Crops should be managed in a way to prevent weed infestations, overuse of herbicides and herbicide resistance. But in the real world, growers will use the most convenient and economical practices until a crisis forces a change. Routine monitoring of weed populations for their resistance to herbicides can avert such a crisis. Making resistance testing protocols widely available can help to attain this goal.
The journal Weed Science reports on methods of testing for herbicide resistance. The authors of a review article discuss testing techniques, including field surveys, plant sampling and dose-response levels. Rapid resistance confirmations are contrasted with more thorough DNA-based tests.
The first report of herbicide resistance came in 1957, but resistance has significantly expanded in the past decade. Today more than 200 plant species worldwide are resistant to herbicides. Modern agriculture depends on resistance testing to determine the best resistance management and mitigation. There are basic steps in testing, but details may vary depending on the herbicide mechanism of action, weed species and timing of application.
The classical approach to resistance confirmation is to collect seeds from surviving plants in suspected fields, plant them in pots and apply appropriate herbicide(s). Multiple doses, rather than a single dose, should be used. This will allow the production of a dose-response curve that will show the magnitude of resistance. The test should include three to four replications to confirm results. It can take up to 2 months to obtain results using this form of testing.
Early and rapid detection of resistance is crucial to averting economic losses. Growers seek resistance tests that allow them to make real-time decisions regarding current weed management. Quick analyses may use whole plants, seedlings, or leaf discs rather than growing new plants from seed. A leaf disc analysis, for example, may yield an initial determination of resistance on the spot or within 24 to 48 hours.
However, unlike the classical approach, these new methods are not applicable in all situations. Different tests may be needed for different weed species or herbicide mechanisms of action. Some of these rapid testing techniques are being developed for commercial use. But most growers currently rely on free testing services provided by academic, extension service, or public research institutions.
Full text of the article, “Review: Confirmation of Resistance to Herbicides and Evaluation of Resistance Levels,” Weed Science, Vol. 61, No. 1, January-March 2013, is available here.