There are currently no known cases of glyphosate resistance in the UK, however, globally, resistance to glyphosate has evolved as a result of repeated use and over-reliance on the herbicide.

New guidelines for minimising the risk of glyphosate resistance have recently been published by the Weed Resistance Action Group and take account of current changes in usage patterns in the UK, which are potentially increasing the risk of glyphosate resistance development, says author of the guidelines, Lynn Tatnell, weed biologist at ADAS.

She says: An over-reliance on a limited group of herbicide modes of action has accelerated the development of herbicide-resistant grass-weeds, particularly black-grass.

This has been mainly due to a lack of new herbicides, regulatory policy changes, a limited crop rotation and the under-exploitation of cultural control practices.

Worldwide, 31 weed species (11 grasses), across 22 different weed families a total of 232 individual biotypes were reported to be glyphosate-resist ant in 2015.

As of 2017 this had increased to 41 weed species.

Glyphosate-tolerant crops were introduced in North and South America in the mid 1990s, resulting in a huge increase in area sprayed with glyphosate, says Mrs Tatnell.

This led to an increase in glyphosate-resistant weeds in the years that followed.

A clear lesson was that reliance on the use of glyphosate alone without mitigation measures, such as cultivation was a key driver for resistance development.

Applications In the UK there are now many reports of multiple glyphosate applications pre-drilling between every crop, every year in the rotation, she says.

This strategy may not be sustainable in the longer term due to resistance development.

There is an increasing number of enquiries regarding poor weed control with glyphosate and possible glyphosate-resist ant weeds in the UK.

The majority refer to black grass, but queries have been raised in the last two years on brome species, sow thistles and annual meadow-grass, says Mrs Tatnell.

Mixtures can form an effective component of resistance management strategies.

However, as many can reduce the efficacy of glyphosate, only ever use mixtures recommended by manufacturers, says Mrs Tatnell.

To be an effective component of a resistance management strategy both components of the mixture must be effective on the same target weeds at the relevant growth stage and timing.

Key grass-weeds black-grass and Italian ryegrass are already resistant to many of the herbicides that can potentially be used.

In addition, there are demands to minimise mixtures total area of use and hence risk of being found in water.

In practical terms, strategies should therefore aim to mitigate the resistance risk from glyphosate by ensuring use is accompanied by cultivation and/or herbicides in sequence.

Source: WRAG

To maximise efficacy, spray at the right dose rate, at the right growth stage and in the right conditions, says Mrs Tatnell.

Dose rate Get the dose rate right for the weed and growth stage.

Annual grasses typically require a minimum of 540g active ingredient per hectare for seedlings up to two to three leaves, 720g ai/ha when tillering and 1,080g ai/ha when flowering.

Growth stage Ideally, spray 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.

Choice of nozzles, water volume and the addition of water conditioner can also be influential.

Water volume Use 80-250 litres/ha, says Mrs Tatnell.

Lower volumes give best results provided correct nozzles are used.

Low dose rates and higher water volumes lead to a low concentration of glyphosate and surfactant.

This can lead to poorer results, especially in conjunction with hard water or other sub-optimal conditions.

Spray quality Medium-coarse BCPC (200-400 microns) are advised.

Use droplets on the finer side of medium for optimum wetting of spike and seedling stage black-grass.

Conventional flat fan nozzles are most suitable for seedlings.

Low drift nozzles can be used on welltillered plants.

Hard water Hard water containing high levels of calcium, magnesium and other polyvalent metal cations, such as iron and aluminium, can lock-up glyphosate by a process known as chelation, explains Mrs Tatnell.

This effectively reduces the applied dose rate of glyphosate and is most noticeable at low application rates and high water volumes.

Addition of a proprietary water conditioner, choosing a low water volume and maintaining the correct dose rate will mitigate the effect.

Source: WRAG