The replacement of perennial grass species by undesirable annual grass weeds not only results in lower productivity but is also contributes to a range of external costs. In particular, shallow rooted annuals result in greater deep drainage and therefore a greater potential for salinity, and greater volumes of runoff of poor quality water to streams. In this paper an economic framework for examining the sustainability issues of a perennial grazing system on the NSW Central Tablelands is presented. This involves a combination of simulation and dynamic programming models, with the state of the system represented by variables for the perennial grass composition and soil fertility. The paper examines a range of management strategies that increase the perennial grass composition in terms of net income from grazing, and the impact upon the externalities.