Palmer amaranth: Why this pigweed should alarm you

Southwest Michigan growers need to look for Palmer amaranth weed escapes in commercial corn and seed corn.

Palmer amaranth, a pigweed species native to the southwestern United States where it has plagued soybean and cotton production areas, was first identified in southwest Michigan in 2010. Michigan State University weed control specialist Christy Sprague and Ph.D. student David Powell have conducted research on controlling this weed in soybeans over the last two years in St. Joseph County. Their research confirmed that the Palmer amaranth populations found in southwest Michigan were resistant to both glyphosate and ALS-inhibiting herbicides. Research conducted in 2012 in soybeans suggests programs that include the use of Liberty Link soybeans in conjunction with an effective preemergence herbicide provided the best control. However, even with the best of these programs, maintaining season-long control levels greater than 85 percent were difficult to obtain. Why?

Palmer amaranth is capable of producing up to 400,000 seeds per plant and can germinate throughout the growing season. Also, there are very few post-emergence herbicide options available for control and they work best when the post-herbicide is applied when Palmer amaranth is 3 inches tall or less. All of these factors make it very difficult to use herbicide programs alone to keep Palmer amaranth infestations from spreading. Production areas in the south rely heavily on subsequent hand-weeding to help keep Palmer amaranth populations in check. That is a very sobering thought.

The relatively good news is that our traditional corn herbicide programs seem to have been keeping Palmer amaranth populations in check in nearly all fields in our area. Note that pesky word “nearly.” While no breakout populations have been found in commercial or seed corn fields, we have been able to find a few individual Palmer amaranth plants in seed corn production fields. Now it is likely that these escapes have been caused by herbicide coverage issues or possibly emergence after the last post-herbicide application. However, there is grave reason for concern.

There are populations of Palmer amaranth that exist that exhibit resistance to the Photosystem II inhibitors (atrazine) and the HPPD-inhibiting herbicides (members of this herbicide family include mesotrione, topramezone, tembotrione and others) that are currently thought to be keeping Palmer amaranth in check in the corn side of the rotation in southwest Michigan. The Weed Science Society of America (WSSA) reported on their herbicide-resistant weeds website that different populations of Palmer amaranth in Kansas were determined to be resistant to these classes of herbicides. Atrazine-resistant Palmer amaranth has also been reported in several states.

University of Nebraska weed control specialist Robert Wilson suggested in a recent article that the movement of Palmer amaranth by equipment, especially combines, or contaminant of seed or livestock feed is possible. They tracked the movement of Palmer amaranth fed through contaminated cottonseed cake and deposited in cattle manure. Populations of Palmer amaranth in Nebraska are also reported to be triazine-resistant.

While there is no immediate reason for panic, this does mean that growers in southwest Michigan really need to be diligent in looking for individual plants or patches of Palmer amaranth growing in corn fields where preemergence herbicides containing atrazine (most of the preemergence herbicide programs) were applied. The same is true where Lumax or Lexar were applied. Growers should be prepared to remove all of the female Palmer amaranth plants found in these fields before they go to seed.

Early identification remains the key to slow down the spread of Palmer amaranth. Look for pigweed escapes that have longer petioles (leaf stems), mostly lack hairiness on the stems and leaves, and typically have “spikes” on the leaf tips of younger plants. Sprague has an excellent publication to help you identify this weed species titled “Palmer amaranth in Michigan: Keys to Identification.” For the most up-to-date information on this and other weed control issues, visit the MSU Weed Science website.

If you find populations of Palmer amaranth on your Michigan farm, give us a call at either Michigan State University Extension’s St. Joseph Count office at 269-467-5511, or the Van Buren County office at 269-657-8213 and we will record the location and severity of the infestation and pass this information along to Sprague. We can also help you to identify the female plants for removal in an initial infestation to reduce the spread of this incredibly hard to control weed.

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