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Futures

Herbicide resistance testing

03/12/2021

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By Mathew Tyhurst,

Crop Sales Specialist
AGRIS and Wanstead Farmers Co-operatives

Herbicide resistance is an ongoing concern for many growers. 

Resistance is being combated on several fronts:

  • Switching up chemistry modes of actions.
  • Timing application strategies, targeting small weeds with pre and post applications to kill weeds before they become too large.
  • Switching up herbicide tolerance trait platforms in seed to accommodate different herbicides to kill problem weeds on the farm.
  • Maintaining complex crop rotations to not allow one crop or one herbicide program to become overused on the field.

Even with all the above strategies there often can be a spot in the field where there are some weeds escapes. These escapes are sometimes written off as a miss with the sprayer or the chemical didn’t work there due to weather or timing of application.

HerbicideResistance_chart1.JPG

This diagram shows that not every weed escape is a resistant weed and some susceptible weeds can escape a herbicide application, but left unchecked after a couple of years with limited changes to farming practices; there is a greater chance that resistance weeds can multiple in a field.

Testing for resistant weeds in past has been a long process of collecting seed and growing it out in a controlled environment and spraying the weed with varying rates of herbicide to see the weed reaction. In some cases, this is still the way it has to be done. In other cases, there is a new way to test for resistance.

DNA testing for suspected resistant weeds is now being offered, this is being made possible thru industry and public support. Currently, these weeds and modes of actions can be tested.

Table 1. Genetic tests available.

WeedSpecies_HerbicideResistance_Chart2.JPG

*Several of these tests were developed by other researchers (Francois Tardif) and reproduced from the scientific literature.
**S264G mutation only induces resistance to Group 5 herbicides, not Group 7.

Take a tissue sample

sample size weed dna.png

If you happen to find some of the weeds that are listed above in your field that were not killed, a simple tissue sample can be taken and submitted to a lab for resistance testing. A quarter-sized piece of the leaf must be pulled off ten plants and then packaged and submitted for sampling.

Lab results

After the lab processes the sample, a report will be sent back with your results.

WeedSpecies_HerbicideResistance-Results_Chart3.JPG

This sample was taken in July of 2020 out of one weed patch in the corner of the field where the grower was unsure why his group 2 herbicide program was not controlling all his common ragweed. Prior to submitting the sample, we could speculate that there was group 2 resistant common ragweed and based on field observation, this would have been a fair assessment but still would leave a little doubt in your mind.

Resistance testing has allowed us to find out that the common ragweed is resistant to multiple modes of action. This could be viewed as a negative that three groups of herbicides have been taken off the table to control the weeds, but the testing really has led to a positive outcome.

Instead of investing in different herbicide programs to find out if something works, which could take several years while the resistant weeds continue to spread. It might be better and more expedient to submit a tissue sample of the weeds for DNA analysis. In a couple of weeks, you will have the results back, and you will know which chemistries can be applied to that field for effective control. This may lead to the possibility of an application of a different mode of action to control the weeds, thus reducing the chance of resistant weeds setting seed.

The results show that not every weed is resistant to all three modes of action; in this case, it shows the heritability in a weed population within a field. Working with your local Wanstead or AGRIS Co-operative Crop Specialist can assist you with scouting, sampling and sample submission to the lab. Based on the results, we can create a resistance management strategy and implement an effective weed control program.

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