Protecting crop yields and preventing increases in weed populations are the primary reasons for controlling weeds. Weed management systems that rely on postemergence control assume that crops can tolerate competition for certain periods of time without suffering yield losses. However, once competition between the crop and weeds begins, the magnitude of yield loss increases rapidly as weed control is delayed. Our inability to accurately predict when weeds first begin to affect yield will always create a level of risk when relying on total postemergence weed control programs. Preemergence herbicides greatly reduce this risk by reducing the populations of weeds that emerge with the crop. The objective of this research was to quantify yield losses associated with delays in controlling weeds that emerge with or shortly following corn.