Resistance

Resistance

1. How will we control black-grass if it becomes resistant to glyphosate?

Glyphosate is a relatively low-risk herbicide for resistance development and there are currently no known cases of glyphosate-resistant weeds in the UK, according to Mr Hull.

But the threat is there and globally resistance has evolved due to repeated use and overreliance on the active in some areas, he warns.

With hardly any chemical alternatives Mr Cotton says growers must do all they can to protect glyphosate for the future by following best practice guidelines, such as AHDB/ WRAG Minimising the risk of glyphosate resistance  

His key tips include:

  • Do not over-rely on glyphosate for black-grass control
  • Use as part of an integrated chemical and cultural strategy that reduces background weed populations – particular focus on delayed autumn drilling and spring cropping
  • Apply glyphosate at a robust rate where used
  • Avoid using where no subsequent cultivation is planned.

Where glyphosate is used to spray off black-grass flushes pre-drilling, Mr Lloyd advises to concentrate on establishing one good flush of weeds, rather than going for repeated flushes and several spray applications.

2. Has increasing resistance of weeds such as black-grass caused the debate over reauthorisation of glyphosate?

In short, no.

Issues around the reauthorisation of products containing glyphosate mainly stem from a report by the International Agency for Research on Cancer (part of the World Health Organisation), which claimed glyphosate – alongside a host of other factors including eating burnt toast - is “probably” able to cause cancer.

Other reports have contradicted this finding, with a joint committee of experts from the U.N.'s Food and Agriculture Organisation and the WHO concluding glyphosate is “unlikely to pose a carcinogenic risk to humans”.

This has given the active a stay of execution for now, but Mr Bradshaw believes the issue highlights the challenges pressure groups pose to agriculture now and in the future.

3. Is resistance becoming a problem even where spring cropping is being used to control black-grass?

Spring cropping is an option for black-grass control, but it does not change the dynamics of resistance, says Mr Cotton. Once resistance has evolved within a population it is embedded in the genetic makeup and is likely to be there forever, even if the seedbank is reduced by cultural and/or chemical means.

Growers must learn to “live with” resistance and key to that is reducing the weed seedbank outside the crop. Failure to do so allows numbers to build and increases the risk of selecting for more resistant individuals.

Spring crops afford more time for cultural measures, but black-grass control within them can be a challenge due to more limited chemical options, he adds. Spring barley offers some of the best options through products such as Avadex (tri-allate), Liberator (diflufenican + flufenacet) and Crystal (flufenacet + pendimethalin), whereas options in spring wheat are fewer.

Mr Drinkwater says spring crops need to be chosen carefully, as those with more open canopies (such as potatoes, sugar beet, peas and beans) are less competitive against black-grass. He also suggests moves to more spring cropping could select-for spring-germinating populations

4. Is there statistical evidence of changing black-grass germination patterns?

Anecdotal evidence suggests more spring cropping could favour spring-germinating black-grass, but Mr Hull says there is currently no scientific evidence to support such a shift in emergence patterns.

Black-grass remains a predominantly autumn germinating weed, with around 90% typically germinating before Christmas, he adds.

However, he acknowledges it is logical that more spring cropping could select-for spring-emerging populations. “Black-grass is a very adaptable weed.”

5. What is the likelihood of resistance to flufenacet?

Like all residual herbicides, flufenacet is at relatively low risk of developing herbicide resistance and therefore remains a cornerstone of many black-grass control strategies, Mr Hull says.

However, it is not immune from resistance and he reports a slight (1% a year) downward shift in the activity seen in research trials.

Mr Anderson-Taylor says no change in field performance has yet been seen.

6. Will science be able to reverse enhanced metabolism resistance (EMR)? If so, when, and will nature find a way around it if scientists succeed?

All forms of herbicide resistance, including EMR, are a “one-way” process that is very difficult to control or reverse, says Mr Hull.

There may be scope to reduce the impact of resistance by using synergists to improve herbicide efficacy, but it is still very “early days” and he acknowledges non-target-site resistance (which EMR is a type of) is less well understood than target-site resistance given the complex gene interactions and multiple resistance mechanisms involved.

Research such as the BBSRC/ AHDB-funded Black-grass resistance initiative [insert link to: http://bgri.info/] is focussed on improving the understanding of the genetic causes and interactions of non-TSR, which remains the “holy grail” for many scientists.

“Nature will always find a way to overcome what we do,” Mr Hull says.

7. If Atlantis hasn’t been used for 3-4 years could its return on farm have a cost-benefit effect?

The nature of resistance evolution means there will always be some susceptible plants within a resistant population, so Atlantis can make a useful contribution to black-grass control even where resistance is confirmed, say Mr Hull and Mr Anderson-Taylor.

However, it must be used as part of an integrated management strategy that first reduces black-grass populations to a manageable level through a range of other chemical and cultural means.

“The key is to know what population you’re dealing with and how representative any survivors are of the entire field population,” says Mr Anderson-Taylor.

“Remember that resistance doesn’t go away, so don’t think any product is going to magically work better by just not using it for a few years.”

Growers could see improved Atlantis performance where land is ploughed after several years of shallow or no tillage as susceptible seed is brought to the surface, notes Mr Hull.

He also points out that Atlantis appears to only select-for ALS-specific non-target-site resistance and does not affect resistance to other non-ALS actives.

8. Is black-grass field scoring worthwhile and if so, what is the best method?

Mr Drinkwater believes field scoring is useful to growers as having access to more information on field populations, resistance status, etc. helps formulate the best control strategy.

It is essential to include the long-term impact across the rotation rather than focussing on a single crop or season, adds Mr Bradshaw.

The ultimate success of any management plan or scoring system relies on the ability to stay flexible and adapt cropping or control measures where needed, Mr Lloyd says.

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