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Posted 26 March 2015. PMN Crop News.

Six Tips for Managing Herbicide-Resistant Palmer Amaranth

Source: United Soybean Board Press Release.

Chesterfield, Missouri (March 10, 2015)--Palmer amaranth, a plant that can grow three inches in one day, is a problem for soybean farmers that literally grows each day. Combine that fast growth with the capacity to produce up to a million seeds per plant and you have a herbicide-resistant weed that can quickly make a devastating impact on soybean fields.


“Unfortunately, glyphosate-resistant biotypes of this weed are very common – it’s hard to find a population of Palmer that is not glyphosate resistant,” says Alan York, Ph.D., professor emeritus at North Carolina State University. “Growers fortunate enough to not currently have this problem need to do whatever they can to avoid letting it get established. For growers who already have the problem, we have effective programs to deal with it and to begin to clean up the fields.”

During a recent Focus on Soybean webinar, Management of Glyphosate-Resistant Palmer Amaranth in Soybeans, York suggests a variety of options to manage herbicide-resistant Palmer amaranth:

1. Minimize the seed bank – Keeping fields, ditches and turn rows clear of weeds that could go to seed is essential. Farmers cannot allow these weeds to go to seed, York says, even if that means pulling them by hand.

2. Diversify herbicides – Utilize a variety of herbicides with different sites of action, including residuals and tank-mixed post-emergence herbicides.

3. Use full rates of herbicides – Reduced rates can sometimes be less effective, allowing weeds to escape and develop resistance.

4. Be timely with post-emergence herbicides – Palmer amaranth grows rapidly, so there is a narrow window of opportunity to effectively treat the weed with post-emergence herbicides.

5. Maximize soybeans’ ability to compete – Plant soybeans in narrower rows with higher seeding rates in order to speed soybean canopy coverage to shade out weeds.

6. Utilize non-chemical controls – Plant cover crops after harvest and utilize tillage, if necessary.

Palmer amaranth is among the most dangerous weed threats in the U.S. and it is important to stay proactive and diverse with weed-management practices in order to keep Palmer at bay.

“Herbicide-resistant Palmer amaranth has been a game changer and has forced some significant changes in our soybean weed-management programs,” adds York. “We can no longer rely on glyphosate alone and must integrate other chemistries into our program. While resistant Palmer amaranth is a formidable pest and is one you want to avoid as long as possible, we can deal with it.”

To learn more about managing herbicide-resistant weeds, visit, a soy-checkoff-funded website that offers a variety of resources on weed management, including the weed identification guide.