Pre-emergence herbicides provide the best opportunity to control grass weeds in winter wheat and barley

Grass weeds are an increasing challenge on tillage farms. In this article, Teagasc Crops Specialist, Ciaran Collins reminds us why effective autumn weed control is essential.

Pre-emergence herbicides generally provide the best opportunity to control grass weeds in winter wheat and barley, reducing early competition and dependence on post-emergence treatments.

The increasing use of pre-emergence herbicides by tillage farmers reflects a greater awareness of the threat posed by grass weeds and herbicide resistance. Pre-emergence herbicides have an important role in resistance management by reducing reliance on post-emergence chemistry where most of the resistance issues are. Getting control right from the start is particularly important in winter barley, where options for controlling established grass weeds are limited.

Planning herbicide application

Plan the herbicide application as part of the drilling programme. Aim to apply at true pre-emergence, ideally within 24-48 hours of sowing.

Prioritise fields with a history of grass weeds, early sown crops and land where travelling later in the autumn may be difficult. Where blackgrass, Italian ryegrass or brome are present, pre-emergence treatment should form the foundation of a planned control programme. A follow-up treatment may still be necessary.

Residual herbicides work best on a fine, firm seedbed with adequate soil moisture. Large clods and uneven seedbeds reduce weed control, while dry conditions can limit herbicide activity.

Ensure seed is fully covered and drilling slots are closed. A minimum of 2-3 cm soil cover is required and avoid applying herbicides when heavy rainfall is forecasted.

Slight crop effects, including leaf whitening or bleaching, can occur following some pre-emergence herbicides and this will be more noticeable on overlaps and headlands. Symptoms are temporary and recovery depends on growing conditions.

Some phytotoxic (herbicide) effects can be expected, typically as leaf whitening or bleaching following the use of pre- emergence herbicides.

Herbicide options

Pre- emergence herbicide options include prosulfocarb, such as Defy, and pendimethalin, such as Stomp. Combination products include Tower, containing pendimethalin, diflufenican (DFF) and chlorotoluron; Mateno Duo, containing aclonifen and DFF and Flight, containing pendimethalin and picolinafen.

Winter wheat

• For annual meadow grass (AMG), do not rely on spring herbicides as the main control strategy. Resistance to ALS herbicides, Pacifica Plus etc. has been detected. Apply products containing Flufenacet/PDM/Prosulfocarb/Aclonifen pre-emergence.
• For brome species, Flufenacet/PDM can give some control but there is a spring follow-up option of Broadway Star/Pacifica Plus.

Winter barley

• Pre-emergence herbicide application is vital as there are no effective spring options for brome species and AMG.
• Where brome is expected, Flufenacet/PDM applied pre-emergence will give partial control.
• Target AMG with pre-emergence application of Flufenacet/PDM/Prosulfocarb/Aclonifen. Early post-emergence applications can still provide some control, but efficacy declines as application is delayed.

Winter oats

• Herbicide choice is limited to Diflufenican (DFF) but does not provide effective grass-weed control.

Check flufenacet stocks

Check pesticide stores for products containing flufenacet, for example Firebird, Navigate, Griffen, Naceto and Reliance etc. The use up deadline is 10 December 2026, after which product use is illegal.

For more information on the control of grass weeds, visit here.