Control and distribution of glyphosate resistant giant ragweed in Ontario
Giant ragweed (Ambrosia trifidia) was the first glyphosate resistant weed in Canada. Giant ragweed interference in soybean has resulted in yield losses of greater than 90%; therefore, control of this competitive weed is essential. In 2011 and 2012, glyphosate resistant giant ragweed was confirmed at 34 additional sites Ontario. In addition, there are 11 locations confirmed with cloransulam-methyl resistant giant ragweed and 5 locations with multiple resistance to both glyphosate and cloransulam-methyl. Glyphosate resistant giant ragweed has been found in four additional counties. Field trials were conducted at 5 locations in 2011 and 2012 to determine the most effective control options. Based on these experiments, glyphosate tankmixes with 2,4-D or amitrole provide the most effective control. These two tankmixes provided greater than 90% control. The minimum dose of 2,4-D required for acceptable control of glyphosate resistant giant ragweed is 500 g a.e. ha-1.