As featured in Arable Farming Magazine
Protecting the efficacy of glyphosate
by Arable Farming August 2020
An AHDB-funded project nearing completion has sought to understand and quantify the risk of glyphosate resistance developing in UK grass-weed populations. Andrew Blake reports.
Preserving glyphosate herbicide’s efficacy on grass-weeds has been the aim of five years of AHDB-backed research which comes to a close in September 2020. The ability of weeds in some parts of the world to survive glyphosate treatment underlies the £0.5 million project*.
ADAS research scientist Lynn Tatnell, who has led the work, explains: “We needed to be sure that the risk of glyphosate resistance developing in UK grass-weeds was understood and could be quantified.”
Resistance to the world’s most widely used herbicide was first identified in rigid ryegrass in Australia as long ago as 1996, according to the International Survey of Herbicide Resistant Weeds. Since then resistance in a wide range of weeds, both grasses and broad-leaved types, has been recorded in many countries.
The reasons have been over-reliance on the chemical and repeated use since it was introduced nearly 50 years ago, believes Mrs Tatnell.
Increasing use of glyphosatetolerant crops since the mid-1990s has inevitably led to an increase in glyphosate-resistant weeds, she adds.
“The clear lesson is that dependence on glyphosate alone is inappropriate.
“The risk of resistance in the UK is real if we continue to use it in the way we have been.
“It is important that we retain this highly valuable herbicide and don’t abuse it. By understanding how to get the best out of it, the industry will retain it for as long as possible because we won’t be causing resistance to occur in the first place.”
Encouragingly the offer of free tests for ‘difficult to control’ grass-weed populations throughout the project’s life have attracted very few samples, she says: “All were well controlled in a dose response glasshouse test.
“This is a good thing reflecting perhaps what’s happening in the field, and that hopefully in the UK we’re still getting good control.”
Complacency must be avoided, she urges, and the project has resulted in key messages. (See the panel above).
- Ideal timing on grassweeds is GS12-13 – when they are small
- Dose: At least 540g per hectare. Should be 720g/ha on tillered plants
- Temperature when applied has big impact on results
How the research was conducted
- Glyphosate use on small plants in stubbles: Work focused on understanding the correct dose and timing (i.e., plant growth stage) to gain maximum efficacy from the herbicide.It included field trials on black–grass alone, and container experiments on both black-grass and Italian ryegrass.
- Glyphosate use on larger plants between crop rows: This work included deliberately applying glyphosate to large black-grass plants at sub-optimal doses. The idea was to mimic an application between crop rows where some plants do not get a full dose but get some exposure and may be too large.The aim was to determine whether resistance builds up more quickly in those situations.
- Adapting resistance test methods to detect glyphosate tolerance shifts using Syngenta’s RISQ agar Petri-dish test: This takes plants from the field early
in the season, and whole plants are tested in agar containing the herbicide. Results are seen in just a few weeks.Mrs Tatnell says: “Our aim is to provide practical management guidelines for farmers and agronomists.”The field trials highlighted some important points. Two applications to stale seedbeds, if the season allows, is beneficial provided the dose is correct for the target weed size.Stale seedbed cultivation is essential. This took place between the field trials’ two glyphosate treatments.
“It adds a cultural control factor, really reducing the grassweed numbers and so putting less pressure on the herbicide.
“We have seen that temperature at and around spray timing makes a huge difference to overall efficacy. That’s perhaps unsurprising, but by carrying out the same experiment over a number of years we have found the results differ greatly between when the autumn/winter has been mild and when it has been very cold.
“For example, in January 2019 the temperature was about 4degC but by mid February it was 17degC. The higher temperature significantly increased glyphosate efficacy.”
It is hoped the results of the project will be incorporated in the Weed Resistance Action Group (WRAG) guidelines.
Key messages from the research
- Optimising the herbicide treatment is essential to reduce the resistance risk
- Do not cut corners – keep the rate up and apply at the correct growth stage
- Monitor suspected resistant patches and if in doubt remove small areas before they spread. Carry out testing on seeds and plants
- *AHDB project
Managing the resistance risk to retain long-term effectiveness of glyphosate for grass-weed control in
UK crop rotations
- October 1, 2015 – September 30, 2020
- Cost: Total £500,000 (AHDB £250,000)
- Project partners: Albaugh, FMC (Headland), Monsanto (Bayer) and Nufarm £40,000 each
- Agrii, Agrovista, Frontier, Hutchinsons and Syngenta – all in-kind contributions including field sites