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This series of two papers describes a mechanistic model that simulates within years the productivity of vegetation and livestock on the communal semi-arid rangeland of the Succulent Karoo of South Africa. The model enables users to evaluate short-term management decisions on the production of milk and meat and to develop sets of equations and rules for long-term models designed to examine the effects of different strategies on the sustainability of the ecosystem. A soil moisture module partitions daily rainfall between runoff, infiltration and drainage and also simulates the loss of soil moisture by evaporation and transpiration. Forage production by different types of plant is modelled in relation to soil moisture and the present potential for growth. Three factors are assumed to influence the animal's preference for a specific type of plant or part of a plant: relative abundance, ease of harvesting and digestibility. The model combines three mechanisms of food intake regulation: the rate at which the animal is able to eat forage, physical capacity of the digestive system, and, in young animals, their growth potential. Metabolisable energy intake is partitioned between maintenance, accretion/depletion of body protein and fat, conceptus growth and milk production. Reproductive and survival rates are simulated in relation to predicted liveweight and liveweight changes for the different age classes of livestock.