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Grassweeds 5: Resistance management

Course: Grassweed management in cereals | Last Updates: 7th October 2015

Stephen Moss
Secretary of the UK Weed Resistance Action Group
Biography >>

Keeping on top of grassweeds, especially blackgrass, is getting harder on many UK farms.

Indeed some infestations are now bad enough to put a question mark over the future of winter cereal growing.

To fight back successfully, given the fact you can no longer rely on a stream of new, more effective herbicides, it is vital to try to learn more about resistance and how to manage it. Once resistance has developed it does not go away.

What causes resistance?

Resistant blackgrass multiplies quickly until it dominates.

Herbicide resistance develops through small numbers of plants that initially survive treatment going on to seed and progressively building up a resistant population.

It is an evolutionary process, the herbicide selecting out naturally occurring plants with resistance mutations that then increase and spread. With repeated selection the resistant plants multiply quickly until they dominate. Just one blackgrass plant can easily shed 50,000 seeds/sq m.

Resistant populations do not typically get noticed until at least 10% of the population is resistant with a corresponding drop in herbicidal control. However, the selection process may have been going on for many years previously.

Unlike cross-pollinating blackgrass and ryegrass, wild oats are self-pollinating which may explain why resistant plants tend to be confined more to patches and the problem is not spreading across the country to the same extent.

How does resistance work?

There are three known ways in which blackgrass and other grassweeds can shrug off the effects of herbicides.

  • Enhanced metabolism resistance

EMR is the commonest type of resistance mechanism and it affects most herbicides to some extent. Resistant plants are able to detoxify the herbicide to varying degrees. It tends to increase fairly slowly and only in extreme cases does it lead to total loss of weed control.

  • ACCase target site resistance

ACCase target site resistance blocks the site of activity specific to most so-called "fop", "dim" and "den" herbicides. These include: Clodinafop, fenoxaprop, fluazifop-P, propaquizalofop, quizalofop cycloxydim, tepraloxydim and pinoxaden. This mechanism affects only these active ingredients, but it often results in very poor weed control. It can also increase rapidly if these types of herbicides are used annually as the main means of weed control.

  • ALS target site resistance

ALS target site resistance blocks the site of activity of sulfonylurea herbicides, such as flupyrsulfuron and mesosulfuron + iodosulfuron, and the related pyroxsulam. It can lead to poor control. To date it is less common than ACCase target site resistance, but it is increasing.

All three mechanisms can be at work in the same field. With Atlantis (mesosulfuron + iodosulfuron), for example, EMR is becoming a bigger issue because it is so widely used, and there have been some quite high levels of resistance.

Is the blackgrass challenge increasing?

Over the past 25 years blackgrass has doubled the number of seed heads/plant in wheat crops to about eight on average.

That has implications for seed return and the rate of increase in blackgrass populations, as well as the spread of resistance.

HGCA-funded work is under way to find out what may be causing the increase in seed heads and to update the blackgrass life-cycle model.

The main reason is probably that farmers are sowing earlier, giving the blackgrass more time to tiller in the autumn and hence produce more heads per plant.

The trend to lower seed rates may also have created less competition from the crop giving the weed more room to grow. It is also possible that climate change may be involved.

What is the impact of fewer herbicides?

The loss of some well-known weedkillers could exacerbate resistance.

Isoproturon (IPU) was always affected by resistance but not to a great degree.

Trifluralin was cheap, unaffected by resistance and useful in herbicide mixtures.

The biggest impact following their withdrawal will be in areas where blackgrass is not a major problem. An IPU/trifluralin mix was an effective, cheap option on many farms.

With those chemicals gone many growers have switched to Atlantis or other higher resistance risk herbicides. The danger is that they could now run into increasing control problems, but be unaware this is happening. In the meantime resistance will continue to rise.

How can you influence resistance?

The main practical step growers can take to counter resistance is to reduce reliance on herbicides that are prone to resistance. This is easier said than done, but crop rotations can extend the range of herbicides that can be used.

Carbetamide and propyzamide, both of which can be used on oilseed rape for example, are as yet unaffected by resistance so they stop the selection process.

To avoid encouraging resistance, do not rely on herbicides affected by ACCase target site resistance or ALS target site resistance as the main means of blackgrass control in successive crops. Their use in mixture or sequence with other lower resistance risk herbicides will help with control which is, of course, a useful benefit, but will not prevent further selection pressure.

Pre-emergence herbicides, for example flufenacet, pendimethalin, prosulfocarb and tri-allate, are all affected by EMR, but generally only to a limited extent.

Products or programmes based on combinations of these active ingredients usually give useful control, and current evidence is that resistance to them does not build rapidly. So they have a valuable role in managing resistance.

With any herbicide it is vital to pay attention to the recommended rate, nozzle choice, water volume and spray timing to maximize its effectiveness.

How do you build an effective anti-resistance strategy?

The essential element in any anti-resistance strategy is to look beyond herbicides – they should be viewed as just one part of a more comprehensive strategy.

An effective strategy should include:

  • A crop rotation to allow the use of alternative chemical controls
  • Ploughing, where appropriate, to reduce numbers of resistant weed seeds in soil surface
  • Stale seed-beds to make the most of non-selective herbicides
  • Delaying drilling for the same reason
  • Increased crop competition to reduce weed seed return

Golden rules


  • Consider introducing more non-cereal crops and spring sowing
  • Think about ploughing – even if only rotationally
  • Consider fallowing fields with very bad grass-weed infestations
  • Be prepared to delay autumn drilling
  • Experiment with a combination of cultural tactics
  • Monitor resistance development through seed tests
  • Consult WRAG guidelines


  • Rely on herbicides alone for grassweed control
  • Rely on high resistance herbicides (fops/dims/dens and sulfonylureas) for blackgrass control
  • Ignore the significance of isoproturon and trifluralin withdrawal if previously dependent on them
  • Ignore the value of pre-emergence herbicides
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