Blackgrass battle set to intensify, AICC members hear

Controlling blackgrass in wheat can only get harder, delegates to an AICC conference were warned on Thursday [8 January]. Pre-emergence treatments will become much more important, and growers will need to pay even more attention to crop competition and herbicide resistance.

Ultimately, it may even be necessary to resort to the modern equivalent of horse-hoeing – spraying broad-spectrum weed-killers between the rows, and perhaps using satellite technology to hit weeds within them, suggested TAG director Jim Orson.

“We shouldn’t panic, but we’ll need to embrace new technology,” he said. “We’ve got to think ahead – we can’t afford to wait trying to sustain a system that can’t be sustained.”

Outlining the list of active ingredients likely to lose their approval in the next few years, including pendimethalin, he said: “The cupboard is beginning to look a bit bare. We’ll be left mostly with pre-emergence herbicides, not good news because people like to see their weed problems before spraying.”

Easing the pressure from blackgrass by maximising crop competition would become more important. Three times as many blackgrass plants could survive in a thin crop as in one sown at the optimum competitive rate, Mr Orson said.

“The higher the population [of blackgrass] the lower the percentage control. If we’re going to rely more on pre-ems we’ve really got to do something about base populations.”

Pointing to the growing problem of resistance, now widespread, Rothamsted Research‘s Stephen Moss likened it to a tractor and sprayer nearing a cliff edge beyond which herbicide control would no longer be possible.

“The question is how near to the resistance cliff are we?” he said.

Already 133 cases of resistance to Atlantis (iodosulfuron + mesosulfuron) had been detected in 21 counties, Dr Moss noted.

Even more worrying was an exercise conducted last season collecting 18 seed samples from blackgrass in fields on 10 farms where customers had expressed themselves “satisfied” with the results.

Only 28% of the plants grown from samples were susceptible to Atlantis. “And on eight of the 10 farms we could detect resistance even though there had been no complaints.”

The farms were geographically widespread, he added. “So it’s very common. Does Atlantis resistance disappear if you stop using it? Basically the answer is ‘no’. It’s a one-way ticket.” Three years’ trials had confirmed that, he said.

Far too few growers tested for resistance. “We’d like them to think ahead and test to show when the writing is on the wall – even where they think Atlantis is working well.”

Trials with various herbicides and mixtures on a field at Chalgrove with high levels of resistance showed how hard it was to achieve satisfactory control. “We got 50-70% from pre-ems, but only 39% from Atlantis,” said Dr Moss.

Even the best mixture offered only 87% control. “The main point is how difficult it is to get 90% control.”

But yield responses to the best control – of the average of 26 blackgrass plants/sq m – was surprisingly good at almost 5t/ha. “That’s nearly five times what some experts would tell you to expect.”

Of that response, 70% came from the flufenacet in Crystal, Dr Moss believed. “Flufenacet is going to be very important, especially as Atlantis resistance builds up.”

Tests applying diflufenican, pendimethalin, prosulfocarb and flufenacet at 75% field rates to susceptible, partially resistant and highly resistant populations showed the latter was consistently the best performer. Control from the others fell to less than a third on the highly resistant populations.

But even flufenacet might be susceptible to resistance, tests using outdoor containers at two sites over three years suggested. “There has been is a trend down in control,” noted Dr Moss.

At Peldon in Essex it slipped from nearly 89% to just under 80% over that period.