Testing for Herbicide Resistance

By Ashley Kaminski, PAg, Crops Extension Specialist, Humboldt

Herbicide resistance in weed populations can create weed management problems for producers. Being aware of how herbicide resistance occurs, how to scout for suspected populations and available testing options can help with future weed management strategies.

Herbicide resistance (HR) happens when the same herbicide active ingredient is used repeatedly on a population of weeds. Using the same active ingredient repeatedly places a selection pressure on the population and allows resistant weeds to survive. Occasionally, there is a change in the genetics of a weed population that makes it resistant, or weeds may develop ways to survive herbicide application, such as evolving to have smaller leaves. Those surviving the current-year herbicide application can produce seed and, as resistance is heritable, will result in an increase in the number of herbicide resistant weed plants in future years. If the same active ingredient is applied year after year, the number of resistant weeds in the field increases and can then present a need for a drastic change in weed management strategy. This is often the case when there are short crop rotations with limited control options.

Fields with many weeds surviving a herbicide application may suggest that the population is resistant, provided the herbicide was applied within product guidelines. Keeping a logbook with your sprayer activity is a good idea. Record what you’ve sprayed, the rate, the herbicide group number and active ingredient, as well as the weather conditions the day spraying was done. This will be useful information if you need to clarify issues of herbicide efficacy or determine if weed seeds should be sent to the Crop Protection Lab for HR testing. You may even look back and realize, without knowing it at the time, that you’ve been depending on one herbicide group a lot. Noticing this pattern ahead of seeing ineffective weed control may decrease the chance of HR in your field.

Talk to your agronomist, crop input retail agronomist, or a crops extension specialist at your local regional office about alternative herbicide group options or alternative crop choices for future years. Alternative crops may allow the use of different herbicide groups or enable different weed management practices altogether. An example would be to include a forage or a winter cereal in your crop rotation.

One strong indicator that HR is a problem is having weeds in your field that haven’t been controlled by herbicide applications. Sending weed seeds from these plants to be tested for HR will help verify the issue and manage it with integrated weed management tools. If the HR test comes back positive, another herbicide option or other cultural control method will need to be considered to continue to manage the surviving weed species. Keep in mind that weed seeds need to be mature and dry to test for HR; green or high moisture content weed seeds will not germinate. Often the time to harvest your weed seeds is right before desiccation or combining – depending on the weed. For example, wild oats often drop their seeds before combining starts, while kochia takes a while to mature and dry down enough for a good seed sample.

Tips on collecting weed seeds for herbicide resistance testing:

• Plan ahead! Fall months come quickly and you might miss the window you need to collect the weed seeds at the ideal time. Plan early which fields you want to collect weed seeds from before harvest starts so you don’t forget.
• Keep tabs on plant maturity. It will be hard to collect seeds if they've dispersed on the ground.
  • Wild oats: usually reach full maturity by mid-August. Mature seeds can range in colour from black to yellowish-brown.The best germination is attained from dark seeds.
  • Kochia: can vary in maturity timing depending on when it germinated. Look for darkened seeds before collecting as green seeds will not germinate for testing purposes.
  • Green foxtail: look for mature plants in late August and early September. Mature seeds are pale in colour.
  • Lamb’s quarters: early maturing plants will produce seed by early August into fall. Seeds will be brown or black when mature.

  

• Use an appropriate collection method for the target weed.
  • Sweep net – wild oats
  • Handheld vacuum – windborne seeds such as thistle
  • Pillowcase - kochia
  Watch Clark Brenzil’s YouTube video for a demonstration on collection methods and sampling tips.
• Collect enough of a sample. Make sure there are enough seeds in case some spoil or don’t germinate at the lab. Target collecting 2,000 seeds.
  • Wild Oats – one coffee can full
  • Smaller seeded weeds – small hot chocolate container full
  • Please do not send seeds in a plastic Ziploc bag! The sample can be killed, spoiled from moisture, or put into dormancy and will not germinate in the lab.
• Collect seeds before a desiccant is applied. If you want to submit seeds for testing, they must be collected before burn-off chemicals are applied, as these can interfere with seed germination and may result in false negatives during testing.

Download the Herbicide Resistance Submission Form.

More information on herbicide resistance in the prairies, including herbicide resistance maps, can be found on the Prairie Weed Monitoring Network online, which is a network of federal, provincial and academic weed scientists and specialists committed to generating new knowledge on troublesome weeds across the Canadian Prairies.

If you have questions or would like more information on herbicide resistance testing, please contact your local crops extension specialist or the Agriculture Knowledge Centre at 1-866-457-2377.