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Extensive grazing lands cover a very large part of the earth's surface and are not only important for livestock: they are often major watersheds and wildlife habitats, as well as sites for the in situ conservation of plant genetic resources. In the developing countries there are at least 40 million pastoralists who depend on natural grazing for their livelihood; most are subsistence herders. Rapid increases in human and livestock populations this century, along with diversion of grazing land to other uses, have contributed to increasing grazing pressures, particularly in arid and semi-arid environments. The assessment of grassland condition and productivity is a complex subject, but a very necessary one for the correct management of grazing lands and the design and execution of projects to improve sustainable extensive livestock production and enhance the living standards of traditional herders.

This publication deals with the study of grazing lands and their livestock, and with the analysis of the results; it is mainly concerned with the traditional sector and with studies for development projects. Modelling is emphasized as a powerful tool for dealing with the complexities of the pastoral system. The potential applications and uses of computer-based pastoral system modelling is far greater than traditional methods since modelling accounts for system complexity and component interrelationships.

A number of software packages are available for modelling agriculture and livestock production. Most are designed for intensive production systems, and only a few are suited to livestock production on extensive grasslands and the forage production systems and analyses of the type described in this document. This publication describes a computer-based model designed specifically for use in pastoral development programmes, and which aids managerial, policy and development decisions, primarily by analysing land and forage resources for their productivity and livestock support capacity, taking system complexity into account.

The draft was prepared by Dr P.S. Harris, who has worked as an FAO consultant over many years and has been developing computer modelling in the traditional livestock sector in Asia and Africa since the 1980s. A series of case studies illustrate the development of the model and its practical use. It should serve as an invaluable reference book in many other parts of the world.

Thanks are particularly due to Stephen Reynolds of the Grassland and Pasture Crops Group for ensuring that the book was brought to publication.

Marcio Porto
Crop and Grassland Service (AGPC)
Plant Production and Protection Division

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