Course: Weed control | Last Updates: 10th October 2015
Herbicide-resistant blackgrass is threatening the future viability of winter cropping in some fields in the UK, and while the situation is currently much less severe in broad-leaved weeds, managing resistance is crucial to maintain their control.
Herbicide-resistant broad-leaved weeds can now be found in England, Scotland and Northern Ireland, where their numbers are increasing.
The weed species affected – chickweed, poppy and mayweed – are increasingly showing resistance to the ALS-inhibiting herbicides, which includes the sulfonylureas, and can now survive treatment with this type of chemistry.
So what is the current situation and extent of the problem? Also, how can growers reduce the risk of resistance developing in their fields, and if they are present, how should they be managed?
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 by researchers. 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.
It usually results in complete resistance to herbicides acting on that target site. This is why the presence of either or both of the two identified mutations allows the affected broad-leaved weeds to survive treatment with most ALS inhibitors.
Unlike grassweeds, there is no evidence of enhanced metabolism resistance in broad-leaved weeds.
Which broad-leaved weeds are resistant?
In Scotland, the main threat is from resistant chickweed. More than 40 farms in 13 counties are officially known to be affected, although it is suspected the problem is far more widespread than the official figures indicate.
The problem stems from an over reliance on sulfonylurea herbicides, and their previous application without a tank-mix partner that works with a different mode of action.
The target-site mutation known to weed scientists as Proline 197 is responsible for resistance in chickweed. Where it is present, there is resistance to the sulfonylureas such as metsulfuron and thifensulfuron, but not to florasulam.
In England, where winter cereals dominate rotations, resistant poppies are the main issue.
First detected in 2000, resistant poppies have now been confirmed on more than 25 farms in nine counties of England, mainly in the east of the country. Like chickweed, the overuse of sulfonylurea herbicides is to blame.
But in contrast to chickweed, both of the target-site mutations have been found in poppies – Proline 197 and Tryptophan 574 – meaning that more herbicides are affected and won’t work where both mutations exist, including florasulam.
Furthermore, since the last official compilation of figures on resistant weeds in 2005, the first cases of herbicide resistance in mayweed have 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.
Work continues on mayweed, as there is plenty of anecdotal evidence the problem is spreading.
Which herbicides can be used?
Not all of the plants that survive herbicide treatments are resistant. In about 90% of suspected cases, 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 (at least three-quarters rate) with a tank-mix partner, they still have a place in weed control programmes and will be effective on other target weed species.
For resistant chickweed, there's an option to switch to products such as Spitfire or Galaxy, which contain florasulam. As there's little evidence of the Trytophan 574 mutation in chickweed, florasulam-containing herbicides are still effective.
Fluroxypyr is another choice, as are the hormone herbicides, such as mecoprop.
The options for control of ALS-resistant poppies, however, will be more limited. Once the ioxynil/bromoxynil products have gone, with their alternative mode of action, there will be a gap in the armoury.
Again, sulfonylureas can still be used, providing care is taken. 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, making good use of pendimethalin-based products such as Stomp Aqua or an Extension of Authorisation for Minor Use (EAMU) covering the use of Liberator (flufenacet + diflufenican) at a lower rate in the spring.
As with all pre-emergence treatments, seed-bed conditions must be good and moisture must be present for them to work effectively.
What else can be done?
Where resistance is suspected, it’s important to send seed or plants off for testing. The results will confirm which herbicides can be used and give direction for a suitable control strategy.
Just like grassweeds, targeting the weeds with herbicides while they are still small gives the best chance of success. That means making spray applications before GS30-31.
It's also essential to avoid using sulfonylureas on their own, even where there are no resistance concerns. Their widespread use across a whole range of crops has created the predicament that many growers are now facing.
What about the future?
With more active ingredients under threat, it's critical to preserve the chemistry that already exists by using tank mixes and sequences.
While there are some new products in the development pipeline, they are mostly mixtures of ALS inhibitors and hormone herbicides, which work in the same way as the existing choices. As a result, their stewardship will be essential for future success.
Cultural control methods can be employed in the spring barley crop, with the use of higher seed rates and more competitive varieties helping to suppress weeds.
- 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
- Galaxy – clopyralid + florasulam + fluroxypyr
- Liberator – flufenacet + diflufenican
- Spitfire – florasulam + fluroxypyr
- Stomp Aqua – pendimethalin
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