Spot blackgrass areas for sampling, growers advised

Many growers were reportedly shocked at the levels of blackgrass in their fields last summer. Herbicide resistance was partly to blame. Sarah Henly finds out how to minimise the risk of it developing.

Now is the ideal time to assess how well your herbicide strategies have worked this season, and to formulate plans for the autumn, suggests James Clarke of ADAS Boxworth.

It is in June that blackgrass heads. Resistance is continuing to increase – the number of heads in herbicide-treated crops last June was greater than the previous season.

“Identifying priorities for sampling and weed seed testing is a vital part of resistance management,” Mr Clarke says. “Only when you know what you are dealing with, can you adopt appropriate tactics to limit the spread of resistant populations.”

The completion of a four-year LINK project, with a levy contribution of £168,664 (total project cost £1m), has helped to ensure growers are as informed as possible on managing the risk cost-effectively, he adds. Guidelines were disseminated last summer as HGCA Information sheet 03.

Appropriate control measures are changing – the pendulum is swinging towards cultural measures such as crop rotation and ploughing, stresses Mr Clarke.

“The value of chemical control to the overall strategy, while still vital, has diminished due to increasing cases of the weed overcoming some of the most effective products. And there are no new active ingredients on the horizon promising to help. If we lose the battle against blackgrass, it could cost the industry more than £500m a year,” he fears.

Most worrying is the increase in target site resistance to Atlantis (mesosulfuron and iodosulfuron) and other sulfonylureas – now confirmed in 132 cases across 21 counties. But the other two types, enhanced metabolism and ACCase target site resistance, are still more common.

Where blackgrass is present, it’s pertinent to adopt a herbicide strategy offering minimal risk of herbicide resistance development. That involves using pre-emergence herbicides to reduce the need for “higher risk” post-emergence products, and relying less on groups active on a specific target site; for example the sulfonylureas, fops and dims, he urges.

If blackgrass is surviving treatment, consider testing. Rapid tests are available for each of the three types of resistance, which can occur independently, in different plants within a single field, or even within the same plant. The most appropriate test depends on which products you have used.

The project has provided guidance on sampling strategies to enable reliable interpretation. Around mid-July, collect different batches of dry weed seeds once they are ripe, he recommends.

“Growers shouldn’t assume a sample taken from one field on the farm will be representative. One sample is likely to be OK for a patch, but be prepared to take several samples within a patchy field or across the farm,” says Mr Clarke.

The outcome will determine how much rotation or cultivation/drilling practices need modifying, to support herbicide programmes. Ploughing and delaying drilling of winter cereals both reduce weed numbers, and higher seed rates of more competitive varieties suppress weeds better.

Furthermore, where resistance is confirmed, prevent blackgrass spreading around the farm by hand rouging, cutting or spraying off patches. Clean machinery and clothing before moving to new areas, and avoid using contaminated straw and manure, he concludes.

Final report available soon on the HGCA website: www.hgca.com/research

Summary

Project no. 3035: Integrated management of herbicide resistance; ADAS, Rothamsted Research; BASF, Bayer CropScience, Dow AgroSciences, DuPont, Syngenta Crop Protection under a Defra Sustainable Arable LINK; from April 2005 to April 2009.

HGCA perspective:

  • Find sustainable herbicide resistance management strategies
  • Identify and target effective herbicide applications
  • Integrate with cultural practices to slow resistance development
  • Understand in-field weed sampling requirements to monitor spread
  • Develop rapid test for ALS herbicide resistance

Crops perspective:This work has identified the numerous mechanisms of herbicide resistance in grassweeds, and highlighted the best ways to monitor and counteract resistant populations in the field. Now it is up to growers to minimise the ever-increasing risk of a blackgrass take-over by giving higher priority to cultural control.

Types of resistance in UK blackgrass

  • Enhanced metabolism – affects most herbicides. Commonest form, though mildest effect
  • ACCase target site resistance – affects fops (eg, Cheetah), dims (eg, Grasp) and dens (eg, Axial). Very common
  • ALS target site resistance – affects sulfonylureas (eg, Atlantis). Least common, but increasing