Mark VanGessel, Extension Weed Specialist; mjv@udel.edu
Control of Palmer amaranth requires the right product, at the right rate, at the right time. Because Palmer has a prolonged germination period, it is important to make soil-applied herbicide applications at planting rather than weeks prior to planting. Research funded by the Delaware Soybean Board consistently shows better control with applications made at planting compared to applications two weeks prior.
Postemergence herbicides are most effective on Palmer amaranth plants less than 4 inches tall. In soybeans, since they are planted later than corn and soil temperatures are warmer, this stage occurs about 4 weeks after planting. For corn (or early-planted soybeans) this timeframe is longer than 4 weeks, but applications need to be made to small Palmer amaranth plants.
We assume that all Palmer amaranth plants in Delaware are resistant to glyphosate and the group 2 herbicides (i.e. Classic, Sandea, Raptor etc.) and so we don’t consider these viable control options. To help prevent additional resistance from developing, two effective modes of action need to be used. Furthermore, our research has more consistent control with two effective herbicides tank mixed together than either herbicide alone. For instance, Valor plus metribuzin is consistently better than either herbicide applied separately. In addition, research on the development of herbicide resistance, tank mixing two effective modes of action is better than spraying them at separate times.
Recommended soil-applied corn herbicides*:
- Acetochlor (Harness) (Group 15)
- S-metolachlor (Dual II Magnum) (Group 15)
- Pyroxasulfone (Zidua or Anthem products) (Group 15)
- Atrazine (Group 5)
- Princep (Group 5)
- Mesotrione (Callisto) (Group 27)
Recommended soil-applied herbicides for soybeans*
- S-metolachlor (Dual II Magnum) (Group 15)
- Pyroxasulfone (Zidua or Anthem products) (Group 15)
- Metribuzin (Group 5)
- Flumioxazin (Valor) (Group 14)
- Sulfentrazone (Authority products) (Group 14)
*Note there are a lot of pre-packaged herbicides that can combinations of these herbicides.
All of these herbicides will provide residual control for Palmer amaranth. On the sandy soils where I conduct many of my trials, pyroxasulfone will provide slightly longer residual control than the other group 15 herbicides.
Likewise, flumioxazin will provide slightly longer residual control than sulfentrazone products. Consider using pyroxasulfone or flumioxazin in fields with high Palmer amaranth seed banks or fields that are difficult to access in a timely fashion.
Fields with a history of Palmer amaranth (or in close proximity to fields with Palmer amaranth issues) should be scouted routinely for postemergence applications. In corn, postemergence control with Group 27 herbicides, which include Callisto/Halex GT (mesotrione), Impact/Armezon, Laudis (or Capreno), or Shieldex, are highly effective when combined with atrazine. Without atrazine, these products are much less effective and often do not kill the emerged Palmer amaranth plants. Dicamba-based products, such as Diflexx/Diflexx Duo or Status, will provide good control of plants up to 4-inches tall.
Nothing will control large Palmer amaranth plants. Diflexx and Status both contain safeners for improved corn safety. Liberty (or other glufosinate products) can also be effective provided there is adequate spray coverage (at least 15 gal/A and medium droplet size) and good sunlight. So, it’s important to scout the fields and treat early.
Postemergence in soybeans is best with a combination of Enlist plus glufosinate (Liberty). This combination outperforms either product applied by themselves. However, neither glufosinate nor Enlist will provide residual control, so adding acetochlor (Warrant), S-metolachlor (Dual Magnum), or pyroxasulfone (Zidua or Anthem products) is recommended. Another option is using Reflex with glyphosate as a postemergence application. Reflex not only controls emerged Palmer amaranth but also provides residual control.
Palmer amaranth plants in different regions of the United States have developed resistance to all of the herbicides mentioned above. It is critical to slow the spread of herbicide-resistant Palmer amaranth in our region and prevent the development of selecting new herbicide-resistant biotypes.
Cover crops can aid in Palmer amaranth control but are not sufficient on their own to provide an acceptable level of control. However, they can reduce the number of emerging Palmer amaranth plants and slow their growth rate, extending the application window for postemergence herbicides. By suppressing Palmer amaranth growth, cover crops play a vital role in reducing the risk of new herbicide-resistant biotypes.