Palmer amaranth

Palmer amaranth

Herbicide resistance has become a major concern in the United States. At a series of Crop Management Roadshow workshops this winter, Nebraska Extension specialists have tried to provide ag producers with tools to deal with pesky weeds.

According to Sarah Sivits, cropping systems Extension educator, herbicide resistant weeds are found in 32 states. There are nine weeds that have been confirmed as herbicide resistant in Nebraska: common ragweed, common waterhemp, giant ragweed, marestail, Johnsongrass, kochia, Palmer amaranth, shattercane and redroot pigweed. Three — common water hemp, Palmer amaranth and kochia — are resistant to multiple herbicides. Of those three, Palmer amaranth is the most problematic agricultural weed in the U.S., Sivits said.

“It is difficult to control because of its long emergence pattern, high water use efficiency and aggressive growth patterns,” she noted. “It can grow as much as 2.5 inches per day. One female plant can produce 600,000 seeds. If not properly managed, yield loss as high as 79 percent in soybeans and up to 91 percent in corn may occur.”

To combat Palmer and other invasive weeds, Nebraska Extension recommends producers follow an Integrated Pest Management (IPM) plan for controlling these pests. The broad-based IPM strategies include: 1) identifying the pest; 2) evaluating the pest damage; 3) determining the need for control; 4) consideration of multiple control options; 5) selecting the best combination of control options; and 6) monitoring selection.

When producers use the same pesticide over and over, individual weeds will begin to carry genes resistant to that pesticide. Agronomists and Extension specialists are working through pesticide applicator trainings and courses such as the roadshow to train producers how to accurately identify the pests, use multiple modes of action when applying a pesticide, how to apply pesticides at the right time during the growing season, and utilizing crop rotation to allow the use of different herbicide groups.

Sivits noted that in cases where herbicides fail to control weeds, other cultural strategies need to be implemented. Options include hoeing, pulling by hand, and tillage.

If pulling plants, she stresses it is important to remove them from the field as they can re-root and produce viable seed if water is available. The bigger the plant, the more seeds it produces, and the harder it is to control.

Small-seeded broadleaf plants like Palmer amaranth need sunlight to stimulate germination. Narrow-row soybeans may help with early canopy closure, reducing sunlight availability to smaller weeds.

Tillage can be a viable option, Sivits said.

“Research has shown that burying seed four inches or more can reduce the seed bank 50-80 percent the first year and as much as 80-100 percent by the third year,” she added.

However, the researchers also stress not to use tillage every year, as this will only re-deposit seeds from the seedbank onto the soil surface. Another control option is to use a cereal cover crop, such as wheat or rye, which helps reduce light availability for seedling germination.

Spraying when weeds are small, less than three to four inches tall, provides better control, Sivits said.

“Pre-emergence herbicides are beneficial in reducing weed populations prior to canopy closure,” she said. “Only burndown followed by early post-applications have been failing in controlling Palmer amaranth.”

Continued research is showing management strategies need to change to burndown, pre-emergence plus a residual herbicide, early post-emergence plus a residual, and potentially a second post-emergence application.

For resources on how best to manage these and all problem weeds, producers in Nebraska should turn to the 2019 Guide for Weed, Disease and Insect Management in Nebraska (EC130), UNL’s

CropWatch website and this website:

Barb Bierman Batie can be reached at