On wheat-sheep farms in the south-west of Western Australia, pasture production in winter is a major limiting factor for the sheep enterprise. This paper describes an economic evaluation of reducing herbicide usage in crops below recommended levels to increase subsequent pasture production in pasture-crop rotations. The analysis was conducted using MIDAS, a whole-farm linear programming model. Inputs to MIDAS were regression models of week kill from herbicide application and yield response to weed removal. The analysis was of ryegrass (Lolium rigidum) control by Hoegrass in the Merredin Region. Results indicated that the gains cost are likely to exceed the losses from reduced grain yields. This conclusion is found to be insensitive to major biological and economic assumptions of the analysis.