Resistance is a familiar word in the arable farming lexicon, but its use in the same sentence as ‘glyphosate’ is not to be taken lightly.
Incidences of resistance to non-selective herbicide glyphosate have been recorded in other parts of the world, including Europe, but not yet in the UK.
Now new guidelines have been published aimed at preventing the development of resistance to the herbicide in UK weeds and significantly, they highlight the increased resistance threat posed by a number of recently-adopted practices, including multiple stale seedbeds and inter-row applications.
The guidelines have been produced by the Weed Research Action Group (WRAG) and are published by AHDB.
WRAG chairman and ADAS science and business development manager James Clarke says the publication of the guidelines is a proactive move ahead of there being any confirmed resistance in the UK.
“The aim is to ensure the efficacy of this important active in weed control is maintained when a number of practices are increasingly being deployed on-farm which could drive the evolution of glyphosate resistance in UK weeds – including multiple glyphosate applications, sub-lethal doses and suboptimal application timing – and we wanted to be proactive in highlighting the risks and promoting best practice.”
Stale seedbeds
One of the main concerns is the recent practice of using multiple stale seedbeds ahead of drilling in autumn in a bid to control large infestations of black-grass or black-grass resistant to selective herbicides.
“We know people are using multiple applications in this pre-drilling window. We also know they are using low rates [of glyphosate],” says Mr Clarke.
He underlines growers’ current reliance on glyphosate, particularly where they are faced with controlling herbicide-resistant black-grass.
David Ellerton, technical director with agchem distributor and agronomy advice business Hutchinsons, acknowledges the increasing use of multiple stale seedbeds. However, growers are being advised also to use other control methods, including cultivations, herbicides with different modes of action and competitive crops, he says.
Work at the company’s black-grass site at Brampton, Cambridgeshire, is enabling Hutchinsons to monitor the impact of glyphosate use over several years on the same field area, he adds.
“We are seeing zero failures, we aren’t picking up any changes,” says Dr Ellerton.
Monsanto, which supplies glyphosate under the Roundup brand, is supportive of the guidelines.
In a statement released at Cereals, the company said the new WRAG guidelines were ‘just what UK growers and agronomists need to safeguard the single most vital weapon in their grass-weed control armoury’.
The guidelines should form the basis for all glyphosate use from now on, says Monsanto northern European technical manager for Roundup Barrie Hunt, who highlights the pivotal value of glyphosate in the face of growing resistance to other herbicide chemistry.
But the close-to 200 different glyphosate products currently approved for professional use in the UK, based on different salts, loadings and adjuvant mixtures and contents made less-than-ideal usage and less-than-effective treatment, which were key drivers of resistance development, more of a risk, he adds.
According to AHDB Cereals and Oilseeds, a potential loss of yield and quality would occur across all cropping systems without the use of glyphosate. In particular, it has been estimated a 20% yield loss would occur without the use of glyphosate pre-drilling.
Monitoring success
Reducing the risk of glyphosate resistance
  • Prevent survivors
  • Repeat application to surviving plants presents the highest risk
Maximise efficacy
  • Use the right dose rate for the target weed on actively-growing plants
  • Reduced dose rates up the risk of reduced efficacy
Use alternatives
  • Use cultivation or other non-chemical control options when practical
  • Use other herbicides in sequence (or in mixture, only if recommended)
Monitor success
  • Consult your agronomist or supplier
  • Remove survivors to prevent spread
  • Test seed samples of survivors
  • Commercially available for 40 years
  • One of the most frequently-used herbicides in the UK
  • Currently no known resistance cases in the UK
  • Resistance has evolved globally as a result of repeated use and over-reliance
  • 31 weed species known to have evolved resistance globally
  • Changes in usage patterns in the UK are potentially increasing the risk of resistance developing
Maximising the efficacy of glyphosate
Spray at the right dose rate, at the right growth stage, in the right conditions
  • Dose rate: Get dose rate right for the weed and growth stage. Annual grasses typically require 540g active ingredient/ha for seedlings up to six tillers and 1,080g ai/ha when flowering
  • Water volume: 80-250 litres/ha. Lower volumes give best results providing correct nozzles are used
  • Spray quality: Medium-coarse BCPC (200-400 microns). Use droplets on finer side of medium for optimum wetting of spike and seedling black-grass. Conventional flat fan (F110-03 or F110-04) are most suitable for seedlings
  • Hard water can lock up glyphosate, effectively reducing the applied dose rate. Addition of a proprietary water conditioner, choosing a low water volume and maintaining the correct dose will mitigate the effect
  • Growth stage: Ideally apply when plants are at least 5cm but before the start of rapid stem extension. Apply prior to ‘shading’ from other plants
  • Conditions: Apply to actively-growing plants, in warm conditions (15-25degC), with at least six hours before any rainfall