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We constructed a simple simulation model (SESS) of the dynamics of forage growth and standing crop and cattle production to evaluate the ecological sustainability of management alternatives for extensive cow-calf production systems in northeastern México and south Texas. Equations were written to estimate annual net primary production based on range condition, annual precipitation, and soil characteristics typical of the region. Simulations were conducted for annual precipitation levels of 300, 500, and 700 mm to estimate total and green standing crop dynamics, cattle grazing efficiency, and range condition trend for different stocking rates. The model-estimated stocking rates to achieve stable or slight improvement of range condition for the three precipitation levels were close to 58, 15, and 6 ha per animal-unit-year (AUY), respectively. With the model parameterized for precipitation and soil characteristics combined with the stocking rates recommended by COTECOCA (1979. Coahuila. Tipos de vegetación, sitos de productividad forrajera y coeficientes de agostadero. Secretaria de Recursos Hidráulicos. Comisión Técnico Consultiva para la Determinación Regional de los Coeficientes de Agostadero. México), we conducted 20-year simulations for three groups of range sites of Coahuila, México (annual precipitation: 1: 270 mm, 2: 351 mm and 3: 467 mm). The trends of body condition score and range condition for years 5, 10, 15, and 20 were similar within each of the three groups. The stocking rates recommended by COTECOCA were too high for sustainability on range site groups with 270 and 351 mm annual precipitation. The simulated probabilities for pregnancy rates at different stocking rates for the three groups indicated that the stocking rates recommended by COTECOCA were too high to achieve pregnancy rates ?80% in 8 out of 10 years with no supplement in the form of hay or concentrated feeds. Model simulations suggested that, in the absence of supplemental feed, ecological sustainability and acceptable livestock production could be achieved simultaneously at light stocking rates.