Abstract
Multiple-herbicide-resistant (MHR) waterhemp has been confirmed and is difficult to control for growers in Ontario, Canada and in the Midwestern United States. The objective of this study was to evaluate early post-emergence (EPOST) herbicides for control of MHR waterhemp in field corn. Five field trials were conducted over a two-year period (2019, 2020) at sites on Walpole Island, ON and near Cottam, ON, Canada. Thirteen herbicide tank-mixtures containing multiple modes-of-action (MOA) were applied EPOST to 5 cm MHR waterhemp in field corn. Control of MHR waterhemp varied by site due to variable plant density, plant biomass, and number of herbicide-resistant individuals across research sites and years. Control of MHR waterhemp ranged from 90% to 100% with glyphosate + S-metolachlor/mesotrione/ bicyclopyrone/atrazine, glyphosate/2,4-D choline + rimsulfuron + mesotrione + atrazine, glyphosate + S-metolachlor/atrazine/mesotrione, glyphosate + mesotrione + atrazine, glyphosate/S-metolachlor/mesotrione + atrazine, glyphosate + S-metolachlor/mesotrione/bicyclopyrone, glyphosate/2,4-D choline + rimsulfuron + mesotrione, and glyphosate + pyroxasulfone + dicamba/atrazine at 4, 8, and 12 WAA. Control of MHR waterhemp ranged from 70% to 100% with glyphosate + topramezone/dimethenamid-P + dicamba/atrazine, glyphosate + isoxaflutole + atrazine, and glyphosate + tolpyralate + atrazine at 4, 8, and 12 WAA. Control of MHR waterhemp was similar for all herbicide programs, except glyphosate + dicamba/atrazine and glyphosate + S-metolachlor/atrazine which resulted in the lowest control at three of five sites that ranged from 63% to 89% and 61% to 76%, respectively. Crop injury was ≤10% for herbicide programs tested, except 28% to 31% corn injury with glyphosate/2,4-D choline + rimsulfuron + mesotrione + atrazine; however, without effect on corn grain yield. Corn yield was comparable with all herbicide programs evaluated in this study. It is concluded that there are herbicide programs that provide control of emerged and full-season residual control of MHR waterhemp in field corn.
Highlights
At 4, 8, and 12 weeks after herbicide application (WAA), control of MHR waterhemp ranged from 61% to 100% across sites and was lower at Site 1 (S1), S3, and S4 due to greater density and biomass compared to S2 and S5 (Table 1, Tables 3-5)
Control of MHR waterhemp was similar amongst all herbicide tank-mixtures, except glyphosate + S-metolachlor/atrazine and glyphosate + dicamba/atrazine which resulted in lower control than all other treatments, control ranged from 61% to 100% and 63% to 100%, respectively
Reductions in MHR waterhemp control with these herbicides resulted in greater MHR waterhemp density and biomass 4 WAA at three of five sites
Summary
Reductions in tillage, greater reliance on herbicides for weed management, and the evolution of resistance to multiple herbicide modes of action (MOA) have contributed to the rapid increase of waterhemp in agricultural cropping systems [1] [2]. More recent reports from Ontario have identified multiple-herbicide-resistant (MHR) waterhemp populations resistant to ALS-, PS II-, EPSPS-, and PPO-inhibiting herbicides. Waterhemp continues to evolve resistance to currently used MOA and is the first weed species to develop resistance to 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides [4] [7] [8] [9]. The first MHR waterhemp population with six-way resistance to synthetic auxins and ALS-, PS II-, EPSPS-, PPO-, and HPPD-inhibiting herbicides was identified in Missouri in 2015 [10]. The ability of MHR waterhemp to rapidly evolve and accumulate traits that confer resistance to multiple MOA makes it difficult to manage in agricultural cropping systems [12]
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