Herbicide resistance is genetic within the plant and should not be confused with poor herbicide efficacy (e.g., poor weather, timing of application, spray quality, spray conditions etc.). Symptoms can include healthy plants beside dead plants of the same species, a susceptible species having poor control when other susceptible species are well controlled, discrete weed patches and a general decline over a number of years. The two main types of resistance mechanisms include target site resistance and enhanced metabolism
So far in the UK resistant populations of weeds have been found in:
- Italian-rye grass
- Wild oats
- Broad leaved weeds
Resistance has also been found by ADAS in an ALS resistant prickly sow thistle
Herbicide resistance tests are key to confirm resistance and help future management of resistant weeds by choosing the correct chemistry. These can be done by collecting seed from suspected resistant weeds and sending to ADAS who will test the seed for its resistance status.