Spray timing and herbicide efficacy in cereals and oilseed rape

Effective grass-weed control is essential in rotations with autumn-sown crops. Although product labels and technical support provide guidance on how to optimise herbicide effectiveness in specific situations, these general principles apply to all crops.

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Herbicide timing in autumn-sown cereal crops

Herbicides form a large part, typically 20–30%, of the variable costs associated with producing a crop. With the need to consider herbicide-resistance risks too, it is important to target each spray required. This table (below) outlines the pros and cons associated with the key herbicide spray timings in autumn-sown cereal crops.

Herbicide timing in autumn-sown crops

Aim

Mode of action

Advantages

Disadvantages

Pre-drilling

Encourage weed growth. Control weeds from harvest to drilling

Contact

Can use non-selective herbicides, which reduce resistance risk

Early drilling shortens time for weeds to emerge and be controlled

At drilling

Apply before crop or weeds emerge, within 24–48 hours of drilling. Control weeds until end of winter

Residual

Prevents weed establishment. Essential building block of grass weed control; only effective timing for some species/herbicides. Limited resistance to pre-em herbicides

Poor weed control where seedbed quality is poor or seedbeds are dry. Crop seed depth, or soil cover, can be an issue with some herbicides

Autumn/winter

Control later-germinating weeds or escapees from pre-emergence treatments. Target weeds when small

Residual Contact

Weeds visible to identify, which aids product choice

Control more difficult if weeds are large. Soils can be too wet. Stressed crops. Large crop canopies. Resistance problems common. Beware cold temperatures, which can reduce efficacy of some herbicides

Spring

Control spring-germinating weeds. Tidy up winter escapees

Contact Some residual

Weed spectrum visible

Large weed size. Sometimes too late for certain species. Target crop growth stage is missed

Pre-harvest

Control late-germinating and perennial weeds

Contact

Ideal timing for perennial weeds

Few species at correct growth stage. Some weed seed set. Some crop market restrictions

Spray timing and herbicide resistance

Key annual grass-weed species at risk from herbicide resistance include black-grass and Italian rye-grass. When applying glyphosate prior to drilling a crop, maximise efficacy (to reduce the number of survivors), always aim to:

  • Use the manufacturer’s recommended dose rate for the weed sizes present*
  • Apply prior to ‘shading’ from other plants
  • Apply to actively growing plants in warm conditions (15–25°C), with at least 6 hours before any rainfall

*Small weeds are particularly challenging targets, especially grass weeds because of their vertical structure. Ideally, spray when plants are at least 5 cm but before the start of rapid stem extension.

An illustration to show that larger weeds are often tougher to kill

Reduced glyphosate doses increase the risk of lower efficacy

Typically, annual grasses require a minimum of:

  • 540g a.i./ha for seedlings up to 2-3 leaves
  • 720g a.i./ha when tillering
  • 1,080g a.i./ha when flowering

How to minimise glyphosate resistance risks in grass weeds (full guidance)

Weed Resistance Action group (general herbicide resistance guidance)

Herbicide programmes

Autumn-sown cereals

Mixtures (several products applied together) or sequencing/stacking (several products applied in close succession) are more effective at controlling grass weed populations than individual products. Pre-emergence options are less affected by resistance and should form a key part of a cereal herbicide programme.

Oilseed rape herbicides

Spring herbicide options are limited in oilseed rape and weed control decisions need to be made prior to drilling. Establishment methods and weeds present affect control options:

  • Where shepherd’s-purse and/or cleavers are predicted, a robust pre-emergence treatment – based on metazachlor – is required. Rapeseed must be well covered by soil to a depth of 15mm
  • For black-grass and other grass weeds, herbicides such as propyzamide are more effective after no or very shallow cultivation. Where deeper cultivations are used, adding a graminicide (‘fop’ or ‘dim’) will improve the level of control.

Spray application and herbicide efficacy in cereals and oilseed rape

Find out how to optimise herbicide application techniques to make every drop count in the fight against arable weeds. This page covers spray equipment (e.g. nozzles) and rates.

Spray application and herbicide efficacy in cereals and oilseed rape


Sprayer on stubble, requiring spray-off with glyphosate
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