Resistance of weeds to chemical herbicides is a problem of rapidly, growing importance in agriculture. In Australia there has been a very rapid growth in the occurrence of resistance in populations of annual ryegrass (Lolium rigidum) growing as a weed in continuous cropping rotations. ln this paper a dynamic model of herbicide resistance development in ryegrass is used to examine various aspects of weed management. The model is used to identify optimal combinations of chemical and non-chemical weed control for a range of scenarios. It is found that the period for which herbicide use can profitably be continued prior to onset of resistance is highly sensitive to the effectiveness of the non-chemical control. It is shown that under plausible conditions, the inclusion of pasture in the rotation to delay the onset of herbicide resistance (with weed seed reductions through grazing) can be a profitable strategy. Copyright (C) 1996 Elsevier Science Ltd.