Evolved herbicide resistance in common waterhemp (Amaranthus tuberculatus) has been a serious issue in Iowa agriculture when this weed first began to appear as a major production problem in soybeans in the 1980's. At that time, ALS inhibitor herbicides (Group 2) were applied to a majority of the corn and soybean acres across the Midwest and common waterhemp populations quickly evolved resistance. In the 1990's, concerns focused on the inevitability of evolved resistance to glyphosate (Group 9). The first glyphosate-resistant common waterhemp populations were identified coincidentally in Badger and Everly, Iowa in 1997. However, glyphosate resistance in common waterhemp was generally scattered in fields and had not become a major concern to agriculture. This changed as the adoption of glyphosate-based corn and soybean production systems became the dominate practice. It was clear that common waterhemp had become the major concern in Iowa soybean production and ultimately in corn production. The more recent identification of common waterhemp populations with evolved resistance to auxinic herbicides (Group 4) and the HPPD inhibitor herbicides (Group 27) as well as the historic resistance to the PSII inhibitor herbicides (Group 5) and PPO inhibitor herbicides (Group 14) leaves few herbicide options for the control of common waterhemp, particularly for postemergence strategies. In order to assess the extent of herbicide-resistant common waterhemp populations in Iowa, a project was initiated in 2011 with support from the Iowa Soybean Association. This project supplements a study conducted in 2008 where resistance to glyphosate was detected in 16% of approximately 220 Iowa common waterhemp populations collected arbitrarily.

Iowa survey of herbicide resistances in common waterhemp including glyphosate and other herbicide groups

246th National Meeting (2013)

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