Wednesday, October 21, 2020

Vigilance needed in battle against blackgrass

September 24, 2020 by  
Filed under Crops

Growers are advised to keep a close eye on blackgrass and other grass weeds that show signs of resistance to pre-emergence herbicides this autumn.

Experts reported signs that blackgrass is developing resistance to some pre-ems last year. Other grass weed species are developing herbicide resistance too, says weed biology specialist John Cussans of NIAB.

With 80% of blackgrass samples from across the country showing resistance to mesosulfuron, fenoxaprop and cycloxydim, the most recent report by the industry-backed Blackgrass Resistance Initiative supports findings by ADAS and BASF.

“There is ubiquitous and widespread resistance in blackgrass,” says Mr Cussans. “More recently, we’ve confirmation of shifts in flufenacet sensitivity in field populations too. It’s a very worrying development.”

In a study conducted jointly with BASF, Bayer and Syngenta, a sample of problematic Italian rye-grass showed variable sensitivity to fluenacet and pendimethalin. Some 50% of the sample had resistance to Axial (pinoxaden) and Atlantis (mesosulfuron and iodosulfuron).

Monitoring and vigilance

“The take-home message here is to keep a close eye on populations of all grass-weeds,” advises Mr Cussans.

“There’s no doubt, we’re seeing a few plants here and there become much bigger patches in fields.  If you don’t have these weeds at the moment, it’s well-worth keeping an eye out for them and removing any odd plants that appear.”

Resistance testing blackgrass seed, mapping populations and monitoring surviving plants will provide the information for developing an effective weed control strategy. Growers should also consider where blackgrass seed is residing – on the surface or deeper in the soil.

“Knowing where the pressure is coming from as well as where the seeds are within the soil profile, is vital information for selecting the most appropriate weed management strategy.”

Delayed drilling remains a key strategy in the battle to control blackgrass.  There was two thirds reduction in blackgrass in 16 field trials where drilling was delayed for a month, says Mr Cussans. Growers should refrain from drilling crops until mid-October, he suggests.

Impact of rain

“On average, every week drilling is delayed in autumn there’s a 15% decrease in blackgrass.  We’ve not yet had a year when delaying drilling from September to October hasn’t reduced blackgrass plants – even in those years with low rainfall.”

The highest levels of control were achieved where there were high levels of rainfall between the early and late drilling dates. Where there was low rainfall before or around the first drilling date blackgrass germination was suppressed and control reduced.

“You can’t spend your way out of early drilling,” says Mr Cussans. “Delaying drilling coincides with better performance of pre-em herbicides – the increased chance of rainfall, increases the likelihood of conditions facilitate improved take-up of residuals.”

Trials suggest the same outcome can be achieved by drilling mid-September and applying Crystal (flufenacet and pendimethalin), Hurricane (diflufenican), Lexus (flupyrsulfuron) and Avadex (tri-allate) – or by waiting 3-4 weeks until mid-October and just applying Crystal alone.

Cultivation choice

The best control comes from a mix of strategies. Achieving 70% control is relatively cost-effective with herbicides. But achieving 97-98% control, which is needed to reduce populations significantly becomes disproportionality expensive with herbicides alone.

Cultivation decisions should be guided by how much blackgrass there is and where it is in the soil profile. Where fresh seed is on the surface, ploughing can reduce head counts by 80-94%. Where all blackgrass is in the seedbank, inverting the soil can produce 34-40% control.

“Where you’ve good control this year, with little fresh seed on the surface, direct drilling the next crop is likely to produce the greatest control. By keeping blackgrass seed deep within the soil profile for longer, you’ll reduce germination and let the seed-bank degrade.”

Spring cropping can significantly reduce blackgrass populations – and delaying drilling until mid-March can reduce populations further still. The difference between planting in mid-February and mid-April was around 80 plants/m².

But there is a trade-off with crop performance, says Mr Cussans. The sweet spot is around mid-March, where blackgrass seedling were reduced to nearly 40 plants/m2 with spring oats yielding nearly 6t/ha and spring barley, 6.5t/ha.

“Not all grass-weeds are as affected by spring cropping though – Italian rye-grass, bromes and wild oats, for example. Yes, there are reduction of plants in the crop, but the overall effect isn’t as profound as it is with blackgrass.”

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