assumes imperfect substitutability, in each sector, between the domestic product and imports. All firms are assumed to be price takers for all imports. What is demanded is the composite consumption good, which is a constant elasticity of substitution (CES) aggregation of imports and domesti­cally produced goods. Similarly, each sector is assumed to produce differentiated goods for the domestic and export markets. The composite production good is a constant elasticity of transformation (CET) aggregation of sectoral exports and domestically consumed products. Such prod­uct differentiation permits two-way trade and gives some realistic autonomy to the domestic price system. Based on the small-country assumption, domestic prices of imports and exports are expressed in terms of the exchange rate and their foreign prices, as well as the trade tax. The import tax rate represents the sum of the import tariff, surcharge, and applicable sales tax for each commodity group. The foreign exchange rate, an exogenous variable in the base model, is in real terms. The deflator is a price index of goods for do­mestic use; hence, this exchange rate measure represents the relative price of tradable goods vis-a-vis nontradables (in units of domestic currency per unit of foreign currency).

A.5.6.3 Application
The GEN-CGE model can be used for studying the impact of tariff changes, removal of nontariff barriers (measured as tariff equivalents), changes in world GDP, changes in world prices, and changes in agricultural technology on employ­ment by sector, prices, household income and welfare. One version of this model has been used for studying the impact of trade reforms in India in 2003 under a project with IDRC in Canada.

A.5.6.4 Uncertainty

A.5.7 The Livestock Spatial Location-Allocation Model (SLAM)

A.5.7.1 Introduction
Seré and Steinfeld (1996) developed a global livestock pro­duction system classification scheme. In it, livestock systems fall into four categories: landless systems, livestock only/ rangeland-based systems (areas with minimal cropping), mixed rainfed systems (mostly rainfed cropping combined with livestock) and mixed irrigated systems (a significant proportion of cropping uses irrigation and is interspersed with livestock). A method has been devised for mapping the classification, based on agroclimatology, land cover, and hu­man population density (Kruska et al., 2003). The classifi­cation system can be run in response to different scenarios of climate and population change, to give very broad-brush indications of possible changes in livestock system distribu­tion in the future.

A.5.7.2 Model structure and data
The livestock production system proposed by Seré and Steinfeld (1996) is made up of the following types:
•     Landless monogastric systems, in which the value of production of the pig/poultry enterprises is higher than that of the ruminant enterprises.
•     Landless ruminant systems, in which the value of pro­duction of the ruminant enterprises is higher than that of the pig/poultry enterprises.
•     Grassland-based systems, in which more than 10% of the dry matter fed to animals is farm produced and in