Three field experiments were conducted from 2013 to 2015 in Barry County, MI to evaluate the effectiveness of PRE, POST, and one- (EPOS) and two-pass (PRE followed by POST) herbicide programs for management of multiple-resistant Palmer amaranth in field corn. The Palmer amaranth population at this location has demonstrated resistance to glyphosate (Group 9), ALS-inhibiting herbicides (Group 2), and atrazine (Group 5). In the PRE only experiment, the only herbicide treatments that consistently provided ~80% or greater control were pyroxasulfone and the combination of mesotrione S-metolachlor. However, none of these treatments provided season-long Palmer amaranth control. Only topramezone provided >85% Palmer amaranth control 14 DAT, in the POST only experiment. Of the 19 herbicide programs studied all but three programs provided ≥88% Palmer amaranth control at corn harvest. Herbicide programs that did not control Palmer amaranth relied on only one effective herbicide site of action and in one case did not include a residual herbicide POST for late-season Palmer amaranth control. Some of the EPOS treatments were effective for season-long Palmer amaranth control; however, application timing and the inclusion of a residual herbicide component will be critical for controlling Palmer amaranth. The programs that consistently provided the highest levels of season-long Palmer amaranth control were PRE followed by POST herbicide programs that relied on a minimum of two effective herbicide sites of action and usually included a residual herbicide for late-season control.

Nomenclature: Atrazine; glyphosate; mesotrione; pyroxasulfone; S-metolachlor; Palmer amaranth, Amaranthus palmeri S. Wats.; corn, Zea mays L.