Glyphosate stewardship
Operators are reminded to take care and stick to guidelines to reduce the risk of resistance when applying glyphosate this season. This follows the first case of glyphosate resistance in a UK field population of Italian ryegrass earlier this year.
Glyphosate is probably the most important herbicide for UK farmers and there is no obvious replacement, which means any case of resistance is of great concern. However, resistance has only been confirmed on one farm, so this has to serve as a firm reminder of the importance of glyphosate stewardship.
While highly effective and relatively easy to use, glyphosate’s benefits are also its biggest dangers. This is because it’s easy to overlook the basics resulting in substandard control, which increases the risk of resistance developing.
The Weed Resistance Action Group (WRAG) advises users to follow four key steps to reduce the risk of resistance:
- Maximise efficiency:
– Use the right dose for the target weed on actively growing plants
– Reduced rates increase the risk of lower efficacy
2. Use alternatives:
– Use cultivation or other non-chemical control when practical
– Use other herbicides in sequence (or mixture, only if recommended)
3. Monitor success:
– Consult your agronomist or supplier
– Remove survivors to prevent spread
– Test seed samples
4. Prevent survivors:
– Repeat applications to surviving plants presents the highest risk.
You can download the full WRAG Guidelines for minimising the risk of
glyphosate resistance in the UK here:
Cultural control
WRAG guidelines recommend avoiding glyphosate applications to any weeds that have survived a previous spray. In practice, this means that you should always cultivate between glyphosate applications when managing fields in the autumn.
An ADAS study on stale seedbeds contributed to the WRAG guidelines, it found that two applications of glyphosate between harvest and drilling is the optimum for weed control without excessive workload, cost and resistance risk.
Best practice applications
An operator’s skill, judgement and attention to detail will have a huge influence on the efficacy of the application. WRAG recommends following best practice to avoid the risk of building resistance.
Dose rate
Apply the correct dose rate for the weed and growth stage. Annual grasses typically require a minimum of 540g a.i./ha for seedlings up to 2-3 leaves, a minimum of 720g a.i./ha when tillering and 1,080g a.i./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
Preferably apply to actively growing plants, in warm conditions (15-25°C), with at least 6hr before any rainfall. Cooler conditions (5–15°C) are also suitable, but it takes longer for symptoms to appear. Avoid applications when it’s very hot or cold.
Water quality
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. This effectively reduces the applied dose rate of glyphosate and is most noticeable at low application rates and high water volumes.
Normal mains water has a hardness in the range of 100-400ppm. In calcareous areas, bore holes and artesian wells may on occasions contain levels up to 1,000ppm. They may also contain iron from sandstone or high organic matter/ acid levels from springs, bogs and moors.
Iron content at high levels, usually above 20ppm, can lock up glyphosate and result in poor performance. Hard water, coupled with high iron levels can also present a problem.
Addition of a proprietary water conditioner, choosing a low water volume and maintaining the correct dose rate will mitigate the effect.
PH Level
The ideal pH for glyphosate performance is between 4 and 5. Note that glyphosate itself is a strong acid (pH of a 1% solution = 2).
Water conditioners should be added to the tank before the glyphosate.
Nozzle choice
Independent trials, carried out by application specialist Tom Robinson, to assess Roundup performance tested four different nozzles:
• Lechler ID3 (90% drift reduction)
• Hypro GuardianAir (75% drift reduction)
• Hypro 3D (angled forward and back)
• 110 fan jet
All were tested at 0.5m and 1m boom heights.
The fan jet increased drift with no improvement in control. The 3D gave the best control but is more prone to drift. The Lechler ID3 was the least drifty but with slightly lower weed control, the Hypro GuardianAir is a good compromise option.
“In ideal conditions, up to Force 2, the 3D is the best option, at higher windspeeds use drift reducing nozzles. Angled nozzles like the 3D improve spray retention because more of the target is hit directly by the spray,” says Tom.
Setting the boom lower, at 0.5m, compared with 1m reduces drift and improves application quality. Tom recommends 12km/h as a forward speed for herbicide applications, going faster can cause turbulence which affects the spray. Follow-up trials in 2023, however, found that drift reducing nozzles may be used up to 13.5km/h.
