21 June 2002

Frustration as PSD holds up new herbicide

By Charles Abel

ON SHOW, but not available for use this autumn – that was the frustrating story behind O4H herbicide project plots at Cereals 2002.

Developed by Aventis CropScience, the O4H winter wheat herbicide project is based on two new sulfonylurea herbicides, mesosulfuron and iodosulfuron, with safener mefenpyr-diethyl, as used in Cheetah Super.

Visitors to the companys plots saw promising activity on key grassweeds and broadleaved weeds. But extra data required by the Pesticide Safety Directorate means O4H will not be launched in time for this autumns herbicide season.

With the French equivalent already on offer as Atlantis that is frustrating, admits project manager Nick Duncan.

"What other herbicide controls blackgrass and Italian and perennial ryegrass, plus wild oats and annual and rough-stalked meadowgrass, and at this level?" he asks. "Unlike ipus 75% control, or Lexuss 85%, O4H achieves up to 99% control.

"It offers a stepwise advance, making it the pinnacle of sulfonylurea herbicide chemistry. It is for sulfonylureas what Opus is for the triazoles," he says.

But that powerful activity has its downsides. While O4H will control grassweed populations resistant to current herbicides it runs the risk of resistance problems itself.

"Its Achilles heel is that because it is so highly effective it will select for resistance, so growers will need to use it in sequences and tank mixes to protect its activity," says Mr Duncan.

"We have seen indications of reduced activity on some populations in the lab, so we know efficacy will drop if it is not used properly."

O4H will need to be preceded by a pre-emergence herbicide, at normal rates to guard against resistance and adverse conditions later in the season.

The post-em tank should then include a residual herbicide to help extend persistence, which is comparable with Lexus, enhance speedwell, poppy, pansy and cleavers control and reduce resistance risks.

Optimum activity is when weeds have one to two leaves, promising more flexibility on spray timings than Lexus and Hawk, adds Mr Duncan.

&#8226 A straight iodosulfuron product for spring broadleaved weed control is also planned, plus a formulated mix with imidosulforn (as in Eagle) to boost cleavers control. &#42

A new standard in grassweed control, but not for sale this autumn, says 04H project manager Nick Duncan (left). Control will include blackgrass populations that are resistant to current herbicides.