5 Tips to Prevent Glyphosate Resistance in Farming

First published in 2015, the Weed Resistance Action Group (WRAG) guidelines ‘Minimising the risk of glyphosate resistance in UK’ have recently been updated following an AHDB project led by ADAS.

1. Tailor dose to target species and growth stage

Repeatedly applying a sub-lethal dose of glyphosate to surviving weeds exacerbates the risk of shifting towards reduced herbicide sensitivity by selecting for less sensitive plants in the field.

Dose rate must therefore be better tailored to growth stage of the annual grass-weed target to get the most effective control from any application, says Bayer’s Roger Bradbury.

The updated WRAG guidelines refine dose rate recommendations for controlling annual grass-weeds at different growth stages. A rate of 540g a.i./ha minimum is recommended at the 2-3 leaf stage, increasing to 720g a.i./ha by tillering and 1080g a.i./ha when flowering.

Roger also highlights the importance of applying an appropriate dose when spraying-off catch and cover crops. Some species, such as vetch, sanfoin and yellow trefoil can be harder to kill and may require a higher dose than species such as buckwheat, linseed or phacelia.

2. Maximum of two stubble applications

The other main revision to WRAG’s guidelines is the suggestion to limit the number of stubble applications of glyphosate to no more than two. Growers need to think carefully about how and when glyphosate is used for controlling weeds in stale seedbeds and managing the green bridge pre-drilling.

“The reason for limiting applications is to reduce the chances of applying repeated doses to weeds that survived earlier treatments,” explains Roger.

3. Select an effective formulation

While dose rate and number of applications are important resistance management factors, Roger says there are many other areas to consider.

In particular, growers should maximise efficacy by selecting a good formulation and achieving quality application to ensure glyphosate reaches, and is taken up by its target. Modern Roundup formulations are highly active with sophisticated surfactant systems that boost uptake and translocation leading to improved speed of activity, rainfastness and long-lasting efficacy.

The balance of surfactants delivers several benefits to spray coverage and penetration. For effective long-term control, Roger says glyphosate must get into plants in a benign way, so it can be translocated to all parts, including the roots:

“This is especially true for challenging perennial weeds with big tap roots or rhizomes. Roundup formulations avoid aggressive tissue damage at the leaf surface which can adversely affect the physiology of the plant and may limit glyphosate translocation, increasing regrowth risk.”

Agrii trials at Stow Longa last season reinforced the importance of formulation and quality application. The work found significantly more black-grass regrowth in plots treated with generic glyphosate compared to plots treated with the same amount of active in a Roundup Powermax formulation. Both treatments used the same flat fan nozzles, water volume of 100 L/ha and 10 kph forward speed.

4. Maximise application quality

Application quality is central to maximising efficacy, although autumn stubble treatments can sometimes lack the attention they deserve, Roger suggests:

“You may be aiming to apply 540g a.i./ha, but if the sprayer’s travelling too fast with the boom moving around too much, some plants may receive significantly less. Delivering sub-optimal doses repeatedly in this way can lead to shifts in sensitivity.”

Roger recommends keeping forward speed to 10-12 kph max, boom height to 50 cm and selecting the correct water volume and nozzle for the target weed and growth stage. While Roundup formulations deliver more consistent responses across a range of application conditions, he says warm weather (15-25 °C) with active weed growth are optimal.

Additionally the latest Roundup formulations offer improved rainfastness, with only 1 hour required for annual grass-weeds.

Results also showed how larger plants can cast a spray shadow, allowing regrowth beneath. This is especially true when spraying off cover crops, where there may be plants of different heights and weeds such as black-grass growing beneath the canopy.

5. Use cultural controls (e.g. cultivations) and other herbicides to reduce reliance on glyphosate

If two glyphosate applications are planned in autumn, Roger recommends cultivating in between the first and second sprays to help destroy any surviving weeds. As well as managing grass-weeds pre-drilling, it’s important to remember that volunteers can provide a green bridge for aphids to migrate into the new crop:

“Managing the green bridge in cereals without Deter is even more important now. Any flush of growth needs to be dealt with in a timely way ahead of drilling, or consider including glyphosate with the pre-emergence herbicide.”

For more information on preventing glyphosate resistance, see the WRAG guidelines.