Considerations for Palmer Amaranth Control in Soybeans

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu and Kurt M. Vollmer, Postdoctoral Researcher, University of Delaware; kvollmer@udel.edu

Control of Palmer amaranth in soybeans requires the right product, at the right rate, at the right time. Residual herbicides are important to slow the growth of Palmer amaranth and provide a wider application window. Since soybeans are planted later than corn, and soybeans are trying to get established when daily temperatures are consistently above 85°F, the Palmer amaranth is growing rapidly. Using a preemergence herbicide applied at planting is important. Our research has shown consistently better control with applications made at planting compared to applications two weeks prior to planting. An herbicide combination has also provided better control than a single active ingredient, and helps reduce the risk of developing resistance to additional herbicide modes of action. Tankmixing Valor or Authority products with metribuzin or a Group 15 herbicide provided more consistent control than either product alone. Remember most of the Palmer amaranth in this region is also resistant to Group 2 herbicides, so those products will not help with Palmer amaranth control (but may improve control of other species).

Using an active ingredient from 2 of the 3 groups will provide better Palmer amaranth control than a single active ingredient.

*Anthem and Anthem Maxx are prepackaged mixtures, but the other components do not provide any residual weed control

**Valor should not be tankmixed with Dual or Warrant when used at planting

Partial list of prepackaged mixtures with two effective herbicides for Palmer amaranth:

• Authority Elite / BroadAxe: sulfentrazone plus s-metolachlor
• Authority MTZ: sulfentrazone plus metribuzin
• Fierce: Valor plus Zidua
• Trivence: Valor plus metribuzin plus chlorimuron

Other prepackaged mixtures may contain more than one of the active ingredients in the table above, but the mixtures may not provide the right rate. Refer to table 4-2 in the Mid-Atlantic Field Crop Weed Management Guide for more information on the components of premixes.

Use the full-labeled rates; but as noted, be sure that a prepackaged mixture provides the right ratios to allow the key herbicide active ingredients to be at the right rate. Reducing herbicide rates will not provide as long of residual control, and using reduced rates could be setting yourself up for developing herbicide resistance due to the weed increasing its ability to metabolize the herbicide (deactivate the herbicide with plant enzymes).

Be prepared to spray your postemergence herbicides in a timely fashion. I like to see Palmer amaranth sprayed by 3 inches in height. It is at this height for a very short time, so be prepared to treat. Research at UD and with Ben Beale in Maryland, clearly shows that Palmer amaranth allowed to grow for more than 4 weeks, even if a preemergence herbicide was used, are often greater than 5 inches tall. If you are not on a routine scouting schedule, then plan on spraying your postemergence herbicide by 4 weeks after the preemergence herbicide was applied. In addition, your postemergence treatment needs to include an herbicide that will provide effective residual control; and this includes Reflex, Dual, Zidua, or Anthem Maxx. Liberty or dicamba products will not provide residual control for Palmer amaranth.

For more information on the right product and right rate, use the Mid-Atlantic Field Crop Weed Management Guide (http://extension.udel.edu/ag/weed-science/weed-management-guides/)