Resistance to Group 2 Herbicides
in Foxtails in Ontario

Since first discovered in 1997, resistance to group 2 herbicides in Ontario has affected almost only broadleaf weeds: green and redroot pigweeds, common ragweed, eastern-black nightshade, common waterhemp, common lambsquarters and common cocklebur. This picture is now changing with the recent confirmation of resistance in green and giant foxtail.

This type of resistance was first reported to us after a sample was received from Lindsay in the fall of 2001. Four more populations of green foxtails and one population of giant foxtail were further reported to us in the fall of 2003. Interestingly, these reports were from a wide geographic area, spanning more than 300 km (Figure 1). Green foxtails resistant to group 2 herbicides have been documented in Lindsay, Brodhagen, Alvinston and in two different field managed by the same producer in Arthur. Giant foxtail was found in Brigden.

Figure 1: Origin of green and giant foxtail populations resistant to group 2 herbicides in Ontario

Green Foxtail

Our investigations revealed all the green foxtail populations to be resistant to imazethapyr (Pursuit). In all the cases, this was the herbicide that was sprayed and that did not provide the desired control and likely was the selective agent. Our dose response curves showed that these populations can withstand much more imazethapyr than a regular susceptible green foxtail as illustrated by the population from Lindsay (Figure 2). All the populations tested had similar response to Pursuit except for one population from Arthur (field 1) that was more resistant than all the others.

Figure 2 - Response of the resistant population from Lindsay and the susceptible population from Cambridge to increasing rates of Pursuit, 11 days after treatment. Herbicide was applied post emergence to two leaf stage seedlings. Rates are increasing from the left to the right and the arrow indicates the field rate. Note that while the susceptible is totally controlled at 1/8 of the field rate, the resistant still produces significant growth at 16 times the field rate.

The next question we wanted to answer was whether these populations had cross-resistance to other group 2 herbicides. We therefore tested the response to nicosulfuron (Accent) and foramsulfuron (Option). All the populations had some resistance to Accent, although, as with Pursuit, resistance levels differed. One population from Arthur (Field 1) had very high level resistance to Accent while the population from field 2 and those from other sites had moderate resistance (Figure 3).

Figure 3 - Response of two resistant populations compared to the susceptible population to increasing rates of Accent, 11 days after treatment. Note that while the susceptible is totally controlled at 1/4 of the field rate, the resistant from Lindsay still produces significant growth at 8 times the field rate. The resistant from Arthur (field 1) is only slightly affected at 32 times the field rate.

There was also differential response to foramsulfuron (Option) although the level of resistance was not as high as with the previous two herbicides (Figure 4). All five populations tested survived higher rates of Option than the susceptible and the population from Arthur (field 1) had the highest resistance.

Figure 4 - Response of two resistant populations compared to the susceptible population to increasing rates of Option, 11 days after treatment. The susceptible is totally controlled at 1/4 of the field rate, while the resistants are still alive, albeit with severe injury, at the field rate.

Enzymatic test conducted in the lab confirmed that all these green foxtail populations have resistance due to alteration of the target enzyme ALS (acetolactate synthase). This is the same type of resistance that was documented in many other weed species worldwide including the pigweeds in Ontario. Genetic tests have also indicated that there are four different mutations in these resistant populations. It is of particular interest that two populations from Arthur, although coming from the same producer, have two different mutations. This indicates that resistance was selected separately in each individual field and further illustrates the difficulty in predicting what type of resistance is likely to be selected following group 2 herbicide uses.

Giant Foxtail

Results with the giant foxtail population are still preliminary. Resistance to Pursuit was confirmed in growth room tests. Enzyme tests indicated a moderate level of resistance to Pursuit, low resistance to Accent and no resistance to Option. How these results translate at the whole plant level remains to be determined.

Management Options

Field test were conducted at Lindsay in 2002 and 2003. These tests were done in the absence of crops so there was no impact of crop competition. However, these results provide a relative comparison of what could be expected from these products. PRE emergence alternatives included Dual and Frontier which both provided excellent control (Table 1). Battalion also provided excellent control, but this was likely due to the Dual component more than the Elim.

Table 1: Effect of PRE herbicides on control of Group 2 resistant green foxtail in Lindsay

DAT: Days after treatment

In general, none of the group 2 herbicides were acceptable POST management methods (Table 2). Control with Accent ranged between 20 to 40%. Glyphosate was providing high control and would be adequate in Roundup Ready crops. Poast also provided good control and would be a good option in soybeans as would other graminicides such as Assure, Select or Excel. In corn, it appears that POST options are limited and in some circumstances, inter-row cultivation might be the only solution that would prevent a significant seed return from the weeds.

Table 2: Control of green foxtail resistant to group2 herbicides in Lindsay

DAT : Days after treatment

Testing for Resistance

With another challenging season for many herbicides, the potential for weed escapes is very high. Many of those will be explained by weather conditions, but resistance could also be a possibility. Resistance is suspected when only one (or a few) species appear to survive the herbicide. It is worth while to look back at field history to determine whether the same herbicide group has been used repeatedly. In other cases though, resistance development could be linked to custom combining, as seeds can be introduced from contaminated fields. The best way to get rid of the guess work is to have the populations tested by calling 1-877-424-1300. Once resistance is confirmed, herbicide programs can be planned accordingly. If resistance is ruled out, then it is time to look back at other conditions that might have caused the escapes.

Acknowledgments

This research was funded in part by the Ontario Ministry of Agriculture and Food and University of Guelph Enhanced Partnership through a grant to François J. Tardif. Julie Laplante is the recipient of an NSERC Scholarship.

If you suspect a Herbicide Resistance Problem contact the: OMAF Agricultural Contact Centre at 1-877-424-1300

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