21 July 2025
The results and knowledge gained from testing suspect weed samples will benefit the entire tillage sector, writes Vijaya Bhaskar of Teagasc Oak Park.
Resistance testing reveals which herbicides work best on your weed population, helping to prevent the further spread of resistance. This valuable service, offered by Oak Park, is currently free for a limited time as part of the DAFM-funded EVOLVE project.
To date, herbicide resistance has been confirmed in five grass weeds (Italian ryegrass, blackgrass, spring wild oats, annual meadow grass, rough-stalk meadow grass) and four broadleaf weeds (poppy, chickweed, common field-speedwell, corn marigold) in Ireland.
Farms with uncontrolled weeds or suspect resistance that have not yet sent seed samples or used this service may have some questions. Here are answers to some common ones:
Q1: What causes herbicide resistance and why test for it?
Herbicide resistance develops when the same herbicide, or those with the same mode of action, are used repeatedly in a field year after year. Susceptible weeds are killed, but resistant ones survive, multiply, and add seeds to the soil seedbank. Testing helps determine if resistance is developing and whether current weed management methods remain effective or need to be changed. Monitoring resistance is also critical to assess the size and severity of the problem nationally.
Q2: How can I tell if herbicide failure is due to resistance?
First, rule out other causes such as reduced rates, sprayer issues, or spraying at the wrong time. If you’ve gone through all these checks and the target weeds still received standard herbicide rates under good conditions but populations remain high, it’s important to test for resistance. All blackgrass and Italian ryegrass should be considered resistance suspects – even if they appear for the first time on your farm.
Q3: What is the process for testing weed samples submitted to Teagasc?
When seed samples arrive, each is assigned a unique identifier based on species, population number, county, and sampling year. Testing begins after 4-7 weeks, following dormancy-breaking treatments for some weeds. Samples are prioritized based on their quantity and quality.
Young plants grown from these seeds are sprayed with recommended rates of post-emergence herbicides, including ACCase, ALS, hormone, and glyphosate, at the 2-4 leaf stage for grass weeds or the 4-6 leaf stage for broadleaf weeds. Survival is visually assessed 30 days after spraying and compared with known sensitive or resistant populations. Genetic testing is also conducted to confirm the type and mechanism of resistance present.
Q4: How is testing information used to tailor weed management?
The test results from glasshouse screening and genetic testing allow us to provide growers/agronomists with tailored feedback. This includes confirming if the population is resistant, identifying the type of resistance, and recommending the most effective herbicide chemistry, if available, to use alongside integrated approaches. Combined with knowledge of field weed pressure, this information guides the necessary actions to eliminate or control resistant populations, including adopting zero-tolerance measures.
Q5: What is the sampling protocol and where should samples be sent?
For grass weeds, collect ripe seeds that easily fall off the seed head when brushed, stroked, or shaken into a paper envelope. For broadleaf weeds, collect ripe seed heads or capsules into a paper envelope. Collect seeds on dry days; if damp, let them dry before sending. Avoid using polythene bags – if used, transfer seeds to paper envelopes immediately. Never store or transport samples in polythene bags.
Large seed quantities are needed to test all herbicides active on that weed.
Be sure to complete the herbicide resistance testing form, including field history, or samples won’t be tested.
Send dry seeds in a paper envelope with the completed form to: Vijaya Bhaskar, Teagasc Oak Park Crops Research Centre, Carlow, R93 XE12 or deliver by hand to Teagasc main reception, Oak Park, Carlow.
For queries or if you want us to collect your population, contact Vijaya Bhaskar (M: 087 060 9409) or Gelu Crisan (M: 086 011 3138).