20 March 2001
Resistance threat to grassweed control

By Tom Allen-Stevens

THE first case of a British weed resistant to one of the most important groups of herbicides has prompted fears for the future of a whole range of sprays.

Researchers at IACR Rothamsted found that samples of chickweed seeds from a field in Scotland had target site resistance to Ally (metsulfuron-methyl).

The scientists discovery has thrown into question the entire future for the group of sulfonylurea herbicides known as ALS inhibitors.

They include products such as Eagle (amidosulfuron), Lexus (flupyrsulfuron-methyl) and the new grass weed herbicide Monitor (sulfosulfuron).

Until now, grassweeds resistant to ALS inhibitors were confined to Australia.

But there has been growing concern that British growers are becoming increasingly dependent on sulfonylureas for grassweed control.

Most sprays about to hit the market are ALS inhibitors. The fear is that there are no new modes of action to take their place if resistance becomes widespread.

“We are more concerned about grassweed control,” said Dr Stephen Moss, principal research scientist at IACR Rothamsted.

“There is a potential risk that resistance may develop, especially if the use of isoproturon is restricted and growers increase their reliance on sulfonylureas.”

Dr Moss is also secretary of the weed resistance action group (WRAG). But he played 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 broad-leaved herbicides in mixtures and in sequences.”

It was Ally manufacturer Dupont which brought the case to light.

Duponts Martyn Rogers said that the Scottish grower in question relied solely on Ally for his broad-leaved weed control.

“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.”

Recent figures show that no one surveyed uses Lexus on its own. “We always encourage its use in a mixture,” said Mr Rogers.

WRAG advice to growers is to monitor the use of their herbicides and consult their agronomist or the manufacturer if they have any concerns.

ALS inhibitors, which interfere with amino acids and halt cell division in the target weed, are favoured herbicides due to their low dose rate.

They have good efficacy and low environmental or operator toxicity.

This is only the fourth field in Europe where target site resistance in chickweed has been found – the spread is not thought to be fast.

Other broad-leaved sprays for chickweed include hormone sprays like CMPP and MCPA, HBN sprays, like bromoxynil and ioxynil, and fluroxypyr (Starane).

Ally and Starane are thought to be the two most widely used brands of herbicide in the UK. About 400,000ha of cereals are treated with Ally every year in the UK.