Top tips to manage herbicide-resistance risks in arable weeds

Herbicide resistance is an irreversible process – once present, it does not disappear (even if the herbicide is no longer used). However, you can slow its development, in rotations dominated by autumn-sown cereal crops, by following these tips.

• Manage resistance threats – even before herbicide efficacy declines
• Reduce reliance on herbicides – a change of herbicide policy alone is unlikely to provide adequate lowering of resistance risk
• Adopt non-chemical control methods
• Maximise the benefit from pre-emergence herbicides, which are associated with partial and relatively slow resistance development
• Place less reliance on post-emergence herbicides – regular use of ACCase-inhibiting and ALS-inhibiting herbicides is associated with a high risk of herbicide resistance
• Where possible, use lower resistance risk post-emergence herbicides in the rotation in oilseed rape and beans
• Note that residual herbicides require moisture and an even seedbed for good efficacy, and most do not perform well in high organic matter (over 5%) soils
• Where soil does not cover crop seed (e.g. in no-till establishment systems), do not apply herbicides until the crop has established.
• Use mixtures and sequences of herbicides with different modes of action to delay resistance
• Be aware that although the use of higher resistance risk herbicides with lower-risk modes of action will help reduce weed populations, this will not prevent further selection for resistance
• Follow the manufacturer’s recommendations on the label (e.g. for optimum dose and spray timing/application)
• Take note of label restrictions – introduced to reduce risk of herbicide resistance – on the sequential use of both ACCase-inhibiting and ALS-inhibiting herbicides
• Avoid treating if heavy rain is forecast, in waterlogged or frosty conditions, or if crop is suffering nutrient stress
• Note that resistance can vary considerably between and, to a lesser extent, within different fields
• Closely monitor herbicide performance to guide management
• Conduct regular – at least once every two to three years – resistance tests on seeds or plants

Regular monitoring will help provide a strong indication of the unique resistance profiles present across a field.

It is much cheaper to stop resistance developing, so spotting the early signs is vital.