May 28, 2024 – Approximately two weeks after spraying, scout to check for surviving weeds that may need a second application. These weed “escapes” have a number of potential causes – from sprayer misses to low water rates.
Another common cause is herbicide resistance. In the case of kochia, herbicide resistance is a safe assumption as many kochia populations have stacked resistance to multiple groups.
One clue that could point to resistance is that the patch has no clear boundary. The patch will somewhat follow machinery pathways, as the patch often spreads by the combine or soil-moving equipment spreading the seeds. It will not have distinct boundaries that we would see in a spray miss. Another clue is that the patch is one species of weed that escaped the spray.
How to reduce the risk of herbicide-resistant weeds?
These steps can help:
- 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. Visit the Canola Encyclopedia weed management chapter to learn more.
How to contain a patch of resistant weeds?
- Use a localized spray with a different product to get rid of it.
- 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.
How to ensure a successful resistance test?
Group 9 resistant kochia is now widespread on the Prairies. To find out for sure, submit suspected glyphosate resistant kochia plants to the PSI lab for tissue testing to confirm resistance.
Follow these steps from PSI to correctly submit a kochia sample:
- Before taking your sample, take a photo of the plants in the suspected patch to show injury symptoms and help further assess between escapes or resistance.
- Sample and bag each green kochia plant separately.
- Gather the top two to three inches from each branch on each plant. Goal: have five to eight tips of leaf material from each plant submitted.
- Bag samples in a sealable Ziploc type bag and place bagged samples on ice immediately after collection, as DNA begins to break down as soon as the leaves are removed from the plant.
- Find all steps to successfully submit samples for testing on the PSI website.
For more on this topic, please read the following Canola Watch fundamentals articles: “How to contain herbicide-resistant kochia” and “Integrated weed management: Best practices”. While there, please sign up to receive the weekly Canola Watch e-newsletter, answering the top canola agronomy questions from across the Prairies.
