Herbicide resistant weeds in Missouri : sources and solutions
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Herbicide resistant weeds continue to invade new territories each year. Two studies were designed to identify both the spread and the current status of herbicide resistant weeds in Missouri. In 2016 and 2017, 98 separate commercially available bird feed mixes were examined for the presence of weed seed. Amaranthus species were present in 94 of the 98 bags of bird feed examined and reached levels as high as 6,525 seeds kg[-1].. Results from linear regression and t test analysis indicate that when proso millet, grain sorghum, and corn were present in feed mixes, Amaranthus seed contamination was increased. The presence of proso millet and grain sorghum also increased contamination of grass weed species while sunflower increased A. artemisiifolia contamination and safflower increased contamination of Bassia scoparia. An additional study collected seed from 112 separate horseweed populations d from infested fields throughout Missouri just prior to soybean harvest in 2015 and 2016. A discriminating dose that represented twice the recommended field use rate of glyphosate, glufosinate, 2,4-D, dicamba, and cloransulam was applied to each population in order to determine the frequency and distribution of herbicide resistances in Missouri horseweed. A population was classified as resistant if visual control 28 days after application (DAA) was less than 60%. Glyphosate resistance was confirmed in all 112 populations while cloransulam resistance was confirmed in 89 of 112, or 79% of the populations. Two populations survived the application of 2,4-D while all populations were found to be susceptible to dicamba and glufosinate. The results of this survey suggest the use of glyphosate and cloransulam for controlling horseweed in Missouri is likely to result in unsatisfactory control and dicamba, glufosinate, and 2,4-D still provide adequate control of horseweed across the state. Both studies draw attention to the distribution and spread of herbicide resistant weed species in the United States.
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