Giant ragweed is becoming an increasingly problematic weed to control in both corn and soybean fields in Wisconsin. In an on-line survey conducted between June and September of this past year (2012), respondents indicated that giant ragweed was the fourth most problematic weed to control in their corn and soybean fields. Moreover, in Wisconsin there has been a giant ragweed population confirmed resistant to glyphosate, and recently one population confirmed resistant to cloransulam-methyl. In total, there are now eleven states in the U.S. and one province in Canada (Ontario) with reported populations of glyphosate-resistant giant ragweed (Heap 2012; Stoltenberg et al. 2012). The populations confirmed resistant to glyphosate were collected in Ohio (2004), Arkansas (2005), Indiana (2005), Kansas (2006), Minnesota (2006), Tennessee (2007), Ontario, CA (2008), Iowa (2009), Missouri (2009), Mississippi (2010), Nebraska (2010), and Wisconsin (2010). Additionally, there are five other states in the U.S. with giant ragweed populations resistant to cloransulam-methyl including Illinois (1998), Indiana (1998), Ohio (1998), Iowa (2000), and Minnesota (2008). Most concerning is that Ohio (2006) and Minnesota (2008) have both reported populations that are multiple resistant to both glyphosate and cloransulam meaning tank-mixtures of these two herbicide mode-of-actions (MOAs) are not effective. There is a very high level of importance to find and evaluate control strategies for giant ragweed in corn and soybean for Wisconsin crop producers.
Soil Science Extension
University of Wisconsin Madison