New approach to blackgrass control in OSR

Adopting a similar mindset for blackgrass control in oilseed rape as in cereals will produce more consistent and better results, visitors to the ADAS Boxworth open day heard.

Using residual herbicides earlier in the season and bringing the first application of a new herbicide sequence forward can improve blackgrass control by up to 20% over current programmes, claimed Stuart Hill of Makhteshim Agan UK.

Making the first application of Crawler (carbetamide) in mid-September at a lower rate of 2.5kg/ha targets blackgrass when it is small and more vulnerable, sensitising the weed for the follow-up Cohort (propyzamide) treatment in November, he said.

“Applying carbetamide earlier at a lower dose may help to lessen the impact of leaching and its detection in water supplies,” he said. “Furthermore, it replaces the need for a contact graminicide, avoiding the use of active ingredients that are already affected by resistance.”

Previous advice of waiting until soil temperatures are at 8C and falling before applying any residual is outdated, he said.

“Oilseed rape crops are being established earlier, often by sub-casting, which results in blackgrass coming up from a range of depths. And as recent autumns have been warm and open, the blackgrass has been well-tillered and deep-rooted by November.

“On top of this, crop canopies tend to be covering the ground, getting in the way and making it more difficult to use later residuals effectively.”

Waiting for a frost to open up canopies can mean applications are not made until January, which is too late for big weeds, he warned.

The new approach targets the first part of the sequence at the Aramo timing, he admitted. “There’s no known resistance to either carbetamide or propyzamide, so it’s making better use of the best herbicides for the job.”

Mr Hill said getting residuals to last as long as possible had been the focus of previous advice. “They still work when soil temperatures are higher; they just don’t last as long in the soil. This could be a benefit in minimising the risk of heavy rainfall and any leaching.”

The first part of the sequence provides immediate activity, slowing blackgrass growth, he pointed out. “The oilseed rape crop is usually at the three to four-leaf stage in mid-September, but, crucially, the blackgrass is at the pre-tiller stage, with a shallow root system.”

Blackgrass control in cereals has moved to earlier timings using stacking of active ingredients, so this development should come as no surprise, he added. “Hitting blackgrass when it is small is the way to go, whatever the crop. The granule formulation of Crawler is well suited to earlier applications, as it will disperse easily in the warmer conditions.”

The propyzamide timing remains unchanged, he said. “But we are investigating the use of lower rates of that active ingredient too.”

Specified two-way tank mixes containing Crawler will be supported by MAUK at the earlier timing, as long as the crop is no smaller than the three to four-leaf stage, he said.

John Cussans of NIAB TAG said the new approach seemed to be a sensible solution. “Getting through vigorous oilseed rape canopies with a residual herbicide can be very difficult. And more consistent results on blackgrass must be every grower’s aim this autumn, after such a difficult year.”

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