To kill Palmer amaranth that first summer, Steinkamp says, “We got out the hoes and chopped them.” Then he worked with a crop consultant to plan a new strategy.

“Palmer amaranth has to be managed in an integrated system,” says Aaron Hager, University of Illinois weed scientist. “You can’t just throw one herbicide at it.” The key elements for control are:

  • Plant into a clean seedbed
  • Use full rates of a preemergence residual herbicide that kills Palmer amaranth
  • Apply timely postemergence herbicide before Palmer pigweeds are 3 inches tall
  • Tank mix another residual herbicide with application to extend control
  • Come back with a second postemerge application if needed
  • Remove surviving plants by hand or mechanically

“The key to management of Palmer amaranth is to control it at its most vulnerable stage — germination,” says Travis Legleiter, Purdue University weed specialist. “The use of preemerge residual herbicides is critical in both corn and soybeans.” 

Steinkamp was already using residual herbicides in corn, where there are quite a few options available for controlling Palmer pigweeds. In heavily infested fields, growing corn for several years is recommended to lower populations, Legleiter says.

preemerge and postemerge options to control Palmer amaranth in soybeans

In soybeans, options for control are limited. Steinkamp’s previous weed program relied on tillage or a burndown of glyphosate plus 2,4-D, followed by one post-emergence glyphosate application. That program was working fairly well before glyphosate-resistant Palmer showed up, he says, although “we were worried about developing waterhemp resistance, but hadn’t taken any action yet.”

Steinkamp’s new plan starts, as usual, with clean fields. Immediately after planting, he sprays a soil-applied residual herbicide on both corn and soybeans. On conventionally tilled soybeans, he’s had good results with Authority XL. His backup plan in case of weather delays is an early postemergence tankmix of Warrant and Roundup PowerMax.

That’s followed about four weeks later by a postemergence application of herbicide with overlapping residual activity. He likes Flexstar GT, which has both burndown and residual action.

Growers should be aware that herbicides may not completely control Palmer pigweeds, notes Mark Jeschke, DuPont Pioneer agronomy research manager. “Cultivation or hand weeding may be necessary to prevent escaped plants from producing seeds.”