Persistent Problems with Pesky Pigweeds

Aaron Hager

Aaron G. Hager
Extension Specialist in Weed Science

Phone: (217) 333-4424


Amaranthus species are among the most troublesome weed species in agronomic production systems. Ten Amaranthus species are regarded as weedy pests across the Great Plains region, including the monoecious (male and female flowers on the same plant) species redroot pigweed (A. retroflexus), smooth pigweed (A. hybridus), Powell amaranth (A. powellii), tumble pigweed (A. albus), prostrate pigweed (A. blitoides), and spiny amaranth (A. spinosus), and the dioecious (separate male and female plants) species common waterhemp (A. rudis), tall waterhemp (A. tuberculatus), Palmer amaranth (A. palmeri), and sandhills amaranth (A. arenicola). Of these, smooth pigweed, redroot pigweed, Powell amaranth, Palmer amaranth, and the waterhemps are most common in Illinois corn and soybean fields.

Contemporary research examining several of these Amaranthus species has provided additional understanding of their biology, herbicide resistance mechanisms, and management options.

Smooth Pigweed: Resistance to Herbicides

Historically, smooth pigweed has been considered the most prevalent Amaranthus species in Illinois. This species continues to be problematic for corn and soybean producers, and the presence of herbicide-resistant biotypes further complicates its management. Triazine-resistant smooth pigweed biotypes in southern Illinois, where atrazine plus simazine tankmixes have been extensively used for broadleaf weed control in corn, were identified several decades ago. More recently (ca. 1998), a smooth pigweed biotype from Edgar County was identified that demonstrated high levels of resistance to imidazolinone (Pursuit, Scepter, and Raptor) herbicides. Molecular analysis of this biotype indicated a single amino acid substitution in the gene encoding the acetolactate synthase (ALS) enzyme (the target site of imidazolinone herbicides) was responsible for the high levels of herbicide resistance noted in greenhouse experiments. In 1999, a smooth pigweed biotype from Massac County was identified that demonstrated resistance to both triazine and ALS-inhibiting herbicides. In contrast to the Edgar County smooth pigweed, the Massac County smooth pigweed biotype was resistant to both imidazolinone (Raptor) and sulfonylurea (Harmony GT) herbicides.

Palmer Amaranth: Becoming More Common

Palmer amaranth is perhaps the most "aggressive" Amaranthus species with respect to growth rate and competitive ability. Palmer amaranth is most common in the southern third of Illinois but, similar to waterhemp's be expanding its range northward. The growth rate and competitive ability of this species exceed that of other Amaranthus species. Horak and Loughin (2000) conducted a two-year field experiment to compare several growth parameters of Palmer amaranth, waterhemp, and redroot pigweed. Palmer amaranth had the highest values for plant volume, dry weight, and leaf area of all species, as well as the largest rate of height increase. Biotypes of Palmer amaranth resistant to ALS-inhibiting herbicides are known to exist.

Waterhemp: How Many Seeds Can It Produce?

Waterhemp continues to cause a great deal of consternation for producers and retail applicators alike. While several herbicides effectively control waterhemp, the most consistent management strategy includes a sequential approach that combines soil-applied herbicides with post-planting cultivation and/or postemergence herbicides.

Herbicide resistance in waterhemp: Resistance to various herbicides in the Illinois waterhemp population has long been known to occur, but little is known about how pervasive herbicide resistance in waterhemp actually is. Over a two-year period, we made approximately 60 waterhemp collections from 30 Illinois counties to examine the extent of herbicide resistance in the Illinois waterhemp population. We randomly selected individual female waterhemp plants from corn and soybean fields (we had no knowledge of herbicide use history for any field we sampled), and we grew seedling plants in the greenhouse that were treated with a triazine herbicide (atrazine), an imidazolinone herbicide (imazethapyr), or glyphosate. Greenhouse results indicate approximately 25 percent of the samples produced progeny resistant to atrazine, while approximately 90 percent of the populations demonstrated resistance to ALS-inhibiting herbicides. Within the atrazine-resistant populations, there apparently are different mechanisms of resistance, cross-resistance to other triazine herbicides, and inheritance of the resistance trait. Similarly, within the ALS-inhibitor resistant populations, there are different mechanisms of resistance that affect patterns of cross-resistance to the various ALS-inhibiting herbicides. Population responses to glyphosate were more uniform compared with responses to atrazine or imazethapyr. However, several populations were identified with reduced sensitivity to glyphosate under greenhouse conditions. These results, coupled with the extensive use of glyphosate for waterhemp control in glyphosate-resistant soybean, suggest a strong potential for selection of waterhemp populations that would not be controlled effectively with field use rates of glyphosate.

Researchers at Kansas State University recently have identified waterhemp biotypes that demonstrated differential response to acifluorfen and lactofen (Al-Khatib et al. 2000). Field reports from several areas of Illinois in 2001 also indicated that waterhemp control was much less than expected following applications of acifluorfen, fomesafen, or lactofen. While no screening of waterhemp collected from Illinois fields with reported diphenylether (DPE) herbicide failures has yet been conducted, the reports from Kansas State University suggest that selection of DPE-tolerant or -resistant waterhemp biotypes can occur.

Effective postemergence control of waterhemp in soybean can be achieved with fomesafen (Flexstar), acifluorfen (Ultra Blazer), lactofen (Cobra/ Phoenix), or glyphosate; the available options are by no means extensive. Loss of effectiveness with one or more of these options will potentially make waterhemp even more problematic. One way to limit the selection of herbicide-resistant waterhemp biotypes is to integrate multiple control tactics, such as utilization of soil-applied and postemergence herbicides, mechanical cultivation, or all three. Historically, when weed resistance reduced the effectiveness of a given herbicide, new products were introduced into the marketplace that provided alternatives to producers. With the current "devalued" soybean market, there is less incentive for herbicide manufacturers to introduce new active ingredients, which heightens the need to manage currently available options so as to reduce the selection for resistant weed biotypes.

Seed production capability: Research was initiated in 2000 to examine the shade tolerance and seed production capability of waterhemp. Shading structures were constructed with treated lumber and measured approximately 8 feet by 8 feet. Shading cloth was attached to the structure frames to provided shade levels within the structures of 40, 67, or 95 percent. Waterhemp seeds were germinated in the greenhouse and transplanted into shading structures on May 23 and June 23. Various plant growth parameters were measured biweekly, and four plants within each shade level (including a zero percent shade control) were allowed to reach maturity, at which time seed production was determined. Waterhemp plants transplanted on May 23 and grown in no shade (and without interference from corn or soybean) produced slightly over one pound of dry matter and just under 1.5 million seeds. Increasing shade or delayed emergence until June 23 reduced plant weight and seeds produced. Even at 95 percent shade, some waterhemp plants reached maturity and produced a small amount of seed. These data suggest that shade is beneficial in suppressing waterhemp growth and seed production, and that crop production practices that hasten crop row closure can be an integral part of an integrated waterhemp management program.

Amaranthus hybridization: Researchers have long known that certain Amaranthus species can hybridize. What is less well documented is how fertile hybrid progeny are, if herbicide resistance can be transferred to hybrid progeny, and characteristics that can be used to identify Amaranthus hybrids. Weed scientists at the University of Illinois have successfully crossed waterhemp with smooth pigweed, produced fertile progeny from these crosses, and demonstrated that resistance to ALS-inhibiting herbicides can be transferred between species. Current research is underway to develop diagnostic procedures that can differentiate Amaranthus hybrids from non-hybrid plants and to determine how frequently hybridization occurs under field conditions.


This presentation illustrates the importance Amaranthus species will continue to have in Illinois agronomic production systems. Resistance to ALS-inhibiting and triazine herbicides is more common nowadays than a decade ago, but concern is heightened about selection of Amaranthus species biotypes that are resistant to DPE herbicides or glyphosate. Reduced effectiveness of these herbicides would present significant new management challenges for Illinois soybean producers, especially considering that the commercialization of new soybean herbicide active ingredients has slowed considerably since the introduction of glyphosate-resistant soybean varieties.

Literature Cited

Horak, M.J. and T.M. Loughin. 2000. Growth analysis of four Amaranthus species. Weed Sci. 48:347-355.

Al-Khatib, K., N. Hoss, D.E. Peterson, and J.A. Dieleman. 2000. Differential response of common waterhemp biotypes to acifluorfen and lactofen. Proc. North Cent. Weed Sci. Soc. 56:32.

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