Variable tolerance among Palmer amaranth (Amaranthus palmeri) biotypes to glyphosate, 2,4-D amine, and premix formulation of glyphosate plus 2,4-D choline (Enlist Duo®) herbicide

Authors: Spaunhorst, Douglas; JOHNSON, WILLIAM - Purdue University

Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/13/2017
Publication Date: 11/20/2017
Publication URL: http://handle.nal.usda.gov/10113/5922801
Citation: Spaunhorst, D.J., Johnson, W.G. 2017. Variable tolerance among Palmer amaranth (Amaranthus palmeri) biotypes to glyphosate, 2,4-D amine, and premix formulation of glyphosate plus 2,4-D choline (Enlist Duo®) herbicide. Weed Science. 65(6):787-797.

Interpretive Summary: Soybean are planted to 5.5 million acres in Indiana annually. Most of these acres are planted to soybean cultivars that possess traits that confer resistance to the herbicide glyphosate. Palmer amaranth is a weed that has been identified in Indiana soybean fields and has a tendency to evolve resistance to numerous herbicides used to control weeds in soybean. Preliminary greenhouse tests confirm most Indiana Palmer amaranth populations are glyphosate-resistant (GR) and some exhibit increased tolerance to 2,4-D amine. Dose response experiments were conducted to determine the level of glyphosate resistance and 2,4-D amine tolerance in four Palmer amaranth biotypes. A premix formulation of glyphosate plus 2,4-D choline (Enlist Duo®) was also evaluated for control of these four biotypes. Results showed that the R1, R2, and R3 biotypes were 31 to 66-fold more resistant to glyphosate (R:S ratio) than the S1 biotype based on the amount of herbicide needed to kill 90% of plants. The maximum postemergence rate of the premix formulation of Enlist Duo labeled in Enlist soybean is 2,195 g ae ha-1. When separated by active ingredient, the maximum postemergence rate of Enlist Duo is equivalent to 1,141 and 1,054 g ae ha-1 of glyphosate and 2,4-D choline, respectively. In the absence of glyphosate, the maximum rate of 2,4-D (1,054 g ae ha-1) in the premix formulation of Enlist Duo controlled S1, R2, and R3 biotypes, but did not control all plants from the R1 biotype. The amount of 2,4-D amine needed to kill 90% of plants from the R1 biotype was 3-fold more than the amount of 2,4-D needed to kill 90% of plants from the S1 biotype. However, no plants survived the 1,155 g ae ha-1 (595 and 560 g ae ha-1 of glyphosate plus 2,4-D, respectively) treatment of the premix formulation of glyphosate plus 2,4-D choline. Overall, results from this experiment suggest GR Palmer amaranth that are also tolerant to 2,4-D amine will be difficult to control with glyphosate or 2,4-D alone, but is controlled postemergence with Enlist Duo® at lower than labeled field rates (1,618 to 2,195 g ae ha-1).

Technical Abstract: Adoption of soybean that is resistant to 2,4-D will result in more use of glyphosate plus 2,4-D premixes and tank-mixtures. Preliminary whole-plant greenhouse assays confirm most Palmer amaranth found in Indiana are glyphosate-resistant (GR) and some biotypes exhibit tolerance to 2,4-D amine. Dose response experiments were conducted to determine the level of glyphosate resistance and 2,4-D amine tolerance in four Palmer amaranth biotypes. A premix formulation of glyphosate plus 2,4-D choline was also evaluated. The R1, R2, and R3 biotypes were 31 to 66-fold more resistant to glyphosate (R:S ratio) than the S1 biotype based on mortality90 values. The maximum postemergence rate of the premix formulation of Enlist Duo labeled in Enlist soybean is 2,195 g ae ha-1. When separated by active ingredient, the maximum postemergence rate of Enlist Duo is equivalent to 1,141 and 1,054 g ae ha-1 of glyphosate and 2,4-D choline, respectively. In the absence of glyphosate, the maximum rate of 2,4-D (1,054 g ae ha-1) in the premix formulation of Enlist Duo controlled S1, R2, and R3 biotypes, but failed to control all plants from the R1 biotype. Mortality90 estimates showed the R1 biotype was 3-fold more tolerant than the S1 biotype to 2,4-D amine. However, no plants survived the 1,155 g ae ha-1 (595 and 560 g ae ha-1 of glyphosate plus 2,4-D, respectively) treatment of the premix formulation of glyphosate plus 2,4-D choline. Overall, results from this experiment suggest GR Palmer amaranth that also exhibit increased tolerance to 2,4-D amine will be difficult to control with glyphosate or 2,4-D alone, but is controlled postemergence with Enlist Duo® at lower than labeled field rates (1,618 to 2,195 g ae ha-1).