Black-grass resistance initiative reports findings

Wednesday, 20 February 2019

Protein quantity and quality may be familiar concepts to growers of grain but pioneering research suggests that getting to grips with protein could also help lay the foundation for the next generation of weed control.

It’s one of many exciting avenues of exploration opened up by the four-year black-grass resistance initiative (BGRI), which has just delivered its final report to the AHDB website. This article takes a look at some of the project’s key discoveries.

The BGRI was set up following a call by AHDB and BBSRC for solutions to the UK’s black-grass management challenge. In response, a unique academic consortium formed in 2014 to combine state-of-the-art genomic approaches with weed ecology and agronomy, and unravel the major driving forces for the evolution of multiple herbicide resistance (MHR).

Protein power

The BGRI delivered a world-first: the first pocket diagnostic tool that can be used to detect MHR in the field. Based on pregnancy test technology, it reveals if a protein known to be associated with MHR is present. Such proteins, in this case the ‘AmGSTF1’ protein, are called ‘biomarkers’.

The test can diagnose the presence of MHR within 10 minutes of taking a black-grass leaf sample. The level of resistance can also be quantified – the stronger the test line on the diagnostic device, the more of the biomarker protein is present. With strong commercial potential, a company (MoLogic) has recently agreed to commit to the long-term manufacture of the rapid in-field test kit.

The AmGSTF1 protein is now known to play a central role in black-grass MHR. In fact, the team believes it is likely to play a similar role in other grasses and further investigations are ongoing to determine if this is the case.

Paul Gosling, who manages weed research at AHDB, said: “The BGRI team identified several proteins associated with herbicide resistance. Interestingly, the mechanism of resistance was found to be similar to the one cancers use to develop resistance to drugs in humans. The AmGSTF1 protein biomarker identified provided the strongest test of the presence of MHR in black-grass.”

Proteins associated with MHR could provide future targets for resistance busting. The BGRI found the first evidence of the role that specific transporter proteins play in MHR. Furthermore, the BGRI’s results show that such transporter proteins work with specific herbicide detoxifying enzymes. It’s certainly high science but understanding proteins and their interactions can help reveal the complex nature of underpinning resistance mechanisms. In fact, based on study of both genes and proteins, the BGRI suggests that there are at least three subtypes of MHR.

  1. Broad-ranging resistance to multiple chemistries
  2. Metabolic-based resistance to single classes of chemistry
  3. A herbicide-specific type

A thorough understanding of these subtypes, combined with sophisticated real-time diagnostics, could, theoretically, be used to identify bespoke herbicide control strategies for specific field populations.

Latent grassweed viruses (those that lie dormant within a plant cell) were also studied within the project. The group discovered two classes of latent viruses that associate with grassweeds but not with crops. Although, these viruses cannot cause MHR directly, they may do so indirectly – by affecting gene control in infected weed populations. Their potential role in enhancing plant stress tolerance mechanisms, including MHR, warrants further investigation.

Weed resistance status

The BGRI was not all about clean labs. In fact, the group frequently put on its boots to conduct one of the most comprehensive audits of the resistance status in UK grassweeds to date.

Resistance to post-emergence herbicides (ALS and ACCase) within UK black-grass populations was found to vary between actives and across regions. Through this variation, however, one thing was extremely clear: resistance to the herbicides tested is extremely widespread. The herbicides tested were:

  • Mesosulfuron-methyl + iodosulfuron (site of action: acetolactate synthase, ALS),
  • Fenoxaprop-p-ethyl (site of action: acetyl-CoA-carboxylase, ACCase)
  • Cycloxydim (site of action: acetyl-CoA-carboxylase, ACCase)

The results of the testing show that resistance to cycloxydim is probably conveyed by TSR alone. Observations from the mesosulfuron + iodosulfuron and fenoxaprop assays, however, suggest both TSR and MHR are important for these actives.

Cross-resistance was also found to be prevalent in black-grass. In fact, 79% of the tested populations were resistant to all three actives.

Black-grass populations were also found to vary in their sensitivity to glyphosate. This variation was shown to have a heritable genetic basis and respond to selection, meaning resistance can evolve. Data on historic glyphosate usage and current levels of glyphosate sensitivity were looked at by the researchers. The results provided clear evidence that selection is occurring in the UK’s black-grass population. However, there remains considerable uncertainty about the speed at which glyphosate sensitivity is changing.

Resistance costs

The economic consequences associated with weeds, including resistant populations, were also analysed in the studies. At the highest black-grass densities, the average cost of resistance (COR) was estimated to represent nearly 40% of the potential gross profit from winter wheat (compared with wheat grown in the absence of resistant black-grass).

Analysis of weed management practices found that herbicide strategies deployed on farms are not usually driven by black-grass density. The report authors suggest this leads to management practices that favour the development of resistance. For example, they say it is likely to be better to use cultural control approaches at low weed densities and reduce herbicide intensity. Although small yield losses would need to be tolerated in the short term, the approach would help slow resistance issues developing in the long term. The BGRI estimates that low black-grass densities account for just over half of England’s wheat producing area. This means the country is potentially half way to the worst-case scenario but, critically, still in a position to manage the situation on vast swathes of land (outside of the black-grass heartlands) to avoid significant losses to black-grass.

Cost-effective integrated pest management (IPM) strategies that are responsive to increasing resistance – by drastically reducing herbicide use as higher levels of resistance evolve – should be the appropriate response, according to the authors. In contrast, multiple herbicide applications per year are the normal response to increasing populations, despite high levels of resistance.

The authors believe that this disparity could arise from a number of contributing factors. For example, some managers may believe that even a little control (mortality of a few susceptible individuals) is better than no control and inaction is seen as the worst approach to weed management. In addition, IPM strategies are often seen as complex, uncertain (especially in the efficacy of alternative approaches) and difficult to implement, compared with the routine application of chemicals.

The national-scale yield loss and costs due to herbicide resistance were estimated for the first time by the BGRI. For example, total annual wheat yield loss in England was estimated at 1Mt. Due to uncertainties in the calculations, the actual figure could range from 0.4Mt to 2Mt. Annual costs to winter wheat production were estimated at £0.4bn, with annual costs as low as £0.3bn or as high as £0.7bn. Even at the lower end, the costs are very large. There is no doubt: herbicide resistance has a severe impact on English arable farming.

As no new herbicide modes of action have been marketed for over 20 years, non-chemical options need to be adopted. But the market and government needs to support the required innovation. In fact, the authors conclude that the BGRI’s findings should act as a catalyst for the UK to develop a national, government-led, pesticide resistance strategy.

About resistant black-grass

There are two broad forms of herbicide resistance in black-grass:

Target site resistance (TSR). Mutations in the proteins targeted by particular chemicals can make weeds less sensitive to them. This form of resistance is relatively well understood. It can be countered by the rotational use of herbicides with differing modes of action.

Metabolic or multiple herbicide resistance (MHR). Weeds become more tolerant of a broad range of herbicides, irrespective of their chemistry or mode of action. Generally, this is due to the weed being better able to detoxify crop protection agents. MHR is also termed non-target site resistance (NTSR). As MHR is poorly understood, the BGRI focused on it.

Pocket diagnostics

Pocket diagnostic tools can be used to detect herbicide resistance in field populations of weeds in as little as 10 minutes.

Pocket diagnostic tools can be used to detect herbicide resistance in field populations of weeds in as little as 10 minutes.

Project details

Full project title and duration

Multiple herbicide resistance in grass weeds: from genes to agroecosystems. Duration (2014–2018)

AHDB investment in these activities

£280,000 (total £2,800,000, remainder funded by BBSRC)

Project Partners

University of Newcastle, University of Sheffield, Rothamsted Research, University of Reading, University of Edinburgh, University of York

Funding Partners

BBSRC

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