1 Introduction

Livestock do not produce soil fertility: they transform feed biomass into livestock products and manure. During this transformation process, biomass and the nutrients in it, can be allocated from a larger area to a smaller area through the manure when deposited. Some concentration of soil fertility might be beneficial, especially in areas with low soil fertility, as it enables crop production ("infield-outfield" systems). However, if concentration is too high, negative effects can occur such as marginalization of grazing land or over-fertilization of the smaller areas where manure is deposited. Negative effects may also result from inevitable losses during the transformation process, such as ammonia emission.

The appreciation of animal manure, consequently, also varies among farming systems. Since ancient times, man has always looked upon manure favourably because of its fertilizing value. Before the introduction of inorganic fertilizers, manure was one of the few fertilizing materials with nutrients in a concentrated form available to plants. Livestock were often kept with the main purpose of manure production. The farmers used grazing stock to collect plant nutrients from a large grazing area and, through manuring, concentrated them on relatively small plots of cropland. Today, such farming systems do still exist. Romney et al. (1994) quote a study in Zimbabwe where farmers reduced grazing time to collect more manure, even though feed intake and thus draught power capacity decreased as a result of this practice.

The introduction of mineral fertilizers has led to the reduction of the value of animal manure as a concentrated form of plant nutrient supply. Also, some very intensive livestock systems have developed where manure production exceeds demand, resulting in "manure surpluses": from an asset, manure has become a liability.

The value farmers attach to manure is related to the manure production versus the agricultural area used. Because crops with higher yields take up larger amounts of nutrients, the use farmers make of nutrients from manure also varies accordingly. Therefore, manure production per unit crop production would be a better characteristic for valuation of manure in a system than manure production per unit utilized agricultural area. This characteristic could, for instance, be expressed as kg phosphorus (P) produced in manure divided by the crop production expressed in kg phosphorus (P) removed in harvested crops. For situations where plant nutrient availability limits crop production (e.g. Sub-Saharan Africa), it would be better to apply the potential crop production with adequate supply of plant nutrients instead of the actual production. This report describes methods for calculating the manure production (Section 3.2). Crop production is usually known. The potential crop production can be estimated with reasonable accuracy; a database exists with the potential crop production per geographical grid cell of 110 km by 110 km for all regions of the world (Penning de Vries, Van Keulen and Luyten, 1994). The valuation of animal manure by farmers depends, of course, also largely on the availability and price of mineral fertilizers.

To evaluate the role of manure in a farming system, the use of manure should be evaluated. As a tool nutrient balances of agricultural systems are suggested: a system should, through animal feed and fertilizers, not import more nutrients than the export in crop products. Since this report is about animal manure management only, the above can be restricted to: a system should, through animal feed, not import more nutrients than what is exported in crop products. This principle should be applied to the macronutrients nitrogen(N), phosphorus (P) and potassium (K) as well as to micronutrients such as copper (Cu) and zinc (Zn). If the amount of nutrients added to the soil is too much, the imbalance will inevitably cause accumulation, and either immediately or at a later stage, emissions to water or the air. Copper contamination of the soil, nitrate leaching and phosphorus leaching are often symptoms of unbalanced nutrient management of a farming system. Ammonia emission can be considered the result of an attitude showing no concern for appropriate nitrogen management. Nutrient management has been defined as a decision-making process with the objective to combine profitable agricultural production with minimal nutrient losses for the present as well as the future (Oenema, 1994). Manure management is an integral part of nutrient management. Nitrogen and phosphorus balance is discussed in this report. Even when nutrient doses with animal manure are adjusted to the level of nutrient removal by the crop, wrong application techniques can cause losses (or emissions). The influence of manure application techniques is, therefore, also discussed.

This study will focus mainly on the effects of manure applied to land. Because in some very intensive livestock systems manure is considered waste and discharged directly into surface water, the consequences of this practice are also dealt with. In addition, emissions from manure in animal housing and during storage are discussed.

Main attention will be given to organic matter (OM), N and P as most positive and negative effects of manure are related to these elements. Where appropriate, attention will also be given to other elements in manure.

Table I indicates which interactions between manure and environment are elaborated in the subsequent chapters of this report, dealt with in another report of this study or not treated in detail.

The various effects of manure on the environment are discussed briefly in Chapter 2, and some globally more important ones are dealt with in detail in Chapter 3. In Chapter 4 conclusions are drawn together with technical, economic and legislative implications for manure management.

Interaction	Discussed in detail in this report	Discussed in another report of this study	Discussed only briefly in this report
Savings of fertilizers by utilization of manure	*
NH3 volatilization	*
N2O emission			*
NO3 leaching	*
P leaching	*
Pollution of surface water by direct discharge or runoff of manure	*
CH4 emission from animals, manure or irrigated agriculture		*
Accumulation in soil of heavy metals	*
Accumulation in soil of organo-chlorines			*
Reduced use of fossil fuel by utilization of manure for biogas			*
Reduced use of feed by utilization of manure as feed			*