Herbicide-resistant blackgrass is threatening the future viability of winter cropping in some fields in the UK and, while the situation is much less severe in broad-leaved weeds, managing resistance is crucial to avoid another blackgrass, writes SRUC senior weeds consultant Mark Ballingall.
Herbicide-resistant broad-leaved weeds can be found in England, Scotland and Northern Ireland with chickweed, poppy and mayweed increasingly showing resistance to the ALS inhibiting herbicides, including sulfonylureas.
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What is herbicide resistance?
Resistance is defined as the inherited ability of a weed to survive a rate of herbicide that would normally kill it. In broad-leaved weeds, two resistance mutations have been identified. Both cause target-site resistance, which occurs when the mutation in the weed blocks the site specifically targeted by the herbicide’s mode of action.
Which weeds are resistant?
In Scotland, resistant chickweed can be found on more than 40 farms in 13 counties. The problem stems from an over-reliance on sulfonylureas, and their previous application without a tank-mix partner.
- Chickweed, poppy and mayweed can be resistant
- All three species have target-site resistance
- Only use sulfonylureas with an appropriate tank-mix partner and at robust rates
- Consider using pre-emergence treatments
- Plan to spray while the weeds are still small
The target-site mutation known as Proline 197 is responsible for resistance in chickweed. This confers resistance to many sulfonylureas, but not to florasulam.
In England, resistant poppies have now been confirmed in nine counties. Again, the overuse of sulfonylurea herbicides is to blame.
In contrast to chickweed, both of the target-site mutations have been found in poppies, meaning that more herbicides are affected and won’t work where both mutations exist, including florasulam.
Furthermore, the first cases of herbicide resistance in mayweed have now been found. In 2010, three farms in Yorkshire were found to have mayweed populations that proved resistant to metsulfuron, although there was no cross-resistance to florasulam.
Which herbicides can be used?
Not all of the plants that survive herbicide treatments are resistant. Often the reason the treatment hasn’t worked is because the herbicide rate applied was too low.
Even where resistance has been confirmed, there’s no need to dismiss the sulfonylureas, providing they are used at robust rates with a tank-mix partner.
For resistant chickweed, there’s an option to switch to products containing florasulam, as these herbicides are still effective.
Fluroxypyr is another choice, as are the hormone herbicides, such as mecoprop.
The options for controlling ALS-resistant poppies are more limited. Once the ioxynil/bromoxynil products have gone, there will be a gap in the armoury.
Again, sulfonylureas can still be used. Rates must be kept high and a good tank-mix partner used. The mecoprop/dicamba mixes are useful for this purpose.
MCPA is also a useful active for controlling poppies. If timing is an issue, certain products can now be used up to growth stage 39 in winter wheat.
Another option is to increase the reliance on pre-emergence herbicides, using pendimethalin-based products or an Extension of Authorisation for Minor Use (EAMU) covering the use of Liberator (flufenacet + diflufenican).
What else can be done?
Where resistance is suspected, it’s important to send seed or plants off for testing. Target the weeds with herbicides while they are still small, before growth stage 30-31.
Avoid using sulfonylureas on their own, even where there are no resistance concerns.
What about the future?
Preserve the chemistry that already exists by using tank mixes and sequences. While there are some new products in development, they are mostly mixtures of ALS inhibitors and hormone herbicides, which work in the same way as the existing choices.