Vince Davis (Assistant Professor) Department of Agronomy, UW-Madison; Ross Recker (Graduate Research Assistant)

Palmer amaranth (Amaranthus palmeri) is a dioecious, summer annual broadleaf weed species in the pigweed (Amaranthaceae) family that is extremely adaptable to environments, including the development of herbicide resistance, and it is extremely competitive with row crops1. Palmer amaranth has been tormenting cotton and soybean producers in the southeast United States for the past decade, and more recently Palmer amaranth has been moving its way north into states such as Iowa^2,3, Illinois⁴, Indiana⁵, and Michigan^6,7,8. This northward movement of Palmer amaranth is alarming, and the movement has often been attributed to spreading contaminated manure from animal production operations that have fed cottonseed feed by-products transported from Southern U.S. production fields, as well as equipment movement, and contaminated seed for Prairie restorations.

Palmer amaranth is not native to Wisconsin. A population was identified in Dane County, WI through the 2013 late-season weed escape survey efforts partially funded by the Wisconsin Corn Promotion Board. During this survey in fall 2013, five plants were distantly distributed in a large soybean field. Four of those plants were male plants (Figure 1), and luckily only one plant was a female plant was present and produced minimal seed in comparison to the seed production they can potentially produce (Figure 2).

Figure 1. Male Palmer amaranth plant from Dane County, WI

Figure 2. Female Palmer amaranth plant from Dane County, WI

Plant tissue from all five plants was sent to the Dr. Pat Tranel at the University of Illinois. Dr. Tranel’s lab conducted molecular techniques to confirm that these plants were in fact Palmer amaranth as well as quantify the number of copies of the EPSPS gene. All five plants were confirmed as Palmer amaranth, and subsequently they produced EPSPS gene amplification ranging from 3-fold to >20-fold. EPSPS gene amplification within those ranges has previously demonstrated to be an effective mechanism for evolved glyphosate resistance in Palmer amaranth9. Whole-plant dose response experiments will be conducted to further confirm if this plant population is in fact resistant to glyphosate, but the molecular findings are at this point a strong indication that it is likely resistant. The origin of how this population established in Dane county, like many others, is difficult to pinpoint, but these plants were found in a field with a history of dairy manure application.

Because this is so far only one confirmed location of questionably resistant Palmer amaranth with minimal plants at this location, this does not necessarily represent a wide-spread catastrophe. However, this does provide further indication that the threat of herbicide-resistant pigweeds in Wisconsin crop production is real. The best approach is to be aware of this threat and implement a robust Integrated Pest Management approach, if you’re not already doing so. This approach should start with intently scouting fields and identifying weeds this spring prior to preplant control. Utilize diverse preplant control methods to ensure starting with a clean field at planting, but make sure scouting and proper identification is done prior to postemergence herbicide applications. Consider interrow cultivation and tank-mix herbicides that provide a second effective mode-of-action for key weed species that need controlled in-crop. Most importantly, intently scout following postemergence applications to look for weeds that were not controlled. Those are the weeds that pose the biggest threat to building a population with herbicide resistance. If any of these scouting trips indicates a pigweed species is one of the main target weeds, then make certain you know what type of pigweed species it is. Redroot pigweed, smooth pigweed, Powell amaranth, and waterhemp are all common pigweed species, however, waterhemp poses the greatest risk of herbicide resistance from that list. As already mentioned Palmer amaranth is not a native, or common, pigweed species, but it poses a significant risk. If Palmer amaranth is identified, then its presence should invoke a “zero tolerance” mindset with eradication as the goal where feasible.

Characteristics of Palmer amaranth include rapid growth rate, high seed production, high degree of genetic diversity, high water use efficiency, and rapid development of herbicide resistance. Palmer amaranth competition with crops has demonstrated yield losses as high as 78% (soybean) and 91% (corn)10,11. Therefore, Palmer amaranth should be of high concern for producers across the state. Below are links to help with the identification and management of Palmer amaranth. Key identification points are: the stems lack hair (like waterhemp, but different than Powell amaranth, redroot pigweed, and smooth pigweed), petioles are often longer than the leaf blade (Figure 3), a long terminal seed head, and seed heads of female plants are very prickly and painful to grab with a bare hand.

Figure 3. Comparison of Palmer amaranth petiole length to leaf blade length

Because this weed has caused such a problem in our surrounding states over the recent couple of years, there are many helpful extension articles we have referenced in this article. Specifically, here are some helpful guides for identification:

• Identification of the weedy pigweeds and waterhemps of Iowa. D.B. Pratt, M.D.K. Owen, L.G. Clark, and A. Gardner. 1999. Available at: Iowa State University Extension
• Guidelines for the identification and management of Palmer Amaranth in Illinois Agronomic Crops. A. Hager; available at: University of Illinois IPM Bulletin
• Palmer amaranth biology, identification, and management. T. Legleiter and B. Johnson. Available at: Purdue University Extension
• Identifying Palmer amaranth in the field — Video. B. Johnson and T. Legleiter. YouTube video
• Palmer amaranth in Michigan, Keys to Identification. C. Sprague, C. Michigan State University Weed Science. Available at: Michigan State University

If you or your crop scout has utilized the identification guides and believe you have Palmer amaranth escapes in your fields, please contact your local county extension agent and/or Dr. Vince Davis (vmdavis@wisc.edu) or (608) 262-1392.

References:

1. Ward, S.M., T. M. Webster, L. E. Steckel. 2013. Palmer Amaranth (Amaranthus palmeri): A review. Weed Tech. 27:12-27.
2. Hartzler, B. and M. Owen. 2013. Troublesome Palmer amaranth expanding its range. Iowa State University, Integrated Crop Management News: article is available here
3. Hartzler, B. 2013. Palmer amaranth update. Iowa State University, Integrated Crop Management News: article is available here
4. Hager, A. 2013. Update on Palmer amaranth distribution in Illinois. University of Illinois, The Bulletin: article is available here
5. Johnson, B. and T. Legleiter. 2013. Palmer amaranth confirmed in 17 Indiana counties. Purdue Agriculture News. Available at: Purdue University
6. Sprague, C. 2012. Palmer amaranth found in more Michigan fields: Now is a good time to scout. Michigan State University Extension News. Available at: Michigan State University
7. Sprague, C. 2013. Palmer amaranth: Why this weed should alarm you. Michigan State University Extension News. Available at: http://msue.anr.msu.edu/news/palmer_amaranth_why_this_pigweed_should_alarm_you
8. Sprague, C. Palmer amaranth: A new invasive weed to watch for in Michigan. Available at: http://www.michigansoybean.org/Media/MichiganSoybean/Downloads/MSPC/Palmer-Amaranth/Palmer-Amaranth-A-New-Invasive-Weed-To-Watch-For-In-MI.pdf
9. Gaines, T.A., W. Zhang, D. Wang, B. Bukun, S.T. Chisholm, D.L. Shaner, S.J. Nissen, W.L. Patzoldt, P.J. Tranel, A.S. Culpepper, T.L. Grey, T.M. Webster, W.K. Vencill, R.D. Sammons, J. Jiang, C. Preston, J.E. Leach, and P. Westra. 2010. Gene amplification confers glyphosate resistance in Amaranthus palmeri. PNAS 107:1029-1034.
10. Bensch, C.N., M.J. Horak, and D. Peterson. 2003. Inference of redroot pigweed (Amaranthus retroflexus), Palmer amaranth (A. palmeri), and common waterhemp (A. rudis) in soybean. Weed Sci. 51:37-43.
11. Massinga, R.A., R.S. Currie, M.J. Horak, and J. Boyer. 2001. Interference of Palmer amaranth in corn. Weed Sci. 49:202-208

This article was posted in WCWS and tagged Agriculture, Amaranthus, integrated pest management, IPM, Palmer amaranth, pigweed, research, Weed, Wisconsin.