Resistance to herbicides now rampant – IACR

26 November 1999

Resistance to herbicides now rampant – IACR

Improved weed control with

less impact on the

environment was the theme

of this years British Crop

Protection Council

conference in Brighton,

which included sessions on

genetically modified herbicide

tolerant crops, new herbicides,

better use of existing products and herbicide resistance.

Over the following pages

Charles Abel, Andrew Blake

and Andrew Swallow report

the highlights

HERBICIDE resistance is now so widespread in grassweeds that no grower in the UK should assume their farm is resistance free.

That is the view of IACR – Rothamsted weed ecologist Stephen Moss, based on resistance testing data. "The message is that resistance is very widespread with black-grass; we have largely stopped counting."

Wild oat and Italian ryegrass resistance to herbicides is also increasingly widespread, and no farm in the country should assume it is resistance free, even if no resistance has been recorded in that county.

"The important thing is to get a sample tested if herbicide activity is sub-standard. Talk to your crop consultant or advisor about how to do this. It is £25 well spent first so growers know what they are dealing with," he stresses.

"For example, if a grower found he had target-site resistance then he would not want to use any fop or dim chemistry. But if resistance is found to be only enhanced metabolism, some fops and dims could give quite good control."

The data was collated by Dr Moss from resistance results supplied to him by all companies and organisations conducting resistance tests.

"We are not saying every farm has got resistant blackgrass, but a lot of farms have had no tests done at all."

The new "R" system for rating herbicide resistance simplifies categorisation of resistance for all weeds, including blackgrass, wild-oats and ryegrass, continues Dr Moss.

"The star system had too many categories and wasnt appropriate for all weeds. The R system should be easier for growers to understand," he says.

Resistance ratings

Old system New R System

5*/4* RRR: Resistance

confirmed, highly

likely to reduce



3*/2* RR: Resistance

confirmed, probably

reducing herbicide


1* R?: Early indications

that resistance may be

developing, possibly

reducing herbicide


S S: Susceptible.

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