Resisting arrest

31 March 2001

Resisting arrest

The UKs first case of a weed with resistance to sulfonylurea herbicides was confirmed last Friday. Tom Allen-Stevens explores the implications

ONE chickweed resistant to the sulfonylurea Ally (metsulfuron-methyl) might not at first appear to be much of a cause for concern to UK arable growers. But the discovery, by researchers at IACR Rothamsted in a field in Scotland, throws into question the future of this group of herbicides – known as ALS inhibitors.

This is the first case of target site resistance to ALS inhibitors recorded in the UK. As well as Ally, the group includes Eagle (amidosulfuron), Lexus (flupyrsulfuron-methyl), Boxer (florasulam) and the new grassweed herbicide Monitor (sulfosulfuron). They work by interfering with amino acids in the plant which halts cell division in the target weed.

Dr Stephen Moss, principal research scientist at IACR Rothamsted and secretary of the weed resistance action group (WRAG), carried out the testing on the resistant chickweed. He plays down the importance of this particular find in chickweed: "It needs to be put into perspective. This is one case in one field and is unlikely to be repeated everywhere. Growers should be aware of it, however, and ensure they use broadleaved herbicides in mixtures and in sequences."

Other broadleaved herbicides outside the ALS group, such as CMPP, bromoxynil and fluroxypyr (Starane), still give satisfactory control of chickweed, although they are being tested for cross-resistance.

The implications for grassweeds could be far more serious, however: "There is a potential risk that resistance may develop, especially if the use of isoproturon is restricted and growers increase their reliance on sulfonylureas," says Dr Moss.

No single case of grassweeds resistant to ALS inhibitors has yet to be found outside Australia. But there is concern that growers are becoming increasingly dependent on sulfonylureas for grassweed control. Whats more, most new grass- weed chemistry, not yet on the market, is sulfonylurea based. With no new modes of action to take its place, controlling blackgrass will become very difficult if sulfonylurea resistance becomes widespread.

So what can growers do to prevent this happening? It was Ally manufacturer DuPont who brought the chickweed case to light. DuPonts Martyn Rogers pins most of the problem down to the way the herbicide was used; the grower in question relied solely on Ally for his broadleaved weed control, year after year. "Farmstat figures show only 6% of growers do this in any one year, so on the whole most growers are using the herbicide more responsibly."

When Lexus was launched, the company was well aware of the more serious implications of sulfonylurea resistant grassweeds: "We dont recommend Lexus as a standalone product specifically to stave off any problems. We always encourage its use in a mixture or sequence," says Mr Rogers. Recent figures show that no-one surveyed uses Lexus on its own.

Mixing the chemistry is also advocated by UAP agronomist Brin Hughes: "Were constantly recommending mixes for blackgrass with at least two modes of action to help prevent resistance develop. A Lexus/Hawk (clodinafop-propargyl + trifluralin) mix would be a good example."

And when it comes to broadleaved sprays Mr Hughes suggests growers should avoid tank-mixes that restrict herbicide choice: "This Scottish grower had a tendency to apply manganese with the spring herbicide. So he couldnt tank-mix in a hormone herbicide such as CMPP."

The way forward may lie in using the best of the new chemistry along with some of the old favourites. Manufacturers Makhteshim specialise in taking up old chemicals and keeping their approval open by continuing with their trials and data collection. "Theres a definite move back to the older chemistry as the over-reliance on fops and dims has brought about so much resistance," says Makh-teshims Adrian Sison.

An example is ryegrass where many growers with resistance problems are turning back to Tolugan (chlorotoluron) and terbutryn to help control. "Blackgrass is a key place where the old chemistry fits in. Sulfonylureas will undoubtedly give you good control, but over-reliance on them may lead to resistance. Look around for suitable older tank-mix partners that will help defend this new chemistry. There are many more tools available than people realise," he says.

WRAGs action plan

&#8226 Crop rotation

&#8226 Cultivation – inversion tillage often helps

&#8226 Sterile seedbeds

&#8226 Crop competition from good, thick crops

&#8226 Prevent resistant weeds going to seed – especially in set-aside

&#8226 Sequences and tank-mixtures using alternative modes of action

&#8226 Avoid use of long, residual ALS herbicides

&#8226 Use short residual herbicides

&#8226 If you suspect resistance, monitor your herbicide use and consult your agronomist or the manufacturer

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