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Issue No. 20, Article 3/August 11, 2006

Waterhemp--What Have We Learned About This Species?

Sometimes to have a better idea of where we're going, it helps to step back and review where we've come. During the early days of January 1994, the Illinois Agricultural Pesticides Conference convened to address the most relevant and contentious issues then facing production agriculture in Illinois. Among the topics presented was a discussion by Dr. Loyd Wax, a weed scientist with USDA/ARS, of the expanding problem of herbicide-resistant weeds. Two significant quotes from Dr. Wax's proceedings paper are reproduced here; the first provides a reference to the status of herbicide-resistant weeds in Illinois during the early years of the 1990s, while the second provides a then-ominous, but now-obvious, mention of a weed species soon to become more problematic for Illinois farmers.

First, the statement about the status of herbicide-resistant weeds in Illinois: "Herbicide resistant weeds, while becoming a significant problem worldwide and in some areas of the United States, are minimal at this time in Illinois. However, there is potential for development of resistant weed biotypes with several classes of herbicides." By the end of 1993, researchers in Illinois had documented herbicide resistance in only three weed species biotypes (common lambsquarters, smooth pigweed, and kochia), with resistance to only one herbicide family (triazines). A decade later, the list of herbicide-resistant weed biotypes in Illinois had grown to include 10 species (eight broadleaf and two grass species), and resistance had grown to encompass four herbicide families (triazines, ALS inhibitors, PPO inhibitors, ACCase inhibitors).

Next, the forward-looking statement (again, published in 1994) of problems and challenges soon to be faced by Illinois farmers: "To date, we have not documented any weed biotypes in Illinois that are resistant to ALS inhibitors. In other greenhouse work, and based on field observations, we have identified substantial differences in tolerance of the various pigweed species, including the water hemps, to postemergence applications of the herbicides Classic and Pursuit." This might have been one of the earliest references to the forthcoming problems with waterhemp, a weed species unfamiliar to and unrecognized by many Illinois farmers in 1994. Fast forward 12 years to 2006, when this once-obscure species is considered the most problematic broadleaf weed species with which weed control practitioners in Illinois (and many other states) must contend.

Although a species indigenous to Illinois, waterhemp was not considered much of a problem weed in agronomic crops until it began to spread across the state sometime beginning about the late 1980s or early 1990s. Today, waterhemp populations continue to infest additional acres of farmland in central and northern Illinois, aided by several adaptations (some of which are unique to this weed species) that allow the species to thrive in contemporary agronomic crop production systems. Indeed, waterhemp has become perhaps the most recognized example of how a weed species is able to adapt to manmade "environments." One adaptation of particular importance that has allowed waterhemp to flourish is its ability to thwart attempts at control with herbicides.

The story of waterhemp management in agronomic crops has been anything but static, and even today the story continues to be written. No weed species in Illinois has demonstrated more unique instances of herbicide resistance. In 1994, Dr. Wax alluded to the forthcoming possibility of selecting waterhemp biotypes resistant to ALS-inhibiting herbicides. By 2002, we reported how pervasive herbicide resistance in waterhemp had come into being across Illinois. 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 (with no knowledge of herbicide use history for any field we sampled), grew seedling plants in the greenhouse, and treated them with a triazine herbicide (atrazine) or an imidazolinone herbicide (imazethapyr).

Greenhouse results indicated approximately 25% of the samples produced progeny resistant to atrazine, while approximately 90% demonstrated resistance to ALS-inhibiting herbicides. Within the atrazine-resistant populations, there appeared to be at least two different mechanisms of resistance, along with variation in patterns of cross-resistance to other triazine herbicides and inheritance of the resistance trait(s). Similarly, within the ALS-inhibitor-resistant populations, there were different mechanisms of resistance that affected patterns of cross-resistance to the various ALS-inhibiting herbicides. Intermingled with the herbicide-resistance screening research, Illinois weed scientists also reported the confirmation of a waterhemp biotype from Bond County that was resistant to both ALS-inhibiting herbicides and triazine herbicides. This marked the inaugural report of multiple herbicide resistance in waterhemp, but the story would continue to evolve.

Weed control practitioners know there are only four active ingredients for postemergence waterhemp control in soybean, and three of these belong to one chemical family. The diphenylether herbicides (PPO-inhibitors) acifluorfen (Ultra Blazer), fomesafen (Flexstar), and lactofen (Cobra/Phoenix) were once used extensively for waterhemp control in soybean, until being largely displaced by glyphosate. These products were often applied alone to control waterhemp, but frequently they were used as tank-mix partners with one or more of the postemergence ALS-inhibiting broadleaf herbicides.

For many years, diphenylether herbicides were the primary weapons against waterhemp in soybean, and we learned that the most consistent control of waterhemp with these herbicides was achieved when applications were made to plants less than 6 inches high. However, during the 2001 growing season, several reports from around Illinois indicated that waterhemp control was much less than expected following applications of diphenylether herbicides. We began investigating a population of waterhemp from western Illinois that was not controlled by postemergence applications of diphenylether herbicides during the 2001 growing season, nor with lactofen (Cobra at 20 fluid ounces plus crop oil concentrate) under greenhouse conditions. Given these observations from the field and our results from greenhouse research, we began experiments to determine how this waterhemp population responded to various soil-applied and postemergence herbicides under actual field conditions.

It soon became obvious that this waterhemp biotype did in fact demonstrate resistance to various PPO-inhibiting herbicides. After several years of extensive field, greenhouse, and laboratory research, in 2005 we reported the confirmation that this waterhemp biotype demonstrated resistance to not simply one herbicide family but three: ALS inhibitors, PPO inhibitors, and triazines. This marked the first-ever report of three-way herbicide resistance in a summer annual weed species in the United States. Additionally, we recently published the results of research that identified a unique mechanism that this waterhemp biotype uses to survive exposure to PPO herbicides. And so the story of waterhemp management in agronomic crops continues to evolve.

The fourth postemergence herbicide option for waterhemp control in soybean is glyphosate. Glyphosate has been very effective against waterhemp since its in-crop utilization rapidly escalated following the commercialization and adoption of glyphosate-resistant soybean varieties. Many soybean farmers have come to rely exclusively, or nearly so, on glyphosate for waterhemp control in lieu of a more integrated management approach. For many years, glyphosate seemed to be the remedy for all of the problems and challenges presented by waterhemp. However, during the past several growing seasons, we have received an increasing number of reports of glyphosate's failing to provide adequate control of waterhemp (and a few other weed species). Other states have reported similar observations. While perhaps not always meeting the criteria for being designated "resistant" to glyphosate, lack of control for whatever reason presents a problem. The story, however, continues to unfold.

The moniker glyphosate-resistant now has been attached to a waterhemp population. Weed scientists from the University of Missouri reported that at least one population has consistently survived either in-field or greenhouse applications of glyphosate, and this resistance characteristic is successfully passed on to succeeding generations. Not surprising, the field from which this population was identified had received numerous applications of glyphosate since 1996. In essence, what some had considered "unlikely to occur" or "less likely to occur than resistance to other herbicide families" now is reality. The occurrence of glyphosate-resistant waterhemp in Missouri begs the question, can it happen in Illinois? While we have yet to confirm any glyphosate-resistant biotypes of waterhemp here, we have no evidence to suggest glyphosate-resistant waterhemp will not occur in Illinois.

Will the incidence of glyphosate-resistant waterhemp be sufficient to persuade changes to weed management programs in the state, especially in soybean production? Only time will provide the 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.

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 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 an easy concept 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 appears to be concerned with stewardship of glyphosate.

In summary, this historical perspective of waterhemp's notorious expansion across Illinois has been given to illustrate an important point. Waterhemp is a very diverse plant species, as is evidenced by the selection of biotypes resistant to ALS-inhibitors, triazine herbicides, PPO-inhibitors, and glyphosate. It's become somewhat old news that much of the Illinois waterhemp population is resistant to ALS-inhibiting herbicides or that many populations are resistant to triazine herbicides. Resistance to PPO-inhibiting herbicides is perhaps more widespread in Illinois than many people assume, but the near-ubiquitous utilization of glyphosate on Illinois soybean acres has likely masked the full extent of PPO-resistant waterhemp. The preponderance of evidence suggests that it is only a matter of time until glyphosate-resistant weeds (waterhemp, in particular) begin to occupy places in the Illinois agronomic landscape.

In years past, many new herbicide active ingredients were commercialized for the soybean market, but that has changed. It is unlikely that many (if indeed any) new active ingredients with good postemergence efficacy on waterhemp will be introduced in the soybean market during the next few years. If the effectiveness of currently available postemergence soybean herbicides continues to be reduced, waterhemp management may reach a new level of difficulty, as there may not be any new solutions that come to market, at least for the foreseeable future.

One way to reduce the selection of herbicide-resistant waterhemp biotypes is to integrate multiple control tactics, such as soil-applied and postemergence herbicides, mechanical cultivation, or all three. Research conducted by weed scientists at the University of Illinois in the mid-1990s indicated that many soil-applied corn and soybean herbicides demonstrated good waterhemp control but few consistently provided season-long waterhemp control. Our recommendation has been, and will continue to be, that the most consistent programs for waterhemp management include soil-applied and postemergence herbicides, along with mechanical cultivation where feasible. Experience has shown that continued heavy reliance on a single herbicide active ingredient, to the exclusion of other management tactics, ultimately speeds the selection of herbicide-resistant weeds. Glyphosate will not be an exception.--Aaron Hager

Aaron Hager

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