A biologically realistic model of crop yield response to herbicide application is presented. It includes functions for weed mortality from herbicide application and yield loss due to surviving weeds. The optimal herbicide rate and two types of decision thresholds are derived theoretically and illustrated with empirical examples. Responses of the various decision criteria to changes in parameters are also examined theoretically and empirically. A multidimensional threshold for weeds based on weed density and weed-free yield is presented. The issue of farmers using other than officially recommended herbicide rates is discussed.