Among the most problematic weeds, Palmer amaranth, or pigweed, is just one in a group of plants in the amaranth family, made up of nearly 75 species. As small-seeded, annual plants, amaranth species grow quickly in open areas with full sun and undisturbed soils, producing from tens of thousands to hundreds of thousands of seeds per plant, according to the Purdue University Extension. 

Palmer amaranth thrives in no-tillage systems because it needs to germinate close to the soil surface. The various amaranth species, including waterhemp and Palmer amaranth, can reduce crop production and even hinder harvesting efforts, due to their rapid growth, aggressive competition, prolific seed production and germination throughout the season, according to the University of Arkansas Division of Agriculture. A program approach with systematic timing is critical for controlling pigweed in soybeans, and you are advised against waiting for pigweeds to fully emerge before attempting to control these problem weeds.

Because pigweed species adapt quickly to new environments, some populations have acquired resistance to many herbicides, increasing the challenge of controlling glyphosate-resistant pigweed. According to researchers with the University of Arkansas, glyphosate-resistant Palmer amaranth can spread rapidly through almost any field if it’s able to get a foothold early in the season.

Palmer amaranth control in soybeans should begin with a preplant or pre-emergence residual herbicide. Good tillage to prepare the seedbed is also helpful, as is spraying Palmer amaranth when it is small  3 inches above the ground, not 3 inches above the crop, according to the University of Arkansas Cooperative Extension.

To reduce the spread of glyphosate-resistant pigweed and to protect Roundup Ready® and LibertyLink® technologies, you are advised to do everything possible to prevent germination, followed by hand removal of any remaining escapes to diminish the weed seed bank.

Syngenta solution

For effective Palmer amaranth control in soybeans, we offer BroadAxe®XC herbicide. BroadAxe XC controls Palmer amaranth and a full range of tough broadleaf and grass weeds through broad-spectrum, long-lasting residual control.

We also offer Boundary® soil-applied herbicide to deliver proven early-season grass and broadleaf control, excellent resistance management, and rotation flexibility. Containing two modes of action, Boundary can extend your post-emergence application window up to five weeks after planting, thereby protecting your yield and keeping your fields cleaner longer.

For full-season weed control in glyphosate-tolerant (GT) soybeans, Boundary and BroadAxe XC should be followed by a timely post-emergence application of Flexstar® GT 3.5 herbicide, which controls the same annual weeds as glyphosate, as well as several glyphosate- and ALS-resistant broadleaf weed biotypes, such as Palmer amaranth. Flexstar GT 3.5 contains Isolink® II Technology, a unique adjuvant system designed to optimize both the systematic activity of glyphosate and the contact activity of fomesafen.

For LibertyLink soybeans, a pre-emergence application of Prefix® herbicide followed by Liberty® plus Dual Magnum® herbicide will provide maximum weed control, while protecting the longevity of Liberty and sustaining a valuable weed management tool.

Boundary, BroadAxe XC, Dual Magnum, Flexstar GT 3.5 and Prefix eliminate weed competition, while helping to sustain both GT and LibertyLink soybean technology. Offering multiple different modes of action to control weeds, these brands, when combined with cultural practices such as crop rotation and tillage, help you reduce the weed seed soil bank, maximize diversity and fight herbicide resistance.

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