2 November 2001

Blackgrass beaten with last minute herbicide switch

A LAST minute change in autumn herbicide strategy helped a Herts wheat grower boost blackgrass control last year.

A delay in spraying fields with known resistant blackgrass prompted Martin Liddell and his agronomist to switch from a pendimethalin programme to one based on contact-acting herbicides.

"Because we had such a wet autumn, we didnt get the fields in question drilled until mid-October," he recalls. "They were then due to get the blackgrass spray applied on Dec 11, by which time the weed had reached a reasonable size."

After consulting ProCam Agricultures Philip Spencer, he decided not to rely on the residual activity of a pendimethalin mix. Instead, the pair opted for Hawk/isoproturon (clodinafop-propargyl + trifluralin/IPU), to get the most from contact action.

"The result was the best blackgrass control we have had on this farm for a while," says Mr Liddell. "We were very busy lifting sugar beet at the time, so a contractor did the spraying."

In previous years blackgrass control at Shrubbs Farm, Shearing was based on IPU. But a gradual build-up of the weed and poor spraying results made further investigation necessary.

A seed test revealed enhanced metabolism resistance as the problem.

"We used to grow continuous wheat," says Mr Liddell. "Blackgrass was never a big problem while we could burn straw, but after that it seemed to escalate. Now we have a 200-acre block of bad blackgrass."

That type of resistance is common in the area, making both cultural and chemical control important, says Mr Spencer. "A good rotation helps, as does using a false seed-bed and glyphosate. A pre-emergence treatment is another good tactic.

"Its also vital that the blackgrass is small, at the 2-3 leaf stage, when it is sprayed. Theres no point trying to control it in spring, when it has tillered."

Avadex (tri-allate) granules were introduced to bad areas four years ago, and IPU has either been replaced or is used with a partner product.

A Lexus/Stomp (flupyrsulfuron-methyl/pendimethalin) mix was applied two years ago, and the plan this autumn is to go for a Hawk/Lexus mix.

Cultural control is also used. All the cereal land is ploughed, except where it follows oilseed rape.

Blackgrass is always left to germinate in the rape, so it can be controlled by chemistry with alternative modes of action. Wheat drilling tends to start in the third week of September, allowing time to spray off the first flush of weeds beforehand. &#42

Pleased with late herbicide switch last season are ProCams Philip Spencer (left) and Herts farmer Martin Liddell.

SHRUBBS FARM

&#8226 Proven herbicide resistance.

&#8226 Delayed residual programme.

&#8226 Change to more contact action.

&#8226 Best control for some time.