Stacking both pre-emergence herbicides and cultural control measures will be essential for getting the best control of grassweeds this autumn, as the countdown to another cropping year begins. Louise Impey reports.

Growers need to embrace cultural control techniques to keep on top of problem grassweeds such as blackgrass as it is no longer possible to use just herbicides.

These techniques, such as delayed drilling, must be used to reinforce pre-emergence herbicide programmes as every bit of help is need to control these grassweeds.

Nick Myers, head of crop production at distributors ProCam, says delaying drilling and creating stale seed-beds where weeds can be controlled are key in the battle against blackgrass.

See also: Beating blackgrass needs a multi-track strategy

“Be patient and be prepared to be flexible. Stale seed-beds – as many as time allows – are an important first step. Take out the following successive flushes of weeds with glyphosate, to reduce weed numbers almost immediately,” he says.

Low dormancy

Fortunately, blackgrass seeds ripened in warm, dry conditions this summer so they should have low dormancy. “That will be helpful. If there’s moisture in late August and early September, the weed seeds should germinate quickly and evenly,” he adds.

Stale seed-beds must provide good seed-to-soil contact for this to happen. “It must be a seed-bed, not just a quick flick over, and it should be consolidated. And you can use them more than once,” Mr Myers says.

Delaying drilling winter wheat until October is important for two reasons. “Not only does it allow you to deal with the weeds that emerge before drilling, but the pre-emergence chemistry will work better at this timing,” he says.

“Soil moisture is a key requirement for getting the best from pre-emergence treatments, and that’s more likely to be present in October. Drilling into warm, dry soils in September is not conducive to getting good residual herbicide performance,” Mr Myers adds.

Many growers stack pre-emergence herbicides, which means applying three, four or five active ingredients before the weeds emerge, and these treatments must be applied before the blackgrass appears.

Flufenacet (as included in Crystal and Liberator) is the most active pre-emergence molecule and should form the base of all programmes, Mr Myers recommends.

“That should give you around 60% control. Then you can add others accordingly, with each one offering a further 10-15% control,” he adds.

So to get into 90% control and over, that is a minimum of three actives. “There’s a choice between DFF (diflufenican), pendimethalin, prosulfocarb and flupyrsulfuron. There’s also flurtamone,” Mr Myers adds.

Applied as granules

Avadex (tri-allate) is another option, although this herbicide is applied as granules in a separate operation.

It doesn’t matter whether the Avadex or the flufenacet mix is applied first. “The order makes no difference to the efficacy. The most important factor is that both are applied before the blackgrass comes up,” he says.

Prosulfocarb (as in Defy) is often added to Liberator. “It fits into the pre-emergence slot alongside flufenacet very well. Another option for this slot is flupyrsulfuron.”

Growers also have the choice of making two applications of flufenacet, providing they are done six weeks apart.

“There’s the opportunity to apply 0.6 litres/ha of Liberator followed by 0.3 litres/ha. Contact materials can be added to the second spray, if blackgrass plants are already showing,” he says.

Avadex worked well in autumn 2013. “Like Defy, it is one of the better choices in dry conditions. It has a sensitising effect,” he says.

Spending up to £90/ha on pre-emergence chemicals, before the crop is through, may seem expensive to some growers, acknowledges Mr Myers.

“The wheat price is under pressure, so this level of expenditure can seem excessive. However, we can’t achieve sustainable results unless we front-load programmes and put more emphasis on residual chemistry,” he says.

“Remember, too, that we need a good, competitive crop to provide additional weed suppression,” Mr Myers adds.

The nozzle of choice

Application of the pre-emergence mix requires good coverage of the soil surface. “It gives better efficacy of the herbicides,” says ProCam’s Nick Myers (pictured).

He believes that Syngenta’s Defy nozzle has a role at this spray timing. “It’s an angled nozzle and if they are alternated forwards and backwards along the spray boom, they give a complete coverage, including any clods.”

Their ability to limit drift also gives more spray days. “And that’s very important where timeliness is needed,” Mr Myers adds.

Independent advice on spray timing

A delay in the application of the pre-emergence spray is the biggest mistake that growers make, believes Lincolnshire-based independent agronomist Sean Sparling.

“Get the timing right. It has to be true pre-emergence,” he says.

That means getting the first spray on within three to four days of drilling. “If the roots of the weeds have time to grow away from the soil zone that contains the herbicide, it’s too late,” Mr Sparling adds.

His preference is for a Liberator/Defy mix, with Avadex applied, too. “Avadex has been a key component of programmes this year and has shown the resilience needed to sit on dry soils. Most of the other pre-emergence herbicides are very dependent on soil moisture,” he adds.

Consolidating the seed-bed is important. “If you roll twice, it’s better than once. An uneven soil surface won’t help,” Mr Sparling says.

A key point is not to reduce water rates. “It’s all about coverage with the pre-ems, so use 200 litres/ha,” he adds.