A bio-economic model is developed that allows a detailed representation of optimal weed management decisions. Focussing on German maize production, we apply the model to the effects of a glyphosate ban on farmers’ income, other herbicide use, maize yields and labour demand. We find that a glyphosate ban has only small income effects. Moreover, our results show that selective herbicides are not used at higher levels, but glyphosate is rather substituted by mechanical practices leading to higher labour demand. Slight yield reduction due to less intensive pre-sowing strategies turns out as more profitable than maintaining current yield levels.