Dow AgroSciences is advising growers to avoid tank mixing residual herbicide carbetamide to propyzamide for blackgrass control in oilseed rape this autumn.

This is because company trials data shows that the same level of weed control can be achieved by using propyzamide (Kerb Flo) alone or in sequence, or mixed with a contact graminicide.

Blackgrass trials, in which carbetamide and Kerb Flo were applied, show that Kerb Flo achieves the best control at any dose, according to Andy Bailey, Dow’s principal biologist.

The full dose of Kerb Flo at 2.1 litres/ha gave around 85% control of blackgrass, whereas a full dose of carbetamide of 3.5kg/ha gave 60% control.

In addition, a tank mix of Kerb Flo with the contact graminicide Laser (cycloxydim) gave the best control of over 92% – similar to the mix of two residuals each at full doses.

In another set of trials, Aramo (tepraloxydim) and Kerb Flo gave 97% control of blackgrass.

The results are from trials carried out at two sites – Cockle Park Farm at the University of Newcastle and Loddington in Leicestershire – from the harvest seasons of 2008 and 2009.

Timing

Based on previous experiences in the field, Mr Willmott recommends avoiding the temptation to make early spray applications.

“Many went on early last year and for that reason they got quite a lot of crop damage,” he says.

“Both residual products will get broken down quicker in warmer soils. However, they also need some moisture to work. That’s why we favour applications from Bonfire Night (November 5) onwards.”

Dow recommends that Kerb Flo should only be applied when the soil temperature at 30cm is 10C and falling, and the soil moisture deficit is less than 50mm and falling 

Commenting on the results, Mr Bailey says: “We found that if you apply a tank mix which equates to a full rate – half rates of carbetamide and propyzamide – we got poorer results than if we applied Kerb Flo alone.”

He strongly discourages growers from applying two full rates of both residuals. “By doubling up on the residual component in a tank mix, you are, in effect, increasing the potential risk of these herbicides finding their way into watercourses,” he says.

Strutt and Parker agronomist Jock Willmott reckons this season most growers in heavy blackgrass situations will be looking to tank mix a contact graminicide, such as Laser (cycloxydim) with propyzamide.

But if this option is not providing adequate control of blackgrass, then they could try to sequence in another product, such as carbetamide.

“Propyzamide tends to give better results and works better on seed-beds which are not as cloddy, whereas carbetamide generally works better on open seed-beds,” says Mr Willmott.

Lincolnshire Farmacy agronomist Philip Vickers says using a full rate of Kerb Flo, sometimes with a contact graminicide such as Laser, is his preferred strategy.

However, in situations where it is inappropriate to use a contact graminicide with Kerb Flo – for example, when the leaf has been too wet – he has applied a tank-mix of carbetamide and propyzamide.

“In a heavy blackgrass situation where you’re not getting the control, this mixture works well,” he says.

“But I never recommend applying the products in more than a full loading of the chemical combination as it goes against stewardship guidelines.”.