Herbicide resistance is the inherited ability of an individual plant to survive the application of a herbicide that would kill a normal population of the same species. This means that a herbicide you used to spray no longer works and the weeds continue to grow.
What to look for
Not all plants will have the same resistance status across the paddock at the same time. Look for live green plants surviving amongst the dead plants after spraying. These plants are likely to be resistant. When the plants within the sprayed area all look similar, this could be herbicide failure rather than resistance.
Herbicide susceptibility testing
Rather than testing for herbicide resistance and what herbicides you can no longer use, you should test for herbicide susceptibility to find what herbicide options you still have.
There are two main methods of testing; seed collected from the suspect area prior to harvest or the collection of live plant samples during the season. As both of these methods require samples to be sent to a laboratory for the testing (with an associated cost), it is advisable to speak to your agronomist to help decide which herbicides to test for.
Herbicide resistance/susceptibility seed tests
This requires collection of suspect weed seed samples from the paddock prior to harvest. Once the seeds are collected from the paddock, they are sent to the lab where they are grown and sprayed with the specified herbicides to give answers relevant to your farming system. Results are normally available 3-4 months after harvest.
Collect seeds or seed heads from suspect areas where plants have survived the herbicide and collect a few seeds from a number of plants. Do not collect seeds/seed heads from a small number of plants as this may bias the sample. If the resistance is widespread, collect across the paddock or problem area in a W-shaped collection pattern.
Approximately 3000 seeds of each weed (an A4-sized envelope full of good seed heads) is required for a multiple resistance test. This equates to about one cup of annual ryegrass seed and six cups of wild radish pods.
Send samples in a paper envelope (not plastic) to either of these two commercial seed testing services:
- Peter Boutsalis, Plant Science Consulting
- John Broster, Charles Sturt University's Graham Centre for Agricultural Innovation
Herbicide resistance/susceptibility Quick-Test™
The Quick-Test™ requires the collection of whole plants collected from a paddock, rather than seeds. This eliminates the problem of seed dormancy and enables a far more rapid turnaround time. It is possible to get the results within 4-6 weeks. In addition, the tests are conducted during the growing season rather than out of season (over the summer). This allows effective management decisions to be made during the same growing season.
- Collect 50-100 plants per paddock or suspect area, aim to collect 20 plants per herbicide test.
- Carefully pull out plants and shake off soil from roots.
- Wrap in a couple of sheets of paper toweling.
- Too much paper towel can dry small plants out.
- Do not wash plants.
- Place the wrapped plants in a waterproof plastic bag, for example, a sealed sandwich bag.
- Aim to post at the beginning of the week so plants are received within the week.
Note: Quick-Test™ is only suitable for post emergent herbicides such as glyphosate or in-crop selective herbicides. To test for pre-emergent herbicide resistance, particularly trifluralin you must use the seed test.
Quick-Tests can be done with Peter Boutsalis, Plant Science Consulting.