Pigweed Identification

Weed identification at the seedling and immature stages can be difficult but is often necessary because scouting should occur before weeds reach 4 inches in height. At emergence before a full set of true leaves appear, pigweeds can be confused with other weed species such as wild buckwheat, eastern black nightshade, and ladysthumb. In addition, the pigweeds: Palmer amaranth, waterhemp, redroot pigweed, Powell amaranth, and smooth pigweed, are not easily separated by species at the immature stages. The first step is to look at the leaves and stems because Palmer amaranth and waterhemp do not have any hairs compared to Powell, redroot, and smooth pigweed, which do have hairs but they may not be obvious at the immature stage. If the plant looks like it may be Palmer amaranth or waterhemp, then the next step is to look at the leaf shape and petiole. Palmer amaranth has a more rounded leaf shape and a petiole that is longer than the leaf itself.

For a list of resources available by species, guides for the most common weed seedlings, and links to the WeedID smartphone apps, take a look at the Weed Info page. There are several Extension resources available to help with pigweed identification including:

  • “Palmer amaranth seedling identification,” Purdue University, 7.5 minute video

The videos provide a nice introduction to identification, particularly, if using the printed guides. Pigweeds present a tough set of management challenges, for instance, very high growth rates, extended emergence over most of the growing season, and high seed production. The United Soybean Board’s TakeAction website has publications on the management of Palmer amaranth and waterhemp. Populations of Palmer amaranth and waterhemp have been confirmed resistant to glyphosate in Wisconsin. To read more about glyphosate-resistant pigweeds in Wisconsin, please consult these fact sheets: Palmer amaranth resistance and waterhemp resistance.

Preliminary data suggests glyphosate resistance of two Wisconsin common waterhemp (Amaranthus rudis) populations

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.

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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).

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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 vmdavis@wisc.edu or (608) 262-1392.

References:

  1. 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
  2. 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
  3. Heap I (2013) The International Survey of Herbicide Resistant Weeds: Web page. http://www.weedscience.com/Summary/home.aspx. Accessed April 01, 2013
  4. Horak MJ, Loughin TM (2000) Growth Analysis of Four Amaranthus Species. Weed Science 48:347-355
  5. 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
  6. Sellers BA, Smeda RJ, Johnson WG, Kendig JA, Ellersieck MR (2003) Comparative Growth of Six Amaranthus Species in Missouri. Weed Science 51:329-333
  7. Steckel LE, Sprague CL (2004) Common waterhemp (Amaranthus rudis) interference in corn. Weed Science 52:359-364
  8. 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