Cesar Cappa, OMAFA Weed Management Specialist – Horticulture (cesar.cappa@ontario.ca)

As spring settles in across Ontario and growers prepare for planting, a persistent challenge continues to emerge: herbicide-resistant weeds. These tough-to-control species are showing up in more fields each year, threatening crop productivity and complicating weed management efforts.

Thanks to support from industry partners and agricultural organizations, free molecular herbicide resistance testing is once again available in 2025. This service allows Ontario farmers to submit leaf tissue samples and receive quick, accurate confirmation of herbicide resistance, often within 10 business days.

Why Test for Herbicide Resistance?

Herbicide resistance occurs when weeds survive herbicide applications that once worked effectively. When standard treatments start to fail, it’s easy to suspect resistance, but visual inspections alone often aren’t enough to be sure. Adding to the complexity, some species like waterhemp can develop resistance to multiple herbicide groups simultaneously. These stacked resistances are really challenging to detect without lab testing.

Molecular testing offers critical confirmation quickly, that will allow to adjust the weed control program.

The Advantages of Molecular Testing

Molecular testing offers several key benefits:

• Highly accurate detection of known resistance mutations.
• Faster results than traditional greenhouse bioassays (approx. 10 business days) — there’s no need to wait for the plants to go to seed.
• Species differentiation, including hard-to-tell members of the Amaranthus and Brassica family.

Frequently Asked Questions

Can I test any weed species for any herbicide?

Not every combination is available. However, the list of tests is extensive and growing. View the current options available in the submission form available at:

Can the test help identify plant species?

Yes. The testing can help distinguish between similar species. Currently, testing is available to distinguish Brassica species (mustard, canola) and Amaranthus species (amaranth, pigweeds and waterhemp).

It’s important to understand that herbicide resistance tests are species-specific. If the wrong species is selected when submitting samples, the results will not be valid. If you’re unsure which weed species you’re testing, request species identification along with the herbicide resistance test. This is especially important for pigweeds, which are often difficult to distinguish from one another.

Can one sample be tested for resistance to multiple herbicides?

Yes, a single sample can be tested for resistance to multiple herbicide groups at once and can also be used to confirm the weed species, if applicable. It’s recommended to request all available tests for the species in question. It’s not uncommon to detect resistance to multiple groups in single plants.

How much does it cost?

The testing cost is $15 to 40 per individual sample and test. Thanks to funding from industry partners, the testing fee is fully covered for Ontario growers.

Just make sure to check “Yes” under the section “Samples to be Billed to OMAFA Survey Program” on the submission form.

Where can I see herbicide resistance trends in Ontario?

An interactive database summarizing resistance by region is coming soon. Stay tuned for updates from OMAFA.

Will my farm data be made public?

No. Individual farm information will not be published or shared. Regional summaries (no finer than the county level) may be used to support province-wide reporting and research.

How to Submit a Sample

Submitting samples correctly is essential for accurate results. Samples must be dried in silica gel and stored in special envelopes included in the testing kits.

You can order kits and find detailed protocols here:

How many samples should I submit?

There are two possible approaches when sampling:

Large sample: multiple plants

If you’re unsure how widespread herbicide resistance is in a field, collecting multiple samples, ideally 10 or more, of the same weed species can make a big difference.

This is especially important for genetically diverse weeds like waterhemp. Testing several plants helps estimate how many carry one or more resistance traits. Relying on just one or two samples could miss the troublemakers entirely, underestimating the real resistance challenge.

Small sample: fewer plants

If sampling multiple plants isn’t practical, or resistance is clearly widespread, a single or small number of samples can still offer valuable insights. Just keep in mind: fewer samples mean a higher risk of missing the full picture if resistance isn’t dominant in the population.

Need more information? Contact Cesar Cappa at cesar.cappa@ontario.ca