Kurt Vollmer, Weed Management Specialist, University of Maryland Extension
and Mark VanGessel, Weed Management Specialist, University of Delaware
Palmer amaranth continues to be a major concern for farmers on the Eastern Shore. Many populations are already known to be resistant to glyphosate (Roundup) and Group 2 herbicides (e.g., Sandea, Raptor). Additionally, in North Carolina, some populations have developed resistance not only to glyphosate and Group 2 herbicides but also to Liberty and Callisto.
To investigate if there is a shift in Palmer amaranth response to commonly used herbicides in our region, we collected seed samples from 17 sites across the Eastern Shore. Most of these fields had been planted with soybeans, and seeds were collected in September. After cleaning, the seeds were stored in a refrigerator for three months to improve germination.
We evaluated each population with four herbicides all applied postemergence: Callisto, Reflex, Enlist One (2,4-D), and Liberty. At least five individual plants from each population were treated with each herbicide.
All the populations were susceptible to Enlist One and Liberty at the normal use rate (1 qt/A for each). However, nine of the populations had at least one plant survive when treated with Callisto, and five populations survived a Reflex application.
We are partnering with Penn State University to conduct further analysis to determine if we are seeing Palmer amaranth resistance developing to Callisto or Reflex in our region.
It is important that Palmer amaranth infested fields get treated with two effective modes of action, applied at full rates. Research has demonstrated that tankmixing two effective modes of action is better for managing resistance than using them in sequence. And do not rely on the same modes of action year after year. Scout fields shortly after application to determine if control was adequate, and if it was not, follow up to avoid Palmer amaranth “escapes” from going to seed.
Review the article from the April 4, 2025 Weekly Crop Update for additional considerations on Palmer amaranth control.
