No. 6 Article 3/May 10, 2012

Update on Multiple Herbicide Resistance in Illinois Waterhemp Populations

Herbicide-resistant waterhemp populations will challenge weed management practices and practitioners this growing season. Further complicating management is the increasing occurrence of waterhemp populations resistant to herbicides spanning more than one site of action, as the efficacy of multiple herbicides is compromised.

Last year we published the results of a herbicide resistance screening program conducted by University of Illinois weed science personnel for waterhemp samples submitted in the 2010 growing season ("Herbicide-Resistant Weeds in Illinois: A Cause for Concern," issue 3, April 22, 2011). The screening uses molecular biology techniques to detect herbicide-resistance traits (glyphosate, PPO inhibitors, and ALS inhibitors) in waterhemp. This article updates survey results for samples submitted for screening in the 2011 growing season. We anticipate being able to provide this screening service again this year; more information will be shared in a future edition of the Bulletin.

Let's start with some discussion on distinguishing between multiple herbicide resistance at the field level and at the individual plant level. With field-level multiple resistance, resistance to herbicides from more than one site of action is present in the population growing in any particular field. Figure 1 illustrates multiple herbicide resistance at the field level: individual plants are resistant to herbicide A, B, or C, but no plant is resistant to more than one of the three herbicides. When multiple herbicide resistance occurs at the plant level, one plant demonstrates resistance to herbicides encompassing more than one site of action. This is illustrated in Figure 2: individual plants are resistant to herbicides A and B; B and C; A and C; or A, B and C. Notice also that some plants in both examples remain sensitive to all three herbicides.

Figure 1. Multiple herbicide resistance illustrated at the field level.

Figure 2. Multiple herbicide resistance illustrated at the individual plant level.

Distinguishing between multiple herbicide resistance at the field and individual plant levels is important and can impact management options. For example, when three types of resistance are present at the field level, a tank mixture of herbicides A, B, and C could (assuming no antagonism occurs with the tank-mix) control all plants, including the sensitive plants, in the field. However, the same tank mixture would not control individual plants resistant to all three herbicides. Figure 3 depicts three-way resistance in a waterhemp biotype, designated here as ACR, from western Illinois. The biotype designated WCS is sensitive to triazines (atrazine), PPO inhibitors (lactofen), and ALS inhibitors (imazamox), whereas the ACR biotype is resistant to all of these herbicides, whether they are applied singly or in two-way or three-way combinations.

Figure 3. An Illinois waterhemp biotype (ACR) with resistance to herbicides from three different site-of-action families.

In reality, multiple resistance often occurs at both the field level and the individual plant level in any given population. In other words, multiple resistance at the field level often includes a mix of plants, some resistant to just one herbicide or herbicide family and others resistant to multiple herbicide families. Simply tank-mixing two or more post┬Čemergence herbicides will not guarantee complete control of herbicide-resistant waterhemp.

Last year 408 waterhemp plants from 97 different fields suspected of having glyphosate resistance were submitted for screening. Screening for resistance to three different herbicide families can result in eight possible outcomes, shown in the Venn diagrams in Figures 4 and 5. The diagrams illustrate all possible outcomes or combinations of multiple factors (or here, all possible combinations of resistance to one, two or three herbicide families; the absence of resistance is the eighth possible outcome). The results from the 2011 survey are presented for individual plants in Figure 4 and on a field basis in Figure 5.

Figure 4. A herbicide resistance profile of samples from 408 waterhemp plants submitted for screening in 2011.

Figure 5. A herbicide resistance profile of 97 fields from which waterhemp samples were submitted for herbicide resistance screening in 2011.

Of the 408 plants submitted and screened (Figure 4), only 13% were sensitive to all three herbicides; thus, 87% were resistant to one or two herbicide families. As indicated in the portions of the circles that do not overlap, 28% of plants were resistant only to glyphosate, 19% only to ALS inhibitors, and 3% only to PPO inhibitors. To determine the total percentage of plants resistant to each herbicide, you simply add the four numbers in the circle for each herbicide family: for glyphosate (GLY), 57% of plants were resistant (28 + 24 + 3 + 2); for ALS inhibitors, 54% (19 + 8 + 3 + 24); and for PPO inhibitors, 16% (3 + 2 + 3 + 8).

How would you determine the percent of individual plants that were multiply resistant, say to both glyphosate and PPO inhibitors? You determine this percentage by adding the two numbers that lie in the overlap of the GLY and PPO circles (5% in this example: 3 + 2). So the diagram shows that 11% of plants were resistant to both ALS and PPO inhibitors (3 + 8), 27% to both ALS inhibitors and glyphosate (24 + 3), and 3% to ALS inhibitors, glyphosate and PPO inhibitors, making them three-way resistant (the percentage in the overlap of all three circles).

Figure 5 shows the same information on a field basis. Only 4% of the 97 fields from which plants were sampled were without herbicide-resistant waterhemp. About 25% of the fields contained waterhemp resistant to PPO inhibitors, 84% had waterhemp resistant to ALS inhibitors, and 66% had glyphosate-resistant waterhemp. Keep in mind that these fields--and plants within fields--were not "randomly" selected but rather were sampled based on suspected resistance to glyphosate.

It is also important to keep in mind that the field-level results are derived from a minuscule sample size (five or fewer plants per field). Sampling more plants per field would undoubtedly increase the percentage of fields with resistance to ALS and PPO inhibitors. Even with the sample-size limitation, over 10% of the sampled fields contained three different types of herbicide resistance, which greatly increases the difficulty of managing these populations using only postemergence herbicides labeled for use in conventional or glyphosate-resistant soybean varieties.

The counties where these samples (and samples screened in 2010) originated are not confined to one particular area in Illinois; Figure 6 illustrates the counties where glyphosate-resistant waterhemp was confirmed in 2010 and 2011 using this molecular biology laboratory assay. The results are not exhaustive; in fact, this and previous surveys suggest that glyphosate-resistant waterhemp occurs in many fields across the southern two-thirds of Illinois.

Figure 6. Illinois counties with confirmed glyphosate-resistant waterhemp populations based on 2010 and 2011 survey data.

This survey is just one project in the ongoing research portfolio on herbicide-resistant weeds in Illinois. Weed scientists at several Illinois universities are working diligently to better understand this phenomenon and find viable solutions for Illinois farmers. Much of the research at the University of Illinois is under the direction of Dr. Patrick Tranel and Dr. Dean Riechers, while Dr. Adam Davis directs most of the modeling and field dissemination research. Postdoctoral researcher Dr. Chance Riggins refined and ran the molecular assays and assembled the resulting data. Weed science colleagues at Southern Illinois University (Dr. Bryan Young) and Western Illinois University (Dr. Mark Bernards) continue to make valuable contributions to the overall research efforts on herbicide-resistant weeds, as do many current and former graduate students and postdocs. Much of this research is made possible by multiple public and private funding sources, including the Illinois Soybean Association.

We thank everyone who took the time to submit samples.--Aaron Hager

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