Resistance research addresses future black-grass control solutions

Resistance to key herbicides continues to spread in black-grass populations, although the rate of resistance development varies between active ingredients. Resistance demonstrated under laboratory conditions may not yet have translated to the field.

Container trials, conducted as part of government-funded research into sustainable grass-weed control, show how black-grass had developed resistance to pre-emergence residual herbicide flufenacet, currently the most important herbicide in the black-grass control armoury, within five years.

Results from the trials show a decline in performance of 5 per cent per year.

However, according to Rothamsted Research grass-weed expert Richard Hull, these results had not yet been found to apply in field conditions.

Speaking at the BCPC Weed Review, Mr Hull described the development of resistance to flufenacet as ‘the really bad news’.

But, he said: “The cheerful part is it appears in the field we are not selecting for resistance.”

In other work, results from random surveys of black-grass populations conducted between 2002 and 2011 revealed a significant increase in development of resistance to Atlantis (iodosulfuron+ mesosulfuron) over the period.

"There is quite a lot of resistance to Atlantis out there now and there is a complete jumble of mutations and resistance mechanisms. But that won’t make much difference to the advice we could give to farmers - the advice we could give doesn’t change depending on the resistance mechanism,’ he said.

The enhanced metabolism resistance mechanism was probably affecting Atlantis to a greater extent currently than target site resistance.

"We now have populations of black-grass that have enhanced metabolism resistance which are just as resistant as target site resistance populations.”

Research under way at Rothamsted was also seeking to determine what effect the relatively recent practice of heavy stacking of residual herbicides might have on future resistance development.

Outlining possible future solutions to black-grass control, Mr Hull presented trials results showing the value of delayed drilling, switching black-grass infested land into spring cropping and tailoring herbicide applications to weed emergence patterns.

"Delaying autumn drilling (from mid-September to early October) gave a 39 per cent reduction in black-grass plants, but planting a spring crop instead of September-drilled wheat reduced plant numbers by 88 per cent,” he said.

Commenting on the timing of herbicide applications, he said: “We don’t really need to worry too much about black-grass plants emerging in the spring; there aren’t really too many of them and they don’t set too many seeds.”

In trials conducted in 2011, 93 per cent of black-grass had germinated by the end of November, even in the dry autumn of that year.

Herbicides for black-grass control should be applied as early as possible in autumn, at the two-leaf stage of the weed, said Mr Hull.