As growers turn their attention to autumn cultivations, what steps should they take to maintain the efficacy of glyphosate?
After a season blighted by black-grass on many farms, growers are turning their attention towards controlling the weed in autumn sown crops, with glyphosate remaining a key weapon in their armoury.
While there are currently no known cases of glyphosate resistance in the UK, repeated use and an over-reliance on the herbicide has led to resistance developing in other countries.
A lack of new herbicides coming on to the market, loss of herbicides due to regulatory changes, a limited crop rotation and lower uptake of cultural control methods has led to over-reliance on a limited number of selective herbicide actives. This has accelerated development of herbicide-resistant grass weeds, particularly black-grass.
Use of glyphosate to control such weeds potentially heightens the risk of resistance developing, but there are steps growers can take to reduce this risk. Lynn Tatnell, ADAS weed biologist, said: “The key is to make sure you use the right dose for the right size of weed. To get a stale seedbed right you need a good flush of weeds before spraying off. Think about the dose and target weed.”
ADAS is currently leading a five-year trial investigating the potential of black-grass and Italian rye-grass to develop resistance to glyphosate. It is also working on a test which could be used to determine resistance during the growing season rather than having to wait for weeds to seed, says Dr Tatnell. “Where people do have concerns, the test would be easy to use and people could have confirmation within the season.”
To maximise glyphosate efficacy, dose, growth stage and conditions are important. Annual grasses typically require 540g.a.i./ha for seedlings up to six tillers and 1,080g.a.i./ha when flowering. Plants should ideally be sprayed when at least 5cm and before the start of rapid stem extension, according to Weed Resistance Action Group (WRAG)/ADHB guidelines.
The chemical should be applied to actively growing plants in warm conditions (15-25degC), at least six hours before any rainfall. Adjuvant use is often associated with use of reduced rates of glyphosate products. However, unless glyphosate product labels specifically recommended this, extreme caution should be exercised as any reduction in efficacy could increase the risk of resistance development, warn the guidelines.
Two applications of glyphosate with sufficient cultivation to kill survivors and effective subsequent herbicide use is likely to be a manageable risk. Multiple applications in the absence of sufficient cultivation should be avoided, say WRAG/AHDB.
ADAS research is also looking at whether given the right timing and effective cultivation, one spray of glyphosate will be as good as more, says Dr Tatnell. “The key thing is that more people are aware of the risk [of glyphosate resistance]. Farmers are so used to having it, if we keep it we want to make sure we are able to use it as we do now.”