Glyphosate Resistant Palmer Amaranth: What Do We Do Now?

Every farming season brings us new challenges. The latest being glyphosate-resistant (GR) Palmer amaranth, a new type of pigweed in Idaho.

The ramifications of identifying GR Palmer amaranth have the potential to alter current cropping systems in Idaho. If left unaddressed, it could change the way we grow crops in the future.

What are the characteristics of Glyphosate-resistant Palmer amaranth?

Palmer amaranth was first found to be glyphosate-resistant (GR) in 2005 in Georgia. Since then, it has spread to nearly 30 states, including Idaho. 

This weed is native to the southwestern U.S. and can grow up to 7 feet tall. It has oval leaves, long petioles, and a green or reddish stem with a possible white V-shaped mark. Palmer amaranth has both male and female plants. Flowers are located on separate plants and are often long, narrow and complex. They can have singular or multiple seedheads, and seedheads usually drop on older plants. Female plants can be identified by sharp bracts on the terminal inflorescence.

Palmer amaranth is a prolific seed producer, with a single female plant capable of producing up to 600,000 viable seeds without competition.

Compared with waterhemp, redroot pigweed, and spiny amaranth - Palmer amaranth has the largest plant volume and leaf area. Overtime, it has developed resistance to several herbicide modes of action, including acetolactate synthesis inhibitors (ALS), EPSP synthase inhibitors, 4-hydroxyphenylpyruvate dioxygenase inhibitors (HPPD), auxin mimic, and photosystem II inhibitors (PSII). 

Why is GR Palmer amaranth an issue?

In recent years, Palmer amaranth has become one of the most problematic weeds for farmers. Palmer amaranth is mainly troublesome due to its competition with crops for light, water, nutrients and space. Additionally, it can tolerate extremely adverse conditions. Because of these factors, many leading chemical companies are developing their new herbicide technologies utilizing GR Palmer amaranth as the driver weed.

What control options are available for GR Palmer amaranth? 

Although GR Palmer amaranth is a new and invasive species to Idaho, many of the current residual control options in potatoes and sugarbeets are staples in controlling GR Palmer amaranth across the country. These include pendimethalin, metribuzin, s-metolachlor and pyroxasulfone. However, postemergence GR Palmer amaranth control options in these crops are very limited.

This means an aggressive residual herbicide program containing multiple herbicide modes of action (MOA) will be paramount in controlling GR Palmer amaranth.

Is GR Palmer Amaranth preventable?

Research and field applications commonly suggest that the most effective and economical techniques for GR Palmer amaranth control are preventative, such as the use of soil residual herbicides. Incorporation of pre-herbicides followed by post-application herbicide systems should be utilized when managing GR Palmer amaranth.

Chemical means of control are the most effective; however, a total management program consisting of mechanical, cultural, chemical and physical control options is necessary to control Palmer amaranth.

Scouting, crop and herbicide MOA rotation, and other cultural practices are all paramount in a total program approach to control GR Palmer amaranth. Additionally, due to the weed's prolific nature, it's highly recommended to sanitize equipment thoroughly from farm to farm to minimize spread. 

Be proactive, not reactive

Looking at the southern U.S., history has shown that achieving 95% control is insufficient for effectively managing Palmer amaranth. There is no silver bullet answer; however, a total management program is required for effective pest control and containment.

Keep in mind that no matter how well a GR Palmer amaranth management program is on a particular farm, infestations can occur from several sources including neighboring property, ditchbank and equipment transportation. If you think it's resistant, it probably is. Be proactive with GR Palmer amaranth, not reactive.

The Takeaway

In instances where resistance is suspected, cases should be reported to the local county extension center and controlled immediately. If you have any questions about developing a plan or managing GR Palmer amaranth, please consult your Agronomy Sales Enablement Team or your local Simplot Grower Solutions Crop Advisor.

Author

Ben Lawrence, Bayou

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