The question “Fertilizing weeds for a profit?” certainly seems illogical because a corn grower would never intentionally fertilize weeds. However, the potential exists that weeds are being fertilized unintentionally in hundreds of fields in Wisconsin each year. The weeds that emerge and grow early in the season are competing with the corn for nutrients, but the amount of competition may not be fully understood. Considering the high cost of nitrogen, perhaps a more refined question to ask is “How do weeds and weed management affect a corn grower’s profitable use of nitrogen?” The University of Wisconsin and other Midwest universities have introduced new nitrogen use guidelines to maximize the returns to nitrogen inputs. At the same time, many corn fields are being treated with postemergence herbicide programs, which increase the potential for early season weed competition. This increases the potential that weeds may compete and limit the nitrogen available for the corn. This may not be a concern when excess nitrogen is applied, which would be more affordable at lower nitrogen prices. However, this could be a significant concern when nitrogen rates are being optimized. Because of this concern, we wanted to determine if early season weed competition shifted the economic optimum nitrogen rates in corn.