Wisconsin Public Radio interviewed Vince Davis about the recent confirmations of herbicide-resistant weeds in Wisconsin. To read or listen to the interview, please visit the WPR website.
Thomas R. Butts and Vince M. Davis
Department of Agronomy, University of Wisconsin-Madison
Common waterhemp (Amaranthus rudis) is a dioecious, small seeded, broadleaf weed species native to North America, specifically common in the Midwest region of the United States. This weed species has become increasingly problematic for corn and soybean growers due to its prolific growth characteristics and highly competitive ability. Among its fellow pigweed (Amaranthaceae) family members, common waterhemp is second only to Palmer amaranth (Amaranthus palmeri) in growth rate and size reaching heights of nearly ten feet 4. Furthermore, common waterhemp can produce over one million seeds per female plant under ideal growing conditions 8. This intensifies the likelihood and speed that herbicide-resistant biotypes can increase in a population and transfer from one location to another through seed dispersal. If common waterhemp is left unmanaged in corn and soybean, growers can see yield reductions of 74 and 56%, respectively2,7.
Control of common waterhemp has become increasingly difficult due to its ability of evolving resistance to numerous herbicide sites-of-action. To date, this weed species has been identified as resistant to six different sites-of-action, including an ALS-resistant biotype located in Wisconsin. Several common waterhemp populations have also evolved resistance to multiple herbicide sites-of-action, further complicating control methods1,5. Glyphosate-resistant common waterhemp biotypes have already been confirmed in fifteen other states including nearby Illinois, Indiana, Iowa, and Minnesota3. Our current research reported here suggests we will add Wisconsin to this list as data from our first greenhouse experiment indicates at least two Wisconsin common waterhemp populations are resistant to glyphosate out of 14 populations examined.
The two weed populations examined were collected from crop production fields in Eau Claire and Pierce counties. They were identified through the Late-Season Weed Escape Survey in Wisconsin Corn and Soybean Fields conducted in 2012 and 2013 by former graduate research assistant, Ross A. Recker. Plants that were collected in the field were likely to have survived a postemergence glyphosate application based on in-field observations of herbicide symptomology, plant locations, personal communication with growers, and other additional data documented during the survey. To confirm glyphosate resistance, seed was collected from 30 mature plants in the field, progeny were grown in the UW-Madison greenhouse, and 10 plants per glyphosate rate were sprayed with Roundup PowerMAX® plus ammonium sulfate at 17 lbs. per 100 gallons of spray solution when they reached three inches tall. Glyphosate rates used were 0, 0.22 (5.5), 0.43 (11), 0.87 (22), 1.74 (44), and 3.48 (88) kg ae ha-1 (fl. oz. ac-1). Plant dry biomass data were collected 28 days after application (DAA). Comparisons between our putative resistant and susceptible biotypes were determined by the effective glyphosate dose needed to reduce plant dry biomass 50% (ED50).
The ten Pierce County plants sprayed at the 0.87 kg ae ha-1 (22 fl. oz. ac-1) rate all survived and grew to an average of three times their spray date height (Figure 1). At the 1.74 kg ae ha-1 (44 fl. oz. ac-1) rate, nine of ten plants survived and grew to an average of two times their spray date height (Figure 2). The ED50 of glyphosate for the Pierce County and susceptible populations was 2.23 and 0.18 kg ae ha-1, respectively (Figure 3). This indicates the Pierce County population has a 12.5-fold level of resistance.
The ten Eau Claire County plants sprayed at the 0.87 kg ae ha-1 (22 fl. oz. ac-1) rate all survived and grew to an average of five times greater than their spray date height (Figure 4). All ten plants also survived the 1.74 kg ae ha-1 (44 fl. oz. ac-1) rate and quadrupled in size from their spray date height (Figure 5). The Eau Claire County population was not able to be analyzed using the log logistic Dose Response Model in R due to inadequate high rates of glyphosate to reduce dry biomass at 28 DAA. Therefore, linear glyphosate response models were established for the Eau Claire County and susceptible populations and analyzed using ANOVA tables which indicated significant differences at all glyphosate rates (Figure 6) (Table 1).
There are several key components to an effective control strategy for glyphosate-resistant common waterhemp. The use of alternative herbicide sites-of-action, such as PPO inhibitors, and tank-mixing multiple herbicide sites-of-action will improve glyphosate-resistant weed control. An early planting date will allow crops to gain a head-start and outcompete common waterhemp due to its late emergence timing6. Herbicide applications should be made at the correct timing when weeds are small and actively growing to ensure the greatest efficacy of the herbicide based on label recommendations. Furthermore, special care should be taken to clean tillage and harvest equipment thoroughly as they can quickly spread weed seed among fields. The focus of these best management practices is to diversify weed control measures, reduce weed seed additions to the soil seedbank, and utilize control measures in the most effective method possible.
This research experiment will be repeated to officially confirm glyphosate resistance in these common waterhemp populations. For updates on Wisconsin weeds please visit our Wisconsin Crop Weed Science website at http://wcws.cals.wisc.edu/. Further information on controlling common waterhemp or other glyphosate-resistant weeds can be found at: http://takeactiononweeds.com/. Finally, if you believe you may be facing glyphosate-resistant weeds in your fields, contact your local county extension agent and/or Dr. Vince Davis at firstname.lastname@example.org or (608) 262-1392.
- Bell MS, Hager AG, Tranel PJ (2013) Multiple Resistance to Herbicides from Four Site-of-Action Groups in Waterhemp (Amaranthus tuberculatus). Weed Science 61:460-468
- Bensch CN, Horak MJ, Peterson D (2003) Interference of redroot pigweed (Amaranthus retroflexus), Palmer amaranth (A. palmeri), and common waterhemp (A. rudis) in soybean. Weed Science 51:37-43
- Heap I (2013) The International Survey of Herbicide Resistant Weeds: Web page. http://www.weedscience.com/Summary/home.aspx. Accessed April 01, 2013
- Horak MJ, Loughin TM (2000) Growth Analysis of Four Amaranthus Species. Weed Science 48:347-355
- McMullan PM, Green JM (2011) Identification of a Tall Waterhemp (Amaranthus tuberculatus) Biotype Resistant to HPPD-Inhibiting Herbicides, Atrazine, and Thifensulfuron in Iowa. Weed Technology 25:514-518
- Sellers BA, Smeda RJ, Johnson WG, Kendig JA, Ellersieck MR (2003) Comparative Growth of Six Amaranthus Species in Missouri. Weed Science 51:329-333
- Steckel LE, Sprague CL (2004) Common waterhemp (Amaranthus rudis) interference in corn. Weed Science 52:359-364
- Steckel LE, Sprague CL, Hager AG, Simmons FW, Bollero GA (2003) Effects of shading on common waterhemp (Amaranthus rudis) growth and development. Weed Science 51:898-903
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 Iowa2,3, Illinois4, Indiana5, and Michigan6,7,8. 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 (email@example.com) or (608) 262-1392.
- Ward, S.M., T. M. Webster, L. E. Steckel. 2013. Palmer Amaranth (Amaranthus palmeri): A review. Weed Tech. 27:12-27.
- Hartzler, B. and M. Owen. 2013. Troublesome Palmer amaranth expanding its range. Iowa State University, Integrated Crop Management News: article is available here
- Hartzler, B. 2013. Palmer amaranth update. Iowa State University, Integrated Crop Management News: article is available here
- Hager, A. 2013. Update on Palmer amaranth distribution in Illinois. University of Illinois, The Bulletin: article is available here
- Johnson, B. and T. Legleiter. 2013. Palmer amaranth confirmed in 17 Indiana counties. Purdue Agriculture News. Available at: Purdue University
- 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
- 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
- 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
- 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.
- 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.
- Massinga, R.A., R.S. Currie, M.J. Horak, and J. Boyer. 2001. Interference of Palmer amaranth in corn. Weed Sci. 49:202-208