By Gared Shaffer
Figure 1. Waterhemp.
Waterhemp Resistance
Herbicide resistance has been an issue for the management of weeds for many years. In South Dakota, resistance in several weed species to multiple sites of action (SOA) of herbicides have been documented. However, the newly published 6-way resistance that is present in Missouri waterhemp has not yet been observed in South Dakota.
Researchers at the University of Missouri were informed of a suspected 2,4D (synthetic auxin) resistant waterhemp population. A replicated field trial and greenhouse trial was developed to test for 2,4D resistance and also other SOA’s. The trial included treatments of the following herbicides; atrazine (PSII inhibitor), chlorimuron (ALS inhibitor), fomesafen (PPO inhibitor), glyphosate (EPSPS inhibitor), and mesotrione (HPPD inhibitor). In this research trial, the population of waterhemp population exhibited very high levels of resistance to atrazine, with visual control never exceeding 27%. Similarly, this population was highly resistant to chlorimuron, with visual control never exceeding 7%, fomesafen had no more than 47% control. Glyphosate provided no more than 24% control and mesotrione was found to have up to 88% control. These results confirmed that some of the waterhemp population had resistance to 6 sites of action.
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