A mathematic, dinamic model of beef production based in grazing is presented. The computer programwasWRlTTEN in FOCAL. The model requires initial information on availability, growth, maximum and minimun monthly digestibility of the pasture, stocking rate, age of the animals expresad in weeks, and live weight of thé animals. A forage growth is generated, that is adjusted by the availability, and then this growth is added to the standing forage, to obtain the total dry matter. Starting from this information, a potential consumption is generated as a function of the age of the animals and adjusted to the availability and digestibility of the forage. Once the real consumption is known, it is substracted from the available forage. Based on consumption and according to maintenance requirements, a weight gain or loss is generated. Once this gain or loss is known, the live weight is updated daily. Qn the basis of comparisons done with real production systems, it can be stated that the present model reflects the behavior of animals and pastures, with acceptable accuracy.