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Development of agriculture in many developed countries has led to serious environmental problems. Most of these are related to the pressure on the land, which may lead to intensification on the one hand and land degradation on the other hand. There is concern that agricultural production in developing countries will cause similar environmental threats in the future as production will have to increase to satisfy the growing demand for food. Intensification leads to high inputs of nutrients in the form of mineral fertilizers and animal feed. Important parts of these inputs leak from the system in the form of nutrient leaching to groundwater and gaseous losses to the atmosphere. Additional environmental problems are land degradation and deforestation in many developing countries, which in many instances are caused by pressure on the existing agricultural land as a result of the growing demand for products and degradation of the existing agricultural land base.

The study presented in this report concentrated on the interactions between livestock production, crop production and land use. The link between livestock and crop production is through the demand for animal feedstuffs. This report presents long-term scenarios describing these interactions and the possible consequences for crop production and animal waste production. As the world population is expected to stabilize in the second half of the twenty-first century, the scenarios must cover a period of 50-100 years to include the impacts of human population numbers. The above crop-livestock interactions play a role in the emission of a number of atmospheric pollutants.

Not all environmental consequences can be quantitatively evaluated. World agriculture is currently responsible for more than half of the atmospheric increase of nitrous oxide (N2O), two thirds of the global ammonia (NH3) input into the atmosphere, and 40% of global methane (CH4) emissions. These compounds play important roles in atmospheric chemistry, ozone depletion, aerosol formation and greenhouse warming. Therefore, a number of examples were selected, including the emission of ammonia (NH3) and nitrous oxide (N2O) from animal waste and mineral fertilizers, as well as projections of the emission of methane (CH4) from ruminant animals. A number of other environmental effects related to livestock and crop production are discussed in a qualitative way.

The scenarios presented in this study are examples showing the comparison of assumptions about demography and agricultural production. Scenarios are tools to assess and compare assumptions about the future. Since ideas about the basic assumptions may change in the course of time, and depending on the purpose of the study for which they are developed, in this report all necessary steps that need to be taken to produce a scenario are described in detail. This will help other scenario developers to adapt the assumptions and construct new scenarios.

The scenarios described in this report, supplemented with scenarios for the developed countries, can be used as input for atmospheric models and integrated models of the environment. The results of this study are of interest for studies on ecological effects of N deposition, atmospheric processes under the influence of pollution. In general, description of the possible future magnitude of the problems related to the use of animal feedstuffs, inputs of nutrients and prevention or reduction of "leaks" in the production system, is of interest to fertilizer specialists, agronomists and workers in the field of rural environment. The description of the construction of the scenarios is of interest to researchers involved in the use and development of scenarios.

Starting from the "World Agriculture: Towards 2010. An FAO study" (AT2010) results, a projection of regional domestic demand and production for groups of agricultural products was made on the basis of scenarios for population, GDP and per capita consumption. The general procedure for developing crop and animal production scenarios involved region-specific assumptions of the yield or production ceilings for irrigated and rainfed crops and animals based on climate, CO2 concentrations, soil suitability, historical developments and new technologies. For the medium scenario the growth rates for yields and animal production parameters of AT2010 were simply extrapolated to the future up to the maximum yield. In the high and low scenarios the rates of increase for crop and livestock productivity are respectively 25% higher and 25% lower than rates for the medium scenario; and yield and animal production ceilings were set higher and lower, respectively. Similar assumptions were made for the rates of increase in irrigated areas and crop intensities.

These combinations of optimistic or pessimistic assumptions on production within a region into regional optimistic or pessimistic scenarios provide an 'outer' range of uncertainty in the projections. In the medium scenario, with a continuation of trends as in AT 2010, for crop and animal production, the pressure on land by increasing arable cultivation will decrease between 2025 and 2050. This is caused by the simultaneous slow down in the growth of demand for agricultural products and continued possibilities for increasing the land productivity. In the low scenario, more land is required for production and larger numbers of livestock are needed to meet the growing demand than in the medium scenario. In the high scenario the opposite occurs, with a lower land pressure and smaller numbers of livestock required for the same total production than in the medium scenario.

The medium scenario for the developing countries, including China, shows an increase of the irrigated area by 50% in the next 50-100 years (90-100 million ha, of which 40 million ha in the first 20 years of the projection period). For the Near East and North Africa the assumptions on total crop production have been adjusted to prevent unrealistic expansion of irrigated and total arable areas. According to the medium scenario, about half of the total increase in irrigated land would occur in South Asia, which may be an overestimate.

The available data on animal feed, which are derived indirectly from country supply-utilization accounts, indicate that at present, of the total domestic demand, about 16% cereals, 20% starchy foods and 3% oilseeds are used to feed livestock in the developing countries. According to the medium scenario the use of cereals as animal feed increases to 30% of the domestic demand in 2050, and similar increases are projected for the other crops. Major increases in the demand for crop and livestock products may occur in Near East and North Africa. In this region there is not enough productive land or water to increase crop production sufficiently to meet the growth in the demand, so in the future there may be major increases in imports of feedstuffs.

According to the medium scenario the excretion of major plant nutrients by animals may double in the coming 5-6 decades. In the high scenario, with growth towards more intensive production, the waste production is lower and even tends to decrease in the period after 2025. In the low scenario, with less growth in animal productivity, the waste production grows faster than in the other two scenarios. As a result of differential growth in the populations of the various animals, and increasing efficiency of N use by animals, the increase in NH3 and N2O emissions is slower than the growth of livestock production. However, particularly in Asia, animal waste production increases rapidly. Combined with a possible concentration of livestock production, high NH3 emission densities may lead to adverse environmental effects such as soil acidification.

Emissions of CH4 from enteric fermentation in developing countries may increase by a factor of 2 in the period up to 2050. If no change in waste management practices occurs, CH4 emission from animal waste will remain unimportant on a global scale. With intensification and concentration of production, however, animal waste may be increasingly stored in lagoons, in liquid form or as slurry, and CH4 from animal waste may become a major global source. The global CH4 emission from rice fields may stabilize if emission rates per unit area do not change. However, if the emission rates are proportional to total biomass production, the global CH4 emission from rice paddies may increase further in the coming decades.

Future fertilizer use may increase towards the current European NPK application level. The current emissions of NH3 from fertilizer use account for about 20% of the total use of mineral nitrogen fertilizers (including urea) in developing countries. The NH3 loss rates may change because they are primarily determined by the type of N fertilizer. Assuming a decrease to the current loss rate of 5% in developed countries, the NH3 emission from fertilizer use projected for 2025 in developing countries may be lower than at present. The emission of N2O from mineral fertilizer will increase by a factor of 3 in the coming 5-6 decades according to the medium scenario.

Conclusions and recommendations resulting from this study are summarized as follows:

- Information on the effect of land degradation on the land's productivity is scarce, in particular for tropical regions. However, such information is crucial for studies such as this one. If yields are negatively influenced by degradation, the crop production scenarios may not be realized, with the arable land areas being affected. Further detailed analysis of the results of the Forest Resources Assessment Project may yield answers to questions on the importance of loss of productivity in shifting cultivation as a driving force of deforestation and other land use changes.

- With the expected impressive increase in animal waste production and the simultaneous increase in demand for plant nutrients, associated emissions of NH3, CH4 and N2O will show significant increases. Abatement of emissions generally involves strategies to increase the efficiency of nutrient use both in crop and livestock production systems. This includes attempts to reduce the synthetic fertilizer use by integration of animal and crop production and closing the nutrient cycles (Mosier et al., in press).

- There is a need for estimates of the geographic distribution of pasture and arable land areas, their prim-iry production and intensity of use. So far, the understanding of the contribution of grazing to animal nutrition is insufficient to fully comprehend the dynamics of pastures and arable land.

- The amounts of concentrates, fodder crops and crop residues used to feed animals are poorly known. Such information is required to improve the understanding of the interactions between livestock and crop production.

- There is a strong need for further study into the irrigation potential of different parts of the world. Projections of the irrigation potential are uncertain due to the sensitivity of river discharges to climate variability and change. In addition, the seasonal distribution of irrigation water use depends on the cropping patterns and water use efficiency, which may change as a result of adaptation to warming, changes in cloud cover and precipitation and increased atmospheric CO2. Scenario analysis of irrigation potential may help in determining future research orientations.

- There is a need for better data on nutrient inputs in crop production from animal waste, recycling of crop residues and biological nitrogen fixation to improve the understanding of changes in mineral fertilizer use and the capability of forecasting fertilizer use.

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