In this report, animal manure management is defined as a decision- making process hyphen aiming to combine profitable agricultural production with minimum nutrient losses from manure, for the present and in the future. Good manure management will minimize the negative and stimulate the positive effects on the environment. Emissions to the air of nutrients, organic matter1 and odour, have adverse effects on the environment. The contribution of manure to plant nutrition and build up of soil organic matter is considered a positive effect. An indirect positive effect is that the use of animal manure may save non-renewable resources used in inorganic fertilizer production.
1 Emission of CH4 from animals and manure is discussed in another report of this studyNegative and positive aspects of manure are closely interrelated because emissions at an early stage inevitably have repercussions on the positive effects on the soil and crop later on. This is schematized in Figure I. The emissions to the air are at the top of the figure, those to surface water and ground water at the bottom. The amounts of nutrients like N, P and K taken up by the crop determine the agricultural value of the manure and depend on the amounts of nutrients emitted on the path from animal to crop. The greater the nutrients emitted the lower agricultural value of the manure. The manure management of a system (see figure 1) should be qualified as poor when the import of nutrients and organic matter (left) is much higher than the export (right). The report advocates application of nutrient balances at plot, farm and livestock system level as a useful tool in nutrient management analysis (Chapter 1 and 3.1).
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The numbers in Figure I indicate those leverage points where manure management decisions can influence the amounts of nutrients emitted. The numbers are in order of the likely impact on the total nutrient balance of the respective measures. The emission processes are discussed in detail in Chapter 3.
Ad 1a: Dosage. P leaching is a result of P overdose over long periods, resulting in its saturation of the soil. The dosage of manure P should not exceed too much the amount of P removed from the field by the crops (3.6.1). Overdose of N is a major cause of NO3 leaching (3.5). Because of the N:P ratio in manure, NO3 leaching due to manure application is usually not a threat if the P balance is maintained. The optimal dose of additional mineral fertilizers should be determined after calculating the N, P and K fertilizer equivalents of the applied animal manure (3.7). This reduces the risk of over-fertilization and consequently NO3 and P leaching. Also the risk of accumulation of heavy metals in the soil is low if the P balance is maintained.The technical recommendations for better manure management are described in Section 4.1. In societies with highly intensive livestock production, demands are higher for better manure management aimed at protection of the environment. Because the farmers involved have little economic incentives to do this themselves (4.2), various laws and regulations have been introduced. Though they resulted in improvement of environmental quality, the balance between effective measures, economic costs and law enforcement remains problematic (4.3).Ad 1b: Timing of manuring. 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 winter precipitation surplus can cause almost complete loss of the mineral N before the planting date in spring. The timing of manuring should be kept as close as possible to the period of nutrient demand of the crop (3.5.2).
Ad 2: Storage method and runoff/leaching. Little quantitative data has been found on runoff and leaching losses from manure storage, but in many publications they are mentioned as a major source of surface water and ground water pollution (3.4.1). Covering manure storage in rainy weather reduces leaching losses considerably. Disposal or leakage of the drained liquid phase of the manure should not be allowed. Direct contact of manure with the underlying soil should be avoided.
Ad 3: Method of application. When manure is applied to the land all its mineral N can be lost by NH3 emission (3.3.3). The fraction mineral N is mostly equal to the urine N and higher than the fraction of organic N (Table V). Volatilization of NH3 is highest when manure is spread on the soil surface and left exposed to the air. Substantial reductions (40-95%, Table VII) in emission can be realized by minimizing the exposure through direct working-in, dilution of the manure, raining-in and the use of special machinery (Annex III). The measures that don't use water also prevent surface runoff of manure (3.4.2).
Ad 4: Storage method and volatilization. Volatilization of NH3 from manure in stables and in storage can vary from 5 to 35% of the total N excreted under Dutch conditions (temperate climate, 6 months of manure storage). Reduction of the NH3 loss requires measures to reduce the NH3 concentration in the manure, and to minimize the exposure of the manure to the air, e.g. by covering (3.3.2).
Ad 5: Feeding. Adjustment of the feeding ration can influence the N content of urine, which is an important determinant for NH3 volatilization. This may imply a major shift in feeding practices (3.3.1). The manure P content can be reduced by adding phytase to the feed and by changing the feed ration to include ingredients with a higher P digestibility (3.6.2).
Finally, suggested as parameters for the assessment of environmental impact of manure in a livestock system are: the ratio "manure-P production"/"P removed in crops", distribution of manure over available farmland, exposure of manure in stables/storage to air and contact with soil, time lapse between manure application and planting date, time lapse between manure application and working in, use of water with manure application, N working coefficient and organic matter saved from being wasted (Section 4.4).
