A prepackaged mixture of desmedipham + phenmedipham was previously labeled for control of Amaranthus spp. in sugarbeet. Currently, there are no effective POST herbicide options to control glyphosate-resistant Palmer amaranth in sugarbeet. Sugarbeet growers are interested in using desmedipham + phenmedipham to control escaped Palmer amaranth. In 2019, a greenhouse experiment was initiated near Scottsbluff, NE, to determine the selectivity of desmedipham and phenmedipham between Palmer amaranth and sugarbeet. Three populations of Palmer amaranth and four sugarbeet hybrids were evaluated. Herbicide treatments consisted of desmedipham and phenmedipham applied singly or as mixtures at an equivalent rate. Herbicides were applied when Palmer amaranth and sugarbeet were at the cotyledon stage, or two true-leaf sugarbeet stage and when Palmer amaranth was 7 cm tall. The selectivity indices for desmedipham, phenmedipham, and desmedipham + phenmedipham were 1.61, 2.47, and 3.05, respectively, at the cotyledon stage. At the two true-leaf application stage, the highest rates of desmedipham and phenmedipham were associated with low mortality rates in sugarbeet, resulting in a failed response of death. The highest rates of desmedipham + phenmedipham caused a death response of sugarbeet; the selectivity index was 2.15. Desmedipham treatments resulted in lower LD50 estimates for Palmer amaranth compared to phenmedipham, indicating that desmedipham can provide greater levels of control for Palmer amaranth. However, desmedipham also caused greater injury in sugarbeet, producing lower LD50 estimates compared to phenmedipham. Desmedipham + phenmedipham provided 90% or greater control of cotyledon-size Palmer amaranth at a labeled rate but also caused high levels of sugarbeet injury. Neither desmedipham, phenmedipham, nor desmedipham + phenmedipham was able to control 7-cm tall Palmer amaranth at previously labeled rates. Results indicate that desmedipham + phenmedipham can only control Palmer amaranth if applied at the cotyledon stage and a high level of sugarbeet injury is acceptable.

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 populations 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 the herbicide dose to cause 90% mortality (LD90). The maximum POST 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 POST 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. Estimates for LD90 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 (600 g ae ha−1 of glyphosate plus 555 g ae ha−1 2,4-D) treatment with the premix formulation of glyphosate plus 2,4-D choline. Overall, results from this experiment suggest GR Palmer amaranth that also exhibits increased tolerance to 2,4-D amine will be difficult to control with glyphosate or 2,4-D alone, but can be controlled POST with Enlist Duo® at lower than labeled field rates (1,618 to 2,195 g ae ha−1).