Did your oilseed rape cultivations go deeper than 5cm? It could make a difference to whether a contact graminicide is needed to get the best from grassweed herbicide Kerb.

Achieving reliable control with Kerb has been the aim of three years trials at ADAS Boxworth, researcher James Clarke says. “In summary, we’ve found where grass weeds are shallow rooted, such as in a min-till situation, Kerb used alone gives very high levels of control.

“But where the weeds are emerging from depth, such as where the plough is used, then you need to add a graminicide to Kerb. In our work, we used Laser.”

The reason why Kerb needs help with deeper rooting weeds is that it sits in the top 5cm of soil, project leader Lynn Tatnell says.

“It doesn’t move at all, which is why it works so well if the ground is disced shallowly. But ploughing occurs to a depth of 10-20cm, so the weed roots may miss the herbicide zone entirely.”

More deeply

Where growers have reported disappointing results, she believes they are working the ground more deeply than they realise. “This is often the case with min-till cultivations. Growers call it shallow, but usually they’ve gone further than 5cm.”

Straw and trash on the surface made little difference to efficacy, she found. “Last autumn trash levels were very high, but Kerb Flo remained unaffected.”

As well as looking at soil conditions, Mrs Tatnell has also investigated a range of timings from September to February. “It seems that mid-November to early December is the optimum timing.”

The blackgrass must be up and there needs to be sufficient soil moisture for optimum performance, she points out. “Kerb needs moisture down to at least 8cm for its activation.”

Going too early risks warm and dry conditions restricting the activity of Kerb. “It also increases the chance of another emergence flush occurring. You also need to be aware of the likely dormancy of the weed seeds.”

Good results

Kerb has also shown good results against ryegrass in the Boxworth trials, notes Mrs Tatnell. “It seems as though it could be very useful for Italian ryegrass, which is becoming another highly resistant grassweed. It may well be as effective on resistant ryegrass as it is on resistant blackgrass.”

The Boxworth work confirms Dow AgroScience’s initial findings that consistency is achievable, the firm’s Rene Pollak says.

“The site has high levels of [fop/dim] target-site resistance,” he comments. “That usually means fops and dims don’t work, but what’s interesting is we got better control from Kerb when Laser was added, either in a sequence or a tank-mix. It shouldn’t give better results, but it does.”

He adds calendar date is not the issue with application timing. “Cold and wet conditions are needed for good Kerb performance. Cold gives the chemical time to get into the plant, while moisture is essential for it to work.”

That means growers may have to wait until January, he believes. “But by then the blackgrass plants are bigger, so the level of control will drop off. It’s a balancing act.”

Moisture is the predominant factor. “Wait for the wet. And then all the relevant VI restrictions apply to Kerb.”

Although Kerb used alone gives good control, Mr Pollak’s advice is to aim for the very highest levels of control. “If you want oilseed rape to be a cleaning crop, it’s worth adding a graminicide. But Kerb should be the pivot of the programme.”

Dow’s recommendation is to use a 2.1 litres/ha of Kerb Flo or 1.7kg/ha of Kerb WP rate in difficult weed situations. “We received approval for these higher rates because there is plenty of evidence that the higher the herbicide loading, the better the control.”

Mr Pollak ends by reminding growers that Kerb controls resistant wild oats and chickweed, too. “Used in the right way, it can really help to stem the rise of resistant weeds.”