Update 2005 on Herbicide Resistant Weeds and Weed Population Shifts

Given that +90% of the soybeans in Iowa are glyphosate-resistant varieties with the concomitant use of glyphosate products, and that recent interest in glyphosate-resistant corn is likely to result in an increasing number of crop acres where glyphosate will follow glyphosate, it is important to understand the level of selection pressure from this weed management plan that will be imposed upon the weed community. Changes in agroecosystems attributable to glyphosate-based systems are already being observed. Notably, recent announcements of glyphosate-resistant weeds suggest that weed populations are reacting to the selection pressure more quickly than anticipated. Local changes include problems with common lambsquarters, common waterhemp, and Asiatic dayflower. The implications of current glyphosate-based production systems on weed communities, and resultant economics need to be understood if sound management systems are to be developed and this important technology preserved.