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Issue No. 1, Article 5/March 21, 2008

Glyphosate-Resistant Waterhemp in Illinois: Recommendations for Management

Illinois farmers have another adversary to consider this year in their annual battle against weeds. Glyphosate-resistant waterhemp is poised to bring new challenges (i.e., problems) to agronomic cropping systems in Illinois, and it might be especially troubling for soybean producers. Weed scientists at the University of Illinois have worked together to develop specific recommendations to help farmers better manage this problem in the 2008 soybean crop. These recommendations were developed using data on waterhemp biology, ecology, and control generated by scientists over the past 15 years. We first presented the recommendations in the 2008 University of Illinois Corn & Soybean Classics; the following information is modified from the resulting proceedings.

Over the past decade, many practitioners have become very proficient at controlling waterhemp but perhaps less proficient at managing it. Potentially serious repercussions are poised to plague Illinois soybean farmers in 2008 due to the widespread adoption of weed control in lieu of weed management. A specific consequence of widespread weed control is the selection of Illinois waterhemp biotypes resistant to glyphosate.


Glyphosate-resistant waterhemp seedlings in an Illinois soybean field, 2007.

A pertinent question to consider is this: How will Illinois soybean farmers manage a waterhemp population no longer susceptible to glyphosate or diphenylether herbicides, the only postemergence soybean herbicide options for waterhemp control?

Why should we pose such a question when we're only on the cusp of the new growing season? University of Illinois weed scientists have conducted field, greenhouse, and laboratory research with an Illinois waterhemp population that is resistant to glyphosate. It is altogether likely that other populations of glyphosate-resistant waterhemp exist across the state.

If the herbicide-resistance profile of a particular waterhemp population is known, appropriate changes in herbicide selection and utilization (particularly postemergence soybean herbicides) can be made well before the beginning of the growing season. However, apart from the general assumption that most Illinois waterhemp populations are resistant to ALS-inhibiting herbicides, the vast majority of populations remain uncharacterized with respect to their susceptibility to the limited number of postemergence soybean herbicides that control waterhemp. We speculate that glyphosate-resistant and PPO-resistant waterhemp biotypes might be encountered across large geographical areas of central and south central Illinois during the 2008 growing season. Additionally, it is altogether possible that waterhemp biotypes resistant to both glyphosate and PPO inhibitors soon will be discovered. These biotypes represent a worst-case scenario, in that there are no postemergence herbicide options for their control in soybean.

The University of Illinois offers the following recommendations for managing glyphosate-resistant waterhemp in the 2008 soybean crop. The recommendations assume glyphosate-resistant soybean are planted. The considerations and justifications employed to develop the recommendations are based on current and previous research on waterhemp biology and management and are discussed after each recommendation. The recommendations are specific to herbicides, but weed management practitioners should strongly consider the benefits of practices that increase the competitive ability of the soybean crop (such as practices that hasten crop establishment and canopy development).

Recommendation 1. Apply a full rate (according to label guidelines for soil type and organic matter content) of a soil-residual herbicide no sooner than 7 days before planting or no later than 3 days after planting.

Justification

  • Why use a soil-residual herbicide? If a waterhemp population is resistant to both PPO inhibitors and glyphosate, there are no other postemergence herbicide options that will control this population. In this scenario, soil-residual herbicides are the only effective herbicide options for waterhemp management.
  • Will there be a return on the investment in a soil-residual herbicide? Waterhemp is competitive with soybean and, left uncontrolled (i.e., not controlled postemergence because of resistance), it continues to reduce yield potential for several weeks after emergence (Table 1).
  • Why use a full rate of a soil-residual herbicide instead of a reduced ("set-up") rate? Research has demonstrated that germination and emergence of waterhemp often extend further into the growing season than is common for other summer annual weed species. A reduced, or set-up, rate of a soil-residual herbicide often can provide some amount of early-season control or suppression, but a full rate generally extends residual control. Even where a full rate of a soil-residual herbicide is used, it is altogether possible that waterhemp emergence will occur at some point after soybean emergence (Table 2). The later into the season that waterhemp emergence can be delayed, the greater the potential for achieving maximum or near-maximum soybean yield.

 

  • Certain soil-residual herbicides are labeled for application several weeks prior to soybean planting. However, considering the extended emergence characteristics of waterhemp, herbicide application closer to planting generally protracts residual control later into the growing season. Conversely, some of the most effective soil-residual herbicides for waterhemp control can cause significant soybean injury if applied after soybean emergence, so applications should occur no later than 3 days after soybean planting (or sooner if specified on the product label).

Recommendation 2. The initial postemergence application of glyphosate (alone at 0.75 to 1.0 pound acid equivalent per acre) must be made when waterhemp is 3 to 5 inches tall.

Justification

  • Why use glyphosate alone instead of tankmixing with a PPO inhibitor? We have only limited data on control of glyphosate-resistant waterhemp with glyphosate + PPO inhibitor tankmixes. Additionally, questions persist about the potential for antagonism with these tankmixes and about which additive(s) should be recommended. For example, nonionic surfactant (NIS) is the preferred additive for certain glyphosate formulations, while crop oil concentrate (COC) is preferred for foliar-applied PPO inhibitors. Would NIS be sufficient for the PPO inhibitor tankmix partner to control a glyphosate-resistant waterhemp population? Would COC antagonize glyphosate sufficiently that it fails to control a PPO-resistant waterhemp population (or other weed species that may also be present)?
  • Why not simply increase the application rate of glyphosate? Field research conducted in 2007 on a confirmed glyphosate-resistant waterhemp population indicated that resistant plants were not adequately controlled with glyphosate at labeled in-crop application rates. Increasing application rate to the maximum allowed by label (1.5 pounds acid equivalent per application) did not consistently improve control compared with lower application rates. Across two application timings (4- and 12- inch waterhemp), control of glyphosate-resistant waterhemp was 50% or less with glyphosate at rates labeled for in-crop application (Table 3).

  • Why apply when waterhemp is only 3 to 5 inches tall? Applying glyphosate before waterhemp exceeds 5 inches generally provides more consistent control compared with applications made to larger plants. Previous research has shown that nonresistant waterhemp less than 5 inches tall is very sensitive to glyphosate at 0.75-pound acid equivalent. Waterhemp plants that survive 0.75- to 1.0-pound glyphosate acid when treated at 5 inches or less should be closely monitored.

Recommendation 3. Fields must be scouted 7 days after the initial glyphosate application to determine treatment effectiveness.

Justification

  • Why scout fields so soon after the initial glyphosate application? Field research in 2007 also indicated that glyphosate-resistant waterhemp plants continued to grow at near-normal rates following treatment with glyphosate. Few herbicide injury symptoms, if any, became noticeable on treated plants, and by the end of the season, treated and nontreated plants were virtually indistinguishable. With this in mind, if the initial application of glyphosate is made when waterhemp plants are 3 to 5 inches tall, it is quite possible that 7 to 10 days might elapse after application before lack of control becomes obvious. During this interval, glyphosate-resistant waterhemp plants will in all likelihood continue to grow and possibly could attain an additional 6 to 8 inches in height.

Recommendation 4. If waterhemp control is inadequate and retreatment is necessary, consider applying a PPO-inhibiting herbicide (lactofen, fomesafen, or acifluorfen) at a full labeled rate (with recommended additives) as soon as possible.

Justification

  • The only remaining herbicide recourse for control of a glyphosate-resistant waterhemp population is the application of a PPO-inhibiting herbicide. Optimal control of waterhemp with PPO inhibitors is usually achieved when plants are 5 inches tall or less, but some control of larger plants is possible. If recommendations 1 through 3 were followed, it is anticipated waterhemp would range between 8 and 12 inches tall by the time a PPO inhibitor is applied.

Recommendation 5: Rescout the treated field within 10 to 14 days to determine effectiveness of the PPO-inhibiting herbicide treatment. If scouting reveals that plants treated with a second herbicide application might survive, implement whatever tactics are available or feasible to rogue these surviving plants from the field before they reach a reproductive growth stage.

Justification

  • Previous research has shown that the PPO-resistance trait can be transferred via pollen. If PPO-resistant male plants reach a reproductive growth stage, the pollen produced could facilitate the spread of this resistance trait to sensitive waterhemp populations. Currently, it is not known whether the trait conferring glyphosate resistance is transferred via pollen, seed, or both.
  • Female waterhemp plants can produce a tremendous amount of seed. Previous research indicates waterhemp may produce in excess of 1 million seeds per plant. If a seed-laden, glyphosate-resistant female plant is mechanically harvested along with the crop, seeds of a resistant population might be sown in fields subsequently harvested with the combine.

These recommendations illustrate the need for an integrated approach to waterhemp management. Integrated weed management introduces multiple tactics to control weeds and to slow the rate at which weeds can adapt to a single control tactic. Introducing an integrated approach to waterhemp management into glyphosate-resistant cropping systems may well stave off some potential new challenges, enhancing the long-term effectiveness of this valuable weed control technology.--Aaron Hager

References

Hager, Aaron G., Wax, Loyd M., Stoller, Edward W., and Bollero, Germán A. 2002a. Common waterhemp (Amaranthus rudis) interference in soybean.Weed Science 50(5):607-610.

Hager, Aaron G., Wax, Loyd M., Bollero, Germán A., and Simmons, F. William. 2002b. Common waterhemp (Amaranthus rudis Sauer) management with soil-applied herbicides in soybean (Glycine max [L.] Merr.). Crop Protection 21(4):277-283.

Author:
Aaron Hager

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