Suspicious weed patches? Check for resistance

Important clues that a patch of weeds are herbicide resistant: One, the patch has no clear boundary. (As Saskatchewan Agriculture weed management specialist Clark Brenzil describes, the patch will have diffuse boundaries that kind of follow machinery pathways, as the patch often spreads by the combine or soil-moving equipment spreading the seeds, but not distinct boundaries that we would see in a spray miss.) Two, the patch will be one species of weed that escaped the spray.

Patches of herbicide-resistant weeds usually develop through a process of ‘selection’. Here are the steps:

  1. A single weed in the populations is naturally resistant to a particular herbicide mode of action or ‘Group’. Mutants like these occur at very low frequencies.
  2. When that particular herbicide is used, it controls all the weeds in a population except for the mutant.
  3. The surviving mutant is completes its life-cycle and leads to the next generation of resistant weeds.
  4. The following year only a portion of the resistant weed seeds germinate and some remain dormant in the soil to emerge in future years.
  5. Applying the same or another herbicide with the same mode of action (Group) in the next or future years enables the resistant population to multiply.

To reduce the risk of selecting for resistant weeds on your farm:

  • Control weeds early.
  • Use tank mixes. Hitting weeds with two modes of action effective on each weed reduces the risk of herbicide resistant weeds escaping and setting seed.
  • Rotate herbicide mixes, using different effective modes of action on the same field and weeds.
  • Alternate between competitive and less competitive crops, and crops that have different seeding and harvest timing (winter cereals, for example).
  • Use the right herbicide at the right rate and apply at the right time. Cutting rates, for example, may reduce herbicide efficacy and increase weed seed return to the soil seed bank.
  • Employ other Integrated Weed Management practices so herbicides are not the only method of weed management used on the farm. The Canola Encyclopedia lists integrated weed management practices that are just good practice for weed management in general.

If you suspect a patch of weeds is resistant:

  • Use a localized spray with a different product to get rid of it.
  • Collect a few for analysis. Canola Watch lists the labs that do herbicide-resistant weed tests.
  • Use mechanical means (tillage, mowing, hand pulling) to remove weeds before they set seed.
  • Adjust herbicide and cropping practices so that weeds can be sprayed with multiple modes of action that are effective on that patch and other at-risk weeds.

Collecting seeds for a resistance test

Tammy Jones, weed specialist with Manitoba Agriculture, explains how the tests are done:

Many different methods have been developed for testing weeds for herbicide resistance by various research groups. There are rapid tests that use transplanted seedlings from the field, hydroponics, agar medium, leaf discs, DNA and even pollen, but few of these tests are commercially available. Instead, most herbicide resistance tests are conducted by growing out plants in a growth chamber/greenhouse in the winter months, providing information for future management decisions. The advantages of the whole plant test are multiple: it is the most similar testing to the in-field situation, relatively cost effective and does not require knowledge of mechanism of herbicide resistance.

The critical step for a successful herbicide resistance test is to collect a quality seed sample. Typically, a sample is taken from a suspicious patch, because the sample is intended to confirm the presence of herbicide resistant weeds. That implies that we are sampling only those weeds that survived the herbicide application, and are avoiding any late emerging weeds. Collect weed seeds when they are mature. Immature seed will result in low or no seed viability, but collecting too late may mean that all of the seeds have dispersed or shed. Knowing what the mature seed should look like will help. For instance, redroot pigweed seed should be black and shiny; if still reddish-brown, the seeds are not mature and will not likely grow. Seeds will drop from the plant as they mature, so there may need to be more than one collection date. 

Collect enough seed. The amount of seed required for the test will vary based on dormancy, germination, and the number of herbicides that are being screened. Aiming for 1,000 to 2,000 seeds is a general recommendation. Each lab has recommendations on how to collect and submit a sample, which should be followed.

Clark Brenzil lists some of the biggest collection FAILS:

  • bag full of chaff but little seed. We can’t test chaff.
  • 1000 to 2000 seeds is required for each test being requested. We often have to call submitters back to tell them we don’t have enough seed for all of the tests they have selected and they have to choose their priorities.
  • seed is not dried before shipment
  • Seed is packaged in a plastic bag and can accumulate moisture. If the submitter insists they want to use a zip lock bag, use the vegetable bag with small air holes. Better yet go by a bunch of heavier gauge paper bags the size of a coffee tin for each test requested. They are more environmentally friendly and will allow the sample to breath.
    dead seed (sprayed with preharvest treatment, heated, cracked) keep in mind on a sunny day the dash of a pickup can reach 160 F (71 C) in 15 minutes. This temperature kills seeds very quickly.

Is a herbicide resistance test worth doing?

Tammy Jones answers: YES! We do a soil test to make fertility recommendations, and in some ways this is the same type of idea. The confirmation of herbicide resistance and information on the proportion of the population that is resistant will help to make effective management decisions. Spraying a quarter section with a chemical that costs $20/acre but doesn’t provide good weed control is a waste of money and time. Herbicide resistance does not just magically disappear from a field and if the problem is ignored, can make a huge impact on productivity and profitability. 

Changing in-field management is still a good idea. Dr. Stephen Powles, University of Western Australia, Australian Herbicide Resistance Initiative, provides this advice: “If something is working, change it.” Based on seed-bank longevity, and seed biology, there may be some challenges on ever fully eliminating a herbicide-resistant weed biotype in the field, but reducing the population to levels that are not economically significant is the goal.

Further reading:
Management tips for glyphosate-resistant kochia
CropLife Canada’s Manage Resistance Now is an online resource developed through industry collaboration to help growers minimize the development of herbicide, fungicide and insecticide resistance on their farms