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Issue No. 3, Article 8/April 14, 2006

The Illinois Outlook: Can It Happen Here?

Since the commercialization of glyphosate-resistant crops, the question of whether glyphosate-resistant weeds will be selected has been extensively bandied around by individuals involved in virtually all phases of production agriculture. Those in academia have (generally) agreed on at least two points: (1) the potential for selecting weed biotypes resistant to glyphosate is less than that associated with selecting biotypes resistant to other herbicide families, and (2) never say it will never happen. Many agrichemical retailers initially expressed high satisfaction with the weed control provided by glyphosate, but some are currently very concerned with what may be described as a "reduced consistency" of performance. Some manufacturers of glyphosate initially expressed confidence that glyphosate-resistant weeds would not become an issue farmers or agrichemical retailers would have to contend with, whereas others voiced near-apocalyptic concerns about the likely consequences of overreliance on a single active ingredient. Whichever position you might have taken on this question during the early years of glyphosate use in-crop, the facts are as follows: Glyphosate-resistant weed populations have been selected; these resistant populations represent more than one weed species; several states, from the East Coast to the West Coast, have reported instances of glyphosate-resistant weeds; and we have no evidence to suggest that Illinois will be immune to this phenomenon (see Table 1).

An important step in avoiding the problems caused by herbicide-resistant weeds is to understand how a resistant weed population develops. Evidence suggests that herbicide-resistant weeds are naturally occurring biotypes that initially exist in very low numbers within the population of a particular weed species. Plants that possess certain traits or characteristics not common to the entire species are referred to as biotypes. When a particular herbicide effectively controls the majority of susceptible members of a species, only those plants that possess a resistance trait can survive and produce seed for future generations. This is often referred to as the selection theory, or (to borrow a phrase from Darwin) "survival of the fittest." Biological organisms (e.g., people, plants, animals) are diverse. The plants in a population that possess characteristics enabling them to survive under a wide range of environmental and other adverse conditions (such as herbicide applications) will be able to produce seed that maintains these survival characteristics. Plants less adapted generally do not survive, and hence only the fittest plants reproduce.

What, then, is meant by the phrase "selection pressure" with respect to herbicide-resistant weeds? Most herbicides are used to control a broad spectrum of weeds. By controlling susceptible members of a weed population, we are essentially using herbicides as agents to select for biotypes that are naturally resistant to the herbicide. These resistant biotypes are better adapted to survive in the environment created by controlling susceptible members of the population. The seed produced by the resistant biotypes ensures that the resistance trait will carry on to future generations. If the same or a similar herbicide is used repeatedly (year after year or several times during the growing season), the resistant biotypes continue to thrive, eventually outnumbering the normal (susceptible) population. In other words, relying on the same herbicide for weed control creates a selection pressure that favors the development of herbicide-resistant weeds.

The development of herbicide-resistant weed populations may thus be summarized by the following sentence: The appearance of herbicide-resistant weeds is the consequence of repeatedly using the same or similar herbicides (usually a herbicide with a single site of action) to control a specific weed species not controlled by any other herbicide or in any other manner. The process of selection for herbicide-resistant biotypes begins the first time a particular herbicide active ingredient is applied to any given field.

In April 2005, we reported on three horseweed (Conyza canadensis) populations from Illinois that were not controlled with glyphosate in greenhouse experiments. The populations we have examined originated from three counties: Douglas, McLean, and Schuyler. Only one of these populations was sent to us by a concerned farmer who noted poor control of horseweed after a burndown application of glyphosate in 2004. The other two populations were collected during weed survey and collection campaigns in 2004. The geographic dispersion of these populations suggests there may not be a "concentrated" area of concern in the state but rather that farmers and applicators across Illinois should remain vigilant. Glyphosate-resistant horseweed was first identified in Delaware in 2000. Since that initial discovery, glyphosate-resistant horseweed populations have been identified in 11 other states, including our neighbors Indiana and Missouri. Weed scientists in the 12 states where the populations are known to exist have conducted extensive field, greenhouse, and laboratory experiments to further our understanding of the mechanism the plants use to survive glyphosate as well as how best to manage these resistant populations under field conditions.

To date, horseweed has been the only weed species we have identified from Illinois fields with resistance to glyphosate. Arguably, farmers have many viable alternative herbicides for control of glyphosate-resistant horseweed (at least before soybean planting). But will there be viable alternatives for control of other glyphosate-resistant species? What if glyphosate resistance became predominant in summer annual weed species? If glyphosate-resistant species are not common in Illinois at this time, should Illinois farmers be concerned? Are there "new" instances of glyphosate resistance Illinois farmers should be aware of?

At least one biotype of Palmer amaranth (Amaranthus palmeri) in Georgia demonstrates resistance to glyphosate. This population was identified growing in a field of glyphosate-resistant cotton, and weed scientists at the University of Georgia estimate this biotype currently infests approximately 500 acres. Palmer amaranth is a summer annual species of pigweed. It is an aggressive species that can reach heights of 4 to 7 feet and produce large amounts of seed. Research has demonstrated that among the pigweed species common to the Midwest, Palmer amaranth has the fastest growth rate and is the most competitive with row crops. The occurrence of glyphosate-resistant Palmer amaranth in Georgia (and suspected cases in other states) should raise awareness among Illinois farmers because (1) this example illustrates resistance to glyphosate can occur in a summer annual weed species that is very competitive with corn and soybean and (2) Palmer amaranth is indigenous to Illinois. If glyphosate-resistant Palmer amaranth can occur in other states, it seems likely that it can occur in Illinois.

The example of glyphosate-resistant Palmer amaranth may not elicit great concern or provide sufficient evidence to justify a significant change in weed management practices in Illinois because the species is not tremendously widespread here. Horseweed resistant to glyphosate may cause some level of concern, but that species is generally more of a problem before planting than after crop emergence, and preplant tillage remains an effective management option. What if some other summer annual weed species with resistance to glyphosate were to be discovered? Would glyphosate-resistant waterhemp be more of a concern? Is it at all possible that glyphosate-resistant waterhemp could become a problem for Illinois farmers? Farmers in Missouri may face this scenario before Illinois farmers do. Researchers at the University of Missouri have reported two populations of waterhemp that have consistently survived after glyphosate applications under field and greenhouse conditions. News releases from Missouri reported that some of these waterhemp plants survived up to 6 lb a.e. glyphosate (a rate approximately equivalent to 170 fluid ounces of Roundup Original Max). If glyphosate-resistant waterhemp can occur in other states, it seems likely that it can occur in Illinois.

Will these "new" cases of glyphosate-resistant weeds be sufficient to persuade changes to weed management programs in Illinois, especially in soybean production? Only time will provide an accurate answer. However, we continue to stress several points related to glyphosate-resistant weeds and glyphosate stewardship:

1. A selection pressure for herbicide-resistant weeds occurs each time the same herbicide is applied to a particular field. Slogans such as "dead weeds don't make seeds" or "use the right rate at the right time" could be interpreted as overlooking the fact that simply using the herbicide results in selection for herbicide-resistant weeds.

2. Increased adoption of glyphosate-resistant corn hybrids, with a concomitant use of glyphosate to the exclusion of other weed management tools, will speed the selection of glyphosate-resistant weeds.

3. Rotating herbicides (sites of action) or tank-mixing herbicides will help slow the selection of glyphosate-resistant weeds, but it is unlikely to completely prevent their selection. Keep in mind that it's nearly impossible to make blanket statements about how effective a particular alternative herbicide or tank-mix partner will be in slowing the selection of glyphosate-resistant weeds.

4. Stewardship of glyphosate herbicide is a concept easy to discuss but more difficult to implement. Manufacturers often have differing messages about stewardship, but it may be wise to ask yourself why a particular manufacturer seems to be concerned with stewardship of glyphosate.

In summary, the preponderance of evidence suggests it is only a matter of time until glyphosate-resistant weeds begin to occupy additional places in the Illinois agronomic landscape. The pace of introduction of new herbicide active ingredients has slowed considerably since the commercialization of glyphosate-resistant crops, so there may not be a novel herbicide solution available to farmers should glyphosate-resistant weeds become increasingly common and problematic. Farmers and retailers alike will likely experience adverse financial consequences as a result of continued heavy reliance on a single herbicide active ingredient.--Aaron Hager and Dawn Nordby

Aaron Hager
Dawn Refsell

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