19 US$) per m³ (Ten Have and Chiappini, 1993), while the required effluent quality (10-15 mg N/l and 1-2 mg P/l; EEC-Council Directive 91/271/EEC, 1991) is still not attained, even if the plants work at optimal level. The conclusion is that large-scale treatment of liquid manure as sewage is not economically justified (Haskoning, 1994), even if management and infrastructural problems are solved.

Second, all losses of nutrients via emission, leaching, direct discharge to the environment, etc., can be valued according to current costs of inorganic fertilizer, by way of replacement costs or foregone benefits. World market price (Jan.-Dec. 1994) for urea is about US$ 365 per ton N while the price of P varies from about US$ 230 - 630 US$ per ton P for rockphosphate and triple superphosphate respectively.

Wijnands et al. (1987) made a cost-benefit analysis of the technologies that reduce NH₃ volatilization from stables, storage and upon application. They concluded that the savings in mineral N fertilizers through reduction of N losses from the manure are, in general, equal to the costs of emission reduction. More recent research indicates that this might be true if the ammonia reduction is only modest, but that costs increase dramatically if the maximum ammonia emission level is set below ca. 35 kg N/ha (in case of land-based livestock production; Van der Putten et al., 1995; Van der Ven, forthcoming) or reduction of more than approximately 50% (in case of landless livestock production; Balthussen and Hoste, 1994).

Little data has been found on the costs and benefits of reduction of leaching. Based on theoretical calculations by Van der Ven (forthcoming) it can be assumed that the economic cost of N-leaching reduction is relatively modest up to a level of ca. 20 kg/ha/y; a further reduction will have more pronounced effects on farm income (Van der Putten et al., 1995).

In a situation where farmers have no economic incentives to adopt emission-reducing technologies, and reduction in emissions from manure is a societal objective, only laws and regulations can bring results.

4.3 Manure legislation

The number and rigour of laws and regulations with respect to animal manure varies among countries and also among states or regions within countries (Schröder, 1993, Hacker and Du 1993). Besides banning the complete livestock industry (like pigs in Singapore), manure legislation may include stipulations with regard to the following:

- Spatial distribution: Many countries try to reduce intensive livestock production near urban areas. Malaysia, for example, has advanced plans to confine all pig production to special pig farming areas (PFAs) at a distance from residential areas, water catchment areas or villages with a predominantly Muslim population. The objectives of this measure are to provide the impetus for profitable pig farming integrated with pollution control and to regulate the development of the industry (Ong, 1991).

- Odour from animal housing: Some countries have provisions for the nuisance caused by odours to people. The prescribed or recommended distance between animal farms and residential areas is dependent on the size of the farm as well as on the number of people that are affected. In Saskatchewan (Canada) there is a similar regulation for the manure spreading areas (Hacker and Du, 1993). George, et al. (1985) observed that in the USA lawsuits against pig producers that emit odour are becoming more common and in most cases are won by the plaintiffs.

- Storage: (a) Some countries have regulations for storage capacity, which should be sufficient to store all manure produced during the period when land spreading is not allowed or undesirable. In the Netherlands, farmers should be able to store all manure produced during autumn and winter. Farmers in Denmark are obliged to have storage capacity for their total annual manure production, since application of manure is only allowed in spring. (b) The storage pit, lagoon, or heap should be built in such a way that it is sealed from the surrounding soil to prevent leaching and runoff. In the Netherlands, concrete walls are stipulated. Future legislation in the USA will, among other things, emphasize the control of all barnyard runoff of manure. To reduce NH₃ volatilization, the storage should also be covered, as is compulsory in The Netherlands.

- Ban on discharge to surface water and dumping: Many countries (e.g. Malaysia, European and North American countries) have explicitly banned the direct discharge of animal manure into surface waters. Law enforcement, however, is often lacking. Some countries (e.g. Italy) set limits for the maximum allowed dose of manure application to land (i.e. the amount of manure from 4000 kg of animals per ha), but omits the dose of the nutrient content, resulting in the possibility to apply over 1000 kg P₂O₅ per ha in the form of poultry manure (Breeuwsma and Silva, 1993).

- Nutrient dose: The addition of nutrients to the soil via both animal manure and mineral fertilizer should not exceed that removed by the crop(s) by too much. In Michigan, for instance, it is stipulated that manure application should not exceed P removal by crops if P levels in the soil are high (Scialabba, 1994). Dutch law only sets a limit for the manure P input, partly because of effective lobbying by the fertilizer industry and arable farmers. As a result, fertilizer application is still high and nutrient surpluses excessive at national level. Belgium, Germany and the Netherlands will in the future probably limit total N and P input from manure and fertilizers. Because of variability in crops, soil and climate, a differentiated system of N and P limitations is desirable, but law enforcement, might be problematical.

- Time of application: Manure spreading on snow or frozen soil is illegal in many temperate countries to prevent runoff during winter. The length of the period varies from country to country, but there appears to be no technical explanations for this variation.

- Method of application: The Netherlands is one of the few countries, if not the only one, where farmers are obliged by law to use manure application techniques with low NH₃ emission. No surface spreading is allowed on grassland: sod fertilization or slurry injection is compulsory. On arable land, the same techniques are also required to be applied, or the manure worked in by other means immediately after spreading.

Legislation regarding animal manure production and use is probably most elaborate in the Netherlands. Here, the high density of very intensive livestock production caused so many pressing problems to society that in 1986 the Fertilizer Law was implemented. In addition to most of the above elements, the main points of this law are:

- Compulsory registration of manure production. The number of animals on each livestock enterprise was registered on 31 December 1986. By multiplying the number of animals by their P excretion, a reference quotum of manure per farm was determined, expressed as kg manure P per ha.

- Ban on expansion of stock. No farmer is allowed to increase his manure production above the quotum on the reference date, unless manure production remains below 125 kg P₂O₅ per ha. New enterprises cannot gain rights to manure production exceeding 125 kg P₂O₅ per ha. Limitations govern the shift from one animal species to another.

- Surplus charge: A manure producer is charged a fee for every kg P₂O₅ per ha over the stipulated manure P production of 125 kg P₂O₅ per ha.

- The practice of dumping of manure surpluses on agricultural land, maize plots in particular, has been banned.

- The distribution of manure over the country has become more homogeneous, as comprehensive manure transportation systems have been developed for transportation from surplus to deficit areas. Manure processing as an option for future export of manure surpluses is being investigated seriously.

- The use of mineral fertilizers has started to decrease, though it remains high.

- Emission of NH₃ from manure application has decreased.

- The combined effect of measures in the agricultural and other sectors has led to improvement in the quality of surface water in the Netherlands.

4.4 Assessment of the impact of manure

Parameters for the assessment of manure impact on the environment can be derived from this report and rated as positive or negative as suggested in Table XII. Livestock systems consultants may select other assessment parameters. The number of plus and minus signs in the various cells may be questioned. Different weights may be attached to different parameters, that may also vary among the different livestock systems.

In parameter (1) the total manure production of a system is related to the capacity of the crops produced in the system to take up nutrients from the manure. It is assumed that when the manure P input is in balance with the P removal by crops, including grass, the same holds for the other nutrients (N, K, and micronutrients).This parameter differs from the animal density in the particular systems or the manure production per ha in the sense that the last two parameters do not take into account differences in (potential) crop production. This report is only concerned with nutrients from manure and not other sources of nutrients such as mineral fertilizers or green manure crops. A proper and complete assessment of the risk of environmental damage, however, should include comparison of the sum of nutrients from all sources with the nutrient removal by crops. If manure is exported to farming areas outside the system, it would seem logical to subtract this quantity from the manure P production. Depending on the importance of the other components influencing the nutrient balance, the evaluation of a situation might be different, e.g. if the ratio is lower than 0.9 and no other nutrient sources are applied, the evaluation might be negative instead of positive as the soil fertility will decrease.

Parameter (2) relates the quantity of manure to the land area, a value equal to or less than one for parameter does not exclude the risk of an overdose of nutrients. This requires a homogeneous distribution of the manure over the available farmland. Although difficult to assess, one could look for measures taken within the system to promote manure distribution or regulations to prevent overdose, dumping or direct discharge to surface water.

Parameter (3) refers to the way in which farmers in the system generally handle manure in the farmyard. Do they allow urine to drain into the soil or surface water. Do they seal the manure pits from the surrounding soil, use bedding material to absorb urine and faeces, or cover the manure storage, etc.?

Parameter (4) refers mainly to the risk of leaching of nutrients from the manure applied. Weight may be defined dependent on the prevailing weather conditions during this time lapse. More weight may be given to livestock systems where conditions during this time lapse are usually rainy, than in systems with dry weather in this period.

Parameter (5) has to do which the influence of NH₃ volatilization immediately after application, as well as the risk of runoff. The use of water with manure application (parameter 6) reduces NH₃ volatilization. The effect of NH₃ deposition on the environment may not be judged as negative in extensive livestock systems, though some additional fertilizer is required to replace the incurred N loss to prevent a drop in soil fertility. For impact assessment, parameters (5) and (6) may then be given a less weight.

The N working coefficient (parameter 7) reflects the efficiency of N utilization from manure by crops (compared to fertilizer), and, thus, the extent to which N emissions have been prevented.

Use of organic matter that would otherwise be wasted (parameter 8), e.g. burning, bedding, must be appreciated positively. Calculation of the amounts for different systems, however, may prove to be impossible because of lack of data.

1) Ratio "manure production (kg P)"/"crop production (kg P)"	Low(<0.9)	Medium(0.9-1.1)	High(>1.1)
Assessment	++ 1	++ 1	--
2) Distribution of manure over available farmland	Homogeneous	Medium	Heterogeneous
Assessment	++	±	--
3) Exposure of manure in stables and storage to air and contact with soil	Low	Medium	High
Assessment	++	±	--
4) Time lapse between manure application and planting date	Short(1-7 days)	Medium(1-4 weeks)	Long(>1 month)
Assessment	++	±	--
5) Time lapse between manure application and working-in	Short(0-1 hr)	Medium(1-12 hrs)	Long(>12 hrs)
Assessment	++	-	--
6) Use of water with liquid manure application	Much	Some	None
Assessment	++	±	-
7) N working coefficient in a typical situation of manure utilization	High	Medium	Low
Assessment	+	±	-
8) Organic matter saved from being wasted	Much	Some	None
Assessment	++	+	--

¹: Assessment can be negative, depending on level of fertilizer application, etc. (see text).
²: Assessment can be positive if major part of manure is transported to other areas.
Explanation: negative signs indicate a negative environmental impact; positive signs indicate little or no negative effect on the environment.