Weed resistance under the spotlight

Most farms routinely spraying against blackgrass are now affected by herbicide resistance to some extent. Resistance in Italian ryegrass, wild oats and several broad-leaved weed is spreading, Northern Ireland has recently experienced its first case of weed resistance, and the first cases of scentless mayweed resistance have also been recorded.

Presenting the first update on the extent of the UK’s herbicide-resistant weeds since 2005, Rothamsted Research‘s Stephen Moss told delegates that about 20,000 farms regularly sprayed against blackgrass. Of those, more than 16,000, in 34 counties, had blackgrass that was resistant to at least one herbicide, some experiencing significant reductions in product activity.

Until 2004 only 2085 farms, in 31 counties, were so affected, Dr Moss noted. “Now it [blackgrass resistance] covers most of England. There’s no resistance positively confirmed in Scotland – but it is highly likely.”

Blackgrass resistance to the widely used Atlantis (mesosulfuron + iodosulfuron), introduced in 2003, had been confirmed on more than 400 farms in 26 counties.

That did not mean, however, that all plants were resistant or that the product no longer worked on those farms, he stressed. “Most farmers are not seeing total field failures. There seem to be few fields where Atlantis fails completely, but reduced activity is becoming more common and this trend will almost certainly continue.”

In the latest survey, resistance in Italian ryegrass, first seen in 1990, had been found on 450 farms in 33 counties – up from 324 farms in 28 counties by 2004.

“It’s not such a big problem as blackgrass at the moment, but this could change as we become increasingly dependent on high-risk herbicides.”

Wild oat resistance had increased slightly less, being found on 250 farms in 28 counties – the 2004 figures were 218 farms in 26 counties.

At first sight that seemed odd because control depended largely on high-risk herbicides, such as the so-called “fops, dims and dens”, said Dr Moss. But, in contrast to blackgrass, the resistance tended to be fop-specific, he explained. “It doesn’t seem to extend to the dims and dens, so herbicides like Axial and Laser still usually work well.”

Isoproturon, only recently withdrawn, was also quite effective against wild oats, he noted. “It may have been limiting resistance build-up more than we thought.”

The picture could change, however, with increasing dependence on high-risk ACCase-inhibiting graminicides such as Axial (pinoxaden), and ALS inhibitors such as Atlantis and Broadway Star (pyroxsulam + cloquintocet-mexyl + florasulam), he warned.

In the latest findings, chickweed resistance occurred on 40 farms in 13 counties – up to 2004 it had been confined to 15 farms in 11 counties. It cropped up most often in Scotland, but had also been detected in 2008 in County Down, noted Dr Moss. “It’s the first instance of a resistant weed in Northern Ireland.

The main reason for its northern bias, he suspected, was cropping. “There’s more spring barley growing there and so greater dependence on sulfonylureas.”

Poppy, the most important herbicide-resistant broadleaved weed in Europe, was also increasingly surviving treatment with sulfonylurea herbicides, he noted. Resistance had been found on 25 farms in nine counties – up from 11 farms in seven counties by 2004. “It needs watching. We’ve also seen our first three cases of resistance in scentless mayweed – all to metsulfuron – and all in Yorkshire. It’s also cropping up in Germany, but there it is in scented mayweed.”

As yet there had been no confirmed resistance to glyphosate (as in Roundup) in the UK. “Most of the cases worldwide are associated with GM crops, and in France and Italy it’s nearly always in vineyards or orchards.”

Countering criticism that recent results, mainly from fields where herbicides had given poorer than expected control, could over-estimate the extent of resistance, Dr Moss pointed out that the findings closely matched those of more random surveys in blackgrass and Italian ryegrass.

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