How to reduce the risk of glyphosate-resistant weeds
© Tim Scrivener Guidelines for minimising the risk of weeds becoming resistant to glyphosate has been updated, based on new research quantifying exactly how and when the herbicide should be used for maximum effectiveness.
Glyphosate is a vital herbicide for managing weeds and burning down cover crops before drilling.
However, its repeated use – such as spraying off several stale seed-beds to control blackgrass prior to barley and wheat drilling – increases the resistance risk.Â
See also: 10 learnings from more than 21 years of blackgrass trials
Fortunately, the UK hasn’t seen the rise of glyphosate-resistant weeds experienced in other countries such as the US and Australia.
However, evidence has started to emerge that certain grassweed populations, such as sterile brome, are becoming more tolerant to the effects of glyphosate.
Concern is spreading that glyphosate resistance will eventually occur, and this active will no longer provide effective control over common grassweeds.
Researchers say there would be a loss of crop production, with one estimate of a 20% yield loss without the use of glyphosate pre-drilling.
New research has prompted the Weed Resistance Action Group to update its guidelines. These outline in practical, clear terms how farmers can administer glyphosate at the right rate, at the right growth stage and in the right conditions.
Below we list the key steps to minimise resistance risk by maximising efficacy.
1. Application
Dose
Make sure the correct rate is chosen for the growth stage and use situation. For example, for annual grasses, a minimum of 540g/ha of the active should be used for seedlings up to two to three leaves.
At the tillering stage, it should be 720g/ha and 1,080g/ha when flowering.
Sub-optimal doses will give less-reliable results, especially if other conditions are also less than perfect. The rate used should be the highest necessary to give effective control for weed species or most advanced growth stage present.
Water volume
Aim for 80-250 litres/ha. Lower volumes give the best results providing the correct nozzles are used (see spray quality). That is because low 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.
Tips for stale seed-beds
Blackgrass and Italian ryegrass are the key weeds at risk of resistance at present.
Glyphosate applied before drilling a crop, such as in a stale seed-bed, is an effective weed control option, but it needs to be used sensibly. The best advice, based on recent research, is:
- Avoid multiple applications of glyphosate to the same weed plants (such as survivors of previous applications)
- Always maximise efficacy and use the manufacturer’s recommended dose rate for the weed sizes present. Reduced rates that allow more survivors may encourage resistance development
- Reduce risks by ensuring that glyphosate use is supplemented by sufficient cultivation to kill survivors and effective herbicides of different modes of action, either in sequence in following crops (pre- and post-emergence) or in a mix (if appropriate and available)
- A maximum of two applications of glyphosate, provided there is cultivation and effective subsequent herbicide use, is likely to be a manageable risk. Multiple applications in the absence of cultivation should be avoided.
Spray quality
Opt for a medium-coarse BCPC (200-400 microns) droplet size. Use droplets on the finer side of medium for the optimum wetting of spike- and seedling-stage blackgrass.
Conventional flat-fan nozzles (such as F110-03 or F110-04) are most suitable for seedlings. Low-drift nozzles can be used on well-tillered 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 through a process known as chelation. This effectively reduces the applied dose rate of glyphosate and is most noticeable when using low application rates and high water volumes.
The addition of a proprietary water conditioner, choosing a low water volume and maintaining the correct dose will mitigate the effects.
Growth stage/timing
- Ensure weeds are green and growing
- Grassweed seedlings should have a minimum of 5cm growth
- Aim to spray dense populations of blackgrass seedlings before shading prevents adequate coverage
- Poor downward translocation will occur during the stem extension phase of growth in April/May. The natural flow of sugars from photosynthesis in the leaf is upwards to the rapidly extending stem for use in the developing flower head
- Treatment in early spring can lead to a dieback of the stem, but subsequent regrowth from the base
- Spraying once the head is formed will give excellent translocation down to the roots and a much higher level of kill
2. Weather
- Efficacy is improved as temperatures rise (15-25C). Avoid periods of prolonged frost
- Avoid spraying in the evening when there is a high risk of run-off from night rainfall from the prolonged drying time
- Spraying onto damp leaves is acceptable, but do not spray if rain is forecast within a few hours. Heavy frontal rain before uptake can lead to very poor results (40-60% of the control achieved without rain)
Read the Guidelines for minimising the risk of glyphosate resistance in the UK online.Â
