Managing Glyphosate-Resistant Palmer Amaranth – Palmer Roundtable Summary
by Bob Hayes (Moderator)
- Palmer amaranth is worse inside levees.
- Palmer amaranth resistance is often first noticed as uncontrolled patches in fields.
- It is costing $6 to 30 more just to manage Palmer amaranth.
- Producers say they’ll do what it takes; just want to know what to do.
- Glyphosate and ALS resistance must be dealt with; other herbicides may also provoke resistance if management is not stressed.
- There is diversity and hybridization in Palmer amaranth populations, so gene movement is a concern.
- Apomixis (males producing seed) does occur in Palmer amaranth.
- We may need to increase emphasis on non-chemical strategies – e.g., wheat (Triticum aestivum) and rye (Secale cereale) cover crops
- If the seedbank is positive, we aren’t doing the right thing.
- Regional- and state-specific recommendations for Palmer amaranth management will be needed because of differing soil types, environment, and management practices.
- Some control options for glyphosate-resistant Palmer amaranth exist for soybean. Unless alternatives are found, the option in cotton is to plow it under, and the Southeast faces a threat to the economy if cotton can’t be grown.
- Tennessee, specifically, would be devastated by the loss of conservation tillage because increased cost of labor and equipment and the environmental impact, especially on soil erosion, would be extreme.
Rye cover and “super conservation” management:
Concern was expressed as to whether, with the development of glyphosate-resistant weeds, conservation tillage can continue to be used, especially if multiple resistance becomes commonplace. A super-conservation program in which a cover such as rye helps in soil conservation and also weed control should at least be considered. This would entail shifting from chemical weed control to agronomic weed control; i.e., using rye as a ‘preemergence herbicide.’ However, rye is a heavy user of water in early spring, and weed control from wheat planted as a cover to prevent erosion of light soils in Arkansas has not been noted. Additionally, the cost of rye would be $22 to 25/acre plus the cost of new equipment and labor for planting rye. However, equipment was changed for conservation tillage, and producers say they will do whatever is necessary to keep farming.
PPO resistance in common waterhemp has spread quickly in some areas, suggesting the need to screen for resistance in Palmer amaranth. The possibility of resistance to dinitroaniline herbicides was also raised. Undoubtedly, resistance to PPOs will occur if they are used for long, but choices now are limited. The need for stacked resistance in crops was expressed, with the idea that it takes longer to develop resistance to two herbicides than to one. Indeed, 2,4-D-resistant cotton (Dow) and dicamba-resistant cotton (Monsanto) are being developed. With the benefit of these new tools, however, is the potential injury to crops that are not resistant, which some say will be an environmental disaster.
A new herbicide by DuPont, KJM-44, may have potential for pigweed control, although early studies in North Carolina indicate that control of Palmer amaranth is short-lived. It is reported to be very active on broadleaf species and has both soil and postemergence activity. The mode of action has not been released, but it is thought to act other than as an ALS inhibitor.
A change in postemergence application method may also improve weed control. When more than one herbicide is applied, more effective weed coverage might be obtained if each herbicide is put out by separate nozzles, allowing a better chance to get the spray droplets in more places on the plant than would a single tank-mix application.
Need for research and education projects:
Regional Palmer amaranth biology and control projects should be developed. One possibility is an ‘across-the-board’ regional evaluation of herbicide treatments and cultural programs to determine if any are applicable to the entire region or to parts of the region of Palmer amaranth resistance. However, although common recommendations for areas with similar production and growing conditions may be effective, for the most part, recommendations need to be state- or region-specific because of different soils and other factors that cause site-specific responses to herbicides.
A 3- to 4-year study on the impact of successful treatments on the weed seed bank is needed. Resistant seed would be needed for studying seed viability in the soil and perhaps seed dispersal. However, adding resistant seed would detrimental to the location. A solution may be to use purple Amaranthus species popular in home gardening as a marker for tracking dispersal. Regional work should include a seed exchange to determine the level of resistance of biotypes under various environmental and cultural conditions.
A quantitative definition of resistance is needed. The multiple (‘fold’) value of resistance is often used to report level of resistance. However, values determined in the greenhouse may be different than under field conditions. Additionally, if plants are truly resistant, their size at time of application may not matter, although techniques and conditions, such as changing pot size, moisture, or temperature, can affect the amount of expressed resistance.
For regional projects, it may be desirable for work to be sanctioned by national and regional weed science societies. For example, the weed resistance committees of the societies could receive ideas from researchers and develop recommendations for research and education that are acceptable across the entire area of resistance.
A national, multidisciplinary project to develop management programs for jointed goat grass was funded in 1994. The project involved more than 35 scientists participating in cooperative research across 11 states (a review of the project can be found in Weed Technology. 2004. Vol. 18:1143-1149. “Developing national research teams: A case study with the jointed goatgrass research program” by Anderson, Hanavan, and Ogg). Funding was requested for four areas: a. education/extension for producers; b. weed biology, ecology, and genetic research needs; c. management programs; and d. bio-economic modeling. It may be possible to use this project as a model for obtaining national funding for glyphosate-resistant Palmer amaranth research.
A committee was selected to draft a proposal for Palmer amaranth research: Jason Norsworthy (temporary chairperson), Trey Koger, Larry Steckel, Ted Webster, Mike Marshall, Amy Lawton-Rauh, Nilda Burgos, and Stanley Culpepper. Support from state and national commodity boards should be solicited for both initial funding and for designating contacts in Washington.
What may be the next resistance problem?
Certainly the attendees of this meeting are aware that resistance to glyphosate appears to be increasing. The weeds that were named as likely to develop resistance to glyphosate included lambsquarters (Chenopodium album), other Amaranthus species, and barnyardgrass (Echinocloa crus-galli). As for the next herbicide that may be selected for resistance, glufosinate is a leading concern for now. Sustainable use of glufosinate must be encouraged, at least by combining it with other herbicide modes of action and controlling all weed escapes possible after glufosinate application.
And what do we do now?
Palmer amaranth resistance to glyphosate is one of the most critical issues weed scientists have had to face. Management programs must be developed for all areas of infestation – whether they be regional or area-specific programs. Resistance-management education programs must be established for Palmer amaranth, but general resistance management strategies should be emphasized as well, with a strong emphasis on rotating herbicide modes of action.
Biology, ecology, and genetics research also must be pursued to understand dispersal, emergence patterns, and inheritance of resistance. A quantitative definition of resistance is also needed, so that all resistance research and reports will have a common base.
Research, extension, and industry need to work together for sustainable solutions to this and future weed resistance problems. ‘Package deals’ offered by industry for resistance management would encourage producers to use more than one or two modes of action, with the enticement of saving a few dollars on the package and getting a future return through effective resistance management practices.
During the meeting several unanswered questions were raised for which research will be necessary. The need to manage Palmer amaranth resistance in order to save conservation tillage and to continue to farm in an economically sustainable way was made imperative. Awareness of the problem in the agricultural community as a whole must continue to be emphasized, and the elected committee should be supported in its efforts at proposals for funding this very critical research.