Waterhemp is a competitive, summer annual, broadleaf weed that poses a considerable threat to Ontario grain farmers. Populations with multiple resistance to Group 2 (ALS-inhibitors), Group 5 (photosystem II inhibitors), and Group 9 (EPSPS inhibitors) herbicides have been confirmed in Ontario. If left uncontrolled, waterhemp competition can result in corn yield losses of up to 74%. The objective of this research was to evaluate preemergence (PRE) and postemergence (POST) herbicides for control of multiple-herbicide resistant (MR) waterhemp. Two field studies at two locations (Cottam and Walpole Island) were conducted in 2016 and 2017. Fifteen PRE and 12 POST herbicides were evaluated for waterhemp control, density, and aboveground biomass and corn yield. At 8 wk after application (WAA), S-metolachlor/mesotrione/atrazine (1393/139/524 g a.i. ha−1) and S-metolachlor/mesotrione/bicyclopyrone/atrazine (1259/140/35/588 g a.i. ha−1) applied PRE were the most efficacious, controlling MR waterhemp 87% and 91%, respectively. At 8 WAA, the most efficacious POST herbicides were mesotrione + atrazine and dicamba/atrazine, controlling MR waterhemp 92% and 87%, respectively. Reduced waterhemp interference with the PRE herbicides evaluated resulted in corn yield that was similar to the weed-free control.
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Vol. 99 • No. 3