Tips on getting the most from pre-em cereal herbicides this autumn

Extracting the maximum efficacy and value from pre-emergence herbicide treatments is vital in the fight against blackgrass, as well as helping controlling costs when margins are tight.

But just as cultural control methods are subject to variability and have their limitations, so do residual herbicides. Yet there are steps that growers can take to mitigate this variation by reducing blackgrass numbers and ensuring the environment is as conducive as possible for activity.

See also: How to create the perfect blackgrass stale seed-bed

The starting point must be to determine as accurately as possible the number and position of seeds present in the seedbank by calculating the seed return in the past three years based on control levels and cultivations, explains Hutchinson’s Dick Neale.

Only when that figure is known can the level of efficacy likely to be obtained from the components of the pre-emergence programme be predicted and decisions made as to which products, additional cultural controls and rotational options should be employed to reduce the blackgrass numbers.

“If you don’t reduce the level of blackgrass you present to the herbicide, you will extract poor value from the herbicide.”

Standard practice when winter cropping cereals in blackgrass situations should be drilling from mid-October onwards, allowing for stale seed-beds and avoiding the peak germination period, thus reducing the number of blackgrass plants that could emerge in the crop and are expected to be killed by the residual chemistry.

The remaining flush that is likely to come through should be below 200 plants/sq m, and even at these figures average control from a single pre-emergence is only 70%, says Mr Neale.

Stacks or sequences of active ingredients will be needed to cope with higher numbers but when levels approach 300 plants/sq m control levels will not be adequate, he warns.

“Only when we get down to 100 plants/sq m can we begin to achieve between 85-100% from chemical input alone.”

Further reduction of plants that will emerge in the crop relies on exploiting crop competitiveness which will enhance overall control. This relies on growers making realistic assessments of crop establishment based on planned drilling date and seed-bed conditions.

Growers often underestimate the crop establishment at later drilling dates, and base rates on 80-85% survival rather than 65-70%, which Hutchinson’s work from Brampton has shown to be a more realistic figure on heavy soil when sown in the second half of October.

“Seed rates need to be high to compete with the blackgrass with 460-500 seeds/sq m required.”

That means up to doubling seed rates in winter wheat or making choices such as hybrid barley to outcompete the blackgrass. Hyvido hybrid winter barley is in a league of its own for blackgrass control, but seed rates above 250/sq m are not required and become too costly, he states.

The increased cost in seed in either of these situations should be valued as herbicide spend rather than seed cost.

“View the extra £40/ha as an additional herbicide that is equally as effective as Avadex (tri-allate), but ensure seed-beds are good enough to minimise establishment losses.”

Optimising activity

Getting the most from residuals is imperative, especially where post-emergence products no longer deliver adequate control. NIAB and Rothamsted trials (see graph below) have shown consistent improvements in herbicide performance with delayed drilling as cooler, wetter conditions are more conducive for the activity of residual chemistry, explains Niab Tag’s weed specialist John Cussans.

pre-em chart

But he stresses that despite a correlation between moisture levels and performance, the value of the increased activity is difficult to quantify.

In trials this year at the NIAB blackgrass site in Cambridgeshire, delaying by a month achieved the same level of control with a pre-emergence as spending an additional £120/ha to achieve that control in the early drilling, which equates to a tonne of wheat.

But this is not consistent and is down to the weather, which can be so variable, he cautions. “In some years it’s possible to control any population, in other years it’s not possible at any level of spend.”

While additional factors such as good seed-bed quality and correct application will help performance, getting the timing right will have a bigger impact on control. Pre-emergence herbicides work better when they are applied to weed seeds that are pre-germination so if you can’t spray it, don’t drill it, says Mr Neale.

“If you delay past seven days you will walk away from 40% of the best control you would have got that year.”

Product choice

Flufenacet is the most efficacious active and remains the cornerstone for control. Research at Rothamsted has demonstrated that over seven years of use its efficacy has drifted from 82%-70% but appears to have stabilised.

Yet it needs protecting, otherwise it could end up like pendimethalin, whose activity has been compromised due its wide use, says Mr Neale. However, pendimethalin can still deliver 50% control in a RR resistant population, making it an important component of a stack.

Defy can also be used to protect other herbicides, adding up to 30% control, as can Avadex, although its contribution is unlikely to be much more than 15%, he adds.

Stack or sequence?

The decision to stack or sequence should come down to how the crop has been established and the quality of the seed-bed.

The move back to ploughing has brought issues in term of seed-bed moisture retention, especially in dry autumns, as well as bringing up populations of unknown quantity and dormancy, which can take anywhere between a month and two years to break, explains Mr Neale.


  • Avadex tri-allate
  • Defy prosulfocarb
  • Liberator diflufenican + flufenacet
  • Stomp Aqua pendimethalin
  • Vigon diflufenican + flufenacet + flurtamone

The loss of moisture in the top 5cm germination zone results in situations where September and early October flushes are missed and the blackgrass seeds germinate in a more protracted manner across the autumn and early winter period some weeks after the crop is drilled.

Front end loading all the herbicides can, if it turns wet, be aggressive on the crop and reduce competiveness. Plus by the time the ploughed up blackgrass breaks dormancy herbicides are already a month old and have lost a lot of efficacy, resulting in poor control.

In this situation Mr Neale advocates spreading out the application in a sequence, applying a pre-emergence at drilling, followed up with another application 10-14 days later and either 10-14 days again or up to six weeks later, as is permitted with a second application of Liberator.

The best treatments at Brampton this year in a plough based seed-bed served to show how overall management of blackgrass allows for flexibility in choice of herbicide stack and timing.

An application of 0.6 litres/ha Liberator pre-emergence followed by 0.5 litres/ha Vigon 10 days later; a pre-emergence stack of 0.6 litres/ha Liberator + 2.0litres /ha Stomp Aqua + 2 litres/ha Defy; or a 0.6 litres/ha Liberator + 15kg/ha Avadex pre-emergence all gave 80-85% in a crop of hybrid barley.

“Although this level of control is not adequate to prevent a blackgrass population from continuing to grow the barley is continuing to suppress seed production and viability in the blackgrass,” says Mr Neale.  

“These additional stack or sequences added 25% control to the base 0.6 litres/ha pre-emergence of Liberator.”

But, Mr Neale warns growers that trials results need to be interpreted with care, because each season, location and population is different, and growers must observe what works best on their farm and experiment to find the most workable solutions for them.

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