Thirteen populations survived the application of glufosinate, which were spread across 12 counties. Cloransulam resistance was confirmed in 99 out of 112 populations and these populations were distributed across 40 counties. Glyphosate resistance was confirmed in all 112 populations and these populations were distributed across 42 counties in Missouri. Horseweed populations were classified as resistant if visual injury assessed at 28 days after application (DAA) was less than 60%. Each population was sprayed with twice the recommended field use rate of glyphosate, glufosinate, 2,4-D, dicamba, and cloransulam. In greenhouse experiments, seed were planted in 10-cm pots and maintained in the greenhouse until they reached a 10-cm rosette for characterization of glyphosate, glufosinate, 2,4-D, dicamba, and cloransulam resistance in a greenhouse experiment. Horseweed seed from 112 separate horseweed populations was collected from infested fields throughout Missouri just prior to soybean harvest in 20. Quantitative data regarding the distribution and frequency of herbicide resistance in horseweed in Missouri is lacking. Horseweed ( Conyza canadensis) has been classified as one of the ten most troublesome and common weeds in the U.S according to a recent Weed Science Society of America survey. Results from a 20 Survey to Determine the Distribution and Frequency of Herbicide- Resistant Horseweed ( Conyza canadensis) in MissouriĮric Oseland, Mandy Bish, Kevin Bradley, University of Missouri, Columbia, Mo.
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