Herbicide-resistant blackgrass affects most wheat-growing regions of the UK at varying levels and is getting harder to control as resistant populations evolve and chemistry becomes more limited.

There are three main types of resistance to a range of different modes of action, with ACCase and ALS target site resistance (TSR) and non-target site resistance (NTSR), especially enhanced metabolism (EMR), now widespread, according to Rothamsted Research.

Combined TSR, where ACCase and ALS target site resistance mutations occur within the same plant, has also been demonstrated and is something researchers expect to increase.

See also: Practical tips from 15 years of blackgrass trials

Resistance issues often focus on the ALS inhibitor herbicide Atlantis (iodosulfuron + mesosulfuron).

However Strutt & Parker agronomist Jock Willmott says it is vital not to overlook the build-up of resistance to other chemistry, notably the ACCase “fops”, “dims” and “dens” that often form the basis of post-emergence herbicides across a range of crops.

“If you’ve had many years of relying on Atlantis, the chances are you have selected for resistance. But it’s the background resistance to everything else testing shows that’s also important and allows you to adjust what you do accordingly.”

Identifying resistance types determines which chemistry is used and how modes of action can be varied throughout the rotation, alongside cultural practices.

Equally, while fields full of blackgrass suggest a resistance problem is already established and confirming this with a test may seem unnecessary, there is still a case for getting samples analysed given ongoing shifts in resistant populations, says Mr Willmott.

“Monitoring these shifts is useful to track how control strategies are progressing. Testing can help streamline actives and identify a window where control may be acceptable.”

What do resistance tests measure?

Many testing centres use the “pot test” to apply different actives to black-grass samples for an indication of the types of resistance present.

The most common tests are for resistance to pendimethalin (indicator of EMR), cycloxydim (indicates ACCase target site resistance) and sulfonylureas (resistance to ALS inhibitors, such as Atlantis).

Other advanced tests to more accurately determine the type of resistance are also available.

For blackgrass, Italian ryegrass and wild oats, most testing centres present the percentage kill achieved using the “R” system (see below).

Understand the results

Confirmation of herbicide resistance may sound alarming, but results need to be interpreted carefully as blackgrass populations are complex, says Gordon Anderson-Taylor of Bayer CropScience.

Sampling blackgrass in July is inherently biased towards blackgrass that has survived a season of chemical and cultural control and may not represent the field population, he says.

“You need to know how the surviving population compares to the total amount you started with. Don’t think that just because resistance is present, the product doesn’t work.

“There may still be 50-70% of the population that’s still susceptible.”

Indeed, in Bayer trials under high blackgrass pressure (500 heads/sq m) at Romney Marsh, Kent, Atlantis reduced blackgrass head count by 50% compared with an untreated control, despite tests confirming widespread resistance.

Half of plants in untreated plots carried TSR to sulfonylureas, while that was up to 100% in Atlantis-treated plots, says Bayer’s Ben Coombes.

He suggests it may be worth resistance testing samples collected from fields where the product concerned (for example, Atlantis) has not been used, to reduce some of this inherent bias and get a more accurate idea of background resistance.

Adapting agronomy

Establishing the type and severity of resistance helps shape management strategies in following crops, where the focus has to be on minimising seed return through cultural or chemical means, says Mr Willmott.

Effective pre-emergence herbicides based on actives such as flufenacet and tri-allate are key to chemical blackgrass control.

Accurate timing is more important than complex pre-emergence “stacks”, so much so that it may be worth delaying seed-bed preparation and drilling until conditions allow a good tilth and effective pre-emergence performance, he says.

“It’s almost impossible to recover control with another residual herbicide or Atlantis if pre-emergences don’t work, so there is a real risk of going on too early to dry, cloddy seed-beds.

“Cost is also a big factor. By applying two or three residual herbicides, followed by Atlantis later, you can soon be looking at spending £130-£140/ha pre-Christmas with no guarantee of achieving effective control.”

While target site resistance is generally fairly clear-cut, in that products either work or fail against susceptible or resistant plants, it is possible to achieve good efficacy in the presence of EMR if products are applied correctly, he notes.

“With enhanced metabolism resistance, generally the bigger the plant, the easier it can overcome the herbicide, so applying a robust dose to small blackgrass [at 1-3 leaf stage] has more chance of working.”

Dr Anderson-Taylor agrees and advises growers to be prepared to spray blackgrass areas at the optimum timing through autumn and winter, providing there is active growth.

“Early applications are certainly better than waiting until spring.

“Although there are populations out there with very high levels of EMR that could be confused with TSR, generally most are at a low to moderate level and an early, well-timed spray still delivers good control,” he adds.

Select and control

Alternating chemistry is central to any anti-resistance strategy, but particularly with TSR, says Dr Anderson-Taylor, who believes growers should not necessarily stop using a product if testing confirms resistance.

“If you stop using a product without effective alternative control, populations of susceptible and resistant blackgrass will continue to build.

“Any strategy that gets the numbers down is worth doing.”

While applying a herbicide in the presence of TSR will select for resistant populations, it will still control susceptible plants and so reduce overall seed return.

Resistant blackgrass should then be targeted with alternative chemistry in following crops (for example, flufenacet in wheat/ barley, or propyzamide/ carbetamide in oilseed rape), or with cultural options, he advises.

Types of resistance

Resistance ratings

Source: Rothamsted Research