Phalaris brachystachys is a common and troublesome weed in winter cereals in Mediterranean countries. A deterministic model was developed to simulate P. brachystachys seedbank dynamics in the wheat-sunflower rotation, a commonly practiced cropping system in southern Spain, under different herbicide-based management scenarios: no herbicide application, full herbicide dose (standard rate) and two reduced dose rates (75 and 50% of the standard rate). Without treatment, a steady increase of the seed bank is predicted up to an equilibrium level of 54,859 seeds m-2 (575 plants m-2) after 25 years. Full dose herbicide applications in wheat years resulted in a progressive seed bank decline over years. Reducing the efficacy of the herbicide by using 50% or 75% of the recommended rate resulted in no long-term seed bank decline. Instead, a population increase until equilibrium densities is predicted. A sensitivity analysis showed that seedling survival and fecundity were the most sensitive demographic parameters under the full dose strategy, whereas fecundity and seedbank mortality were the most sensitive parameters under reduced dose strategies. Reduced dose strategies tended to be less sensitive than the full dose strategy. Simulations indicated that long-term control of this weed may be attained under full dose, highly effective, herbicide applications. Unless effectiveness could be maintained at very high levels, reduced herbicide doses may not be a recommendable option for the long-term control of this species in a wheat-sunflower rotation.