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.

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.

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.

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

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.

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.