Win the battle against resistant blackgrass

Cereal growers trying to control blackgrass need to start by understanding why it has become such a problem, advises Stephen Moss of Rothamsted Research.

To do this, they should cast their minds back and look at how wheat drilling dates have changed over time, he suggests.

“We’ve seen the amount of winter wheat drilled in September move from just 5% in the 1970s to almost 60% today,” he reveals. “That’s played right into the hands of blackgrass, as most of it emerges in September or October within crops, rather than before drilling.”

This autumn-emergence pattern would not be as relevant if herbicides still worked well on the weed, he acknowledges. “But they don’t. Resistance to them is widespread and is still increasing, so control from them will continue to decline.”

Added to this, no new active ingredients are expected in the near future, while some existing herbicides are likely to be withdrawn for regulatory reasons, he adds.

Another key consideration is that shallow, non-inversion tillage tends to favour blackgrass, as weed seeds are retained in the surface soil layer from where they can readily emerge.

“These factors combined show why non-chemical control methods, which can be used with herbicides, are important and should be included in any programme.”

Dr Moss outlines five key points that should be used to formulate an integrated control strategy.

“The first is weed emergence. Some 80% of blackgrass emerges in the autumn – something which is very relevant where spring cropping is to play a role.”

His second point is that weed seeds emerge from the top 5cm of soil. “If they’re buried deeper than that, they don’t come through.”

Third, blackgrass seed longevity in the soil means it declines by 74% a year, he says. “This is an important point where fallows are being considered. One year of fallow is not enough.”

Population dynamics is his fourth point. “You need to get 95% control just to stand still. That’s why herbicides alone won’t be sufficient and will have to be backed up.”

His final reminder is that blackgrass is a very competitive weed. “This means growers should be aiming for less than five plants/sq m if they want to avoid yield loss. Once that figure is more than 10, wheat production is not sustainable.”

Work on the different contributions made by cultural control techniques has shown that they can be used effectively, but will usually require combining with others for best results and to reduce the reliance on herbicides, he says.

Average contribution from cultural control methods

  • Ploughing – gives an average 69% control by burying weeds seeds and its rotational use has potential 
  • Delayed drilling – this allows the use of stale seed-beds and helps give an average 31% control of blackgrass. The longer the delay, the better the results, but the greater the risk.
  • Higher seed rates – increasing the crop seed rate creates more competition and gives an average of 26% control. The higher the seed rate, the better, but lodging issues have to be considered.
  • Competitive varieties – choosing a competitive variety will help smother emerging blackgrass and gives an average of 22% control.
  • Spring cropping – an effective method of reducing blackgrass, giving an average of 88% control, but can be difficult to achieve on heavy soils.
  • Fallow or grass ley breaks – providing no new seeding takes place, these can give a 70-80%/year reduction in the seed bank, making them valuable techniques. A one-year fallow or break is not enough to substantially reduce blackgrass infestations – two or even three years are much better.

Dr Moss also suggests leaving some unsprayed patches in blackgrass-infested fields. “These can be very useful in quantifying the level of control you have achieved.”

This strategy also shows the true situation on any individual farm, he adds. “For reasons we don’t fully understand, some herbicides work better than others on certain farms. And some techniques vary enormously from year to year.”

His colleague Richard Hull has been involved in a three-year research project on the role of drilling dates and seed rates in reducing blackgrass populations.

“The risks associated with these have also been examined,” he explains.

By far the biggest effect on blackgrass numbers has come from drilling date, he reports. “Higher seed rates only had a marginal effect.”

Three sowing dates – mid-September, early October and late October – were used in five field trials, he says.

“In four of the five sites there was significantly less blackgrass where drilling was delayed into October. It didn’t matter which of the October dates was used – there was a huge benefit of more than 2t/ha from drilling in the first week of October.”

Only in a very dry autumn did the same number of blackgrass plants come up at all three dates, he says. “That’s very relevant. If you can’t get a chit and then spray it off, the drilling date doesn’t work as well.”

Furthermore, the pre-emergence herbicide programme applied in the trial gave better results at the later drilling dates, he says. “There was more soil moisture then, so the control from the pre-emergence herbicides was better. This was a very consistent effect.”

He advises that the pre-emergence programme should always have flufenacet in it, at full rate.

“Later drilling has two benefits,” he concludes. “You get fewer blackgrass plants in the crop and better pre-emergence control. Another point is that blackgrass which emerges in September has 40% more heads, so is more damaging than later-emerging plants.”

Higher seed rates and delayed drilling can help control blackgrass.

Case study: Andrew Ward, Lincolnshire

Range of techniques is needed

Making the decision to spray off a winter wheat crop midway through the season isn’t an easy one, admits Lincolnshire grower Andrew Ward.

But it is just one of a range of techniques that he has employed on his 650ha farm at Leadenham, near Lincoln, to help get resistant blackgrass under control and prevent further weed seed return.

“It’s a case of short-term pain for long-term gain,” he explains. “We’ve been spraying bad blackgrass patches off for the past four years, and it seems to be working. We’ve got less blackgrass on the farm this year than we’ve had for quite a while.”

Last year, Mr Ward resorted to making silage out of more than 40ha of blackgrass-infested wheat. “In that particular case, we had already spent £560/ha on the crop,” he recalls. “But there wasn’t a satisfactory alternative if we were going to be able to get things back on track. The weed pressure was just too great and our control efforts had failed.”

The ground was then fallowed for the rest of the year, to allow successive flushes of blackgrass to be controlled with glyphosate, before being put into spring barley.

“I can make more money from a good crop of spring barley than I can from any wheat crop with poor blackgrass control,” he says. “And it allows me to use cultural control methods to best effect, to bring weed numbers down.”

For this reason, winter barley has been ruled out, due to its lack of effective blackgrass control opportunities, he says.

As well as the introduction of spring barley – which Mr Ward has done across the farm’s three different rotations as necessary – wheat drilling has been delayed where possible and stale seed-beds have been used extensively, he says.

“We’ve had to become more flexible,” he says. “This year, instead of using post-emergence herbicides, any blackgrass is being pulled up by hand by a gang, to limit its spread.”

Delayed drilling has thrown up some challenges, especially on the heavy land, he admits. “We ended up having to drill one block in the second week of December this year at a seed rate of 500 seeds/sq m, which was later than I wanted. But it did allow us to achieve three kills of blackgrass beforehand.”

It didn’t, however, receive any pre-emergence herbicides, as it rained heavily immediately after drilling, he recalls. “That block of land has had no herbicides whatsoever. Where there is blackgrass in any numbers, the bad patches have been sprayed off.”

Mr Ward has tried introducing ploughing, but has been quick to dismiss it. “It was a disaster. So much blackgrass came through, it made the situation worse.”

Instead, he prefers to use his Simba Solo immediately behind the combine, even where a spring crop is planned.

Pre-emergence herbicide use in wheat is carefully planned, with a Liberator or Crystal/Defy/DFF mix being applied after drilling, as well as Avadex granules. “We haven’t used any Atlantis on the farm this year. The money we would have spent on it has either gone to the gang of pullers or towards the cost of spraying off crop patches.”

Mr Ward is confident that his varied efforts are producing rewards. “There aren’t any quick remedies. We’ve seen a big reduction in blackgrass numbers, but we won’t be relaxing our guard just yet.”

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