Oilseed growers are being urged to look at applying residual herbicides such as Kerb and AstroKerb despite dense crop canopies.

Both products can be used through until the end of January as long as soil temperatures are below 10C and falling, and soil moisture deficits are below 50mm and falling.

Although some growers have questioned how effective these largely blackgrass products will be on thick lush crops, the manufacturer Dow AgroSciences says control is not impaired.

A full oilseed rape crop canopy at the time of application makes no difference to the level of blackgrass control, says Dow’s oilseed rape agronomist David Roberts.

This is because the active ingredient in Kerb, propyzamide, is quite soluble and is readily washed off canopy leaves and on to the soil where the residual herbicide remains active.

At this time of year, conditions are generally good to maximise the efficacy of propyzamide as long as a sprayer is able to travel, Mr Roberts adds.

He advises that if growers are still concerned about the level of weed control, then they can wait until frosts have opened up the crop canopy before applying these residual herbicides.

Kerb can be applied in frosty weather, but Dow advises not spraying on to frozen ground if rain is forecast as this could result in run-off into watercourses.

Earlier this year, Dow introduced AstroKerb which added the contact herbicide aminopyralid to propyzamide to give added control of poppies and mayweed.

For this product, Dow advises growers to wait until frost is off the weed-leaf and the leaf is dry before spraying.

Both herbicides can be mixed with a number of fungicides for control of phoma and light leaf spot, such as prothioconazole.

“The ultimate decision to spray, however, should be based on local conditions, including the ability to travel and consideration of the potential risk to watercourses,” says Mr Roberts.

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