Spray early for optimum weed control

Timing of pre-emergence and post-emergence herbicide application needs to be precise to get the best results, writes Ciaran Collins

Weeds have different competition patterns so prioritise ones with the greatest impact on yields
Weeds have different competition patterns so prioritise ones with the greatest impact on yields
Blackgrass cases have been increasing in recent years

Ciaran Collins

Pre-emergence and early post-emergence herbicide applications in wheat and barley will give the best weed control this autumn.

In the past, isoproturon (IPU) offered farmers a degree of flexibility in the timing of herbicide applications, but since its use was prohibited in September 2017, farmers need to be more precise with herbicide applications to get satisfactory weed control.

This is particularly relevant for the control of annual meadow grass (AMG) in winter barley where pre-emergence or early post-emergence timings are required to achieve the most reliable control.

Know your target

Knowledge of the weeds that are most likely to occur in a field is key to herbicide selection and timing of application. Good records from previous seasons are an important component of any weed control strategy.

Blackgrass cases have been increasing in recent years
Blackgrass cases have been increasing in recent years

Weeds have different competition patterns so priority will need to be given to weeds that will impact greatest on yield. The table (right) outlines the relative competitiveness of weeds in winter wheat and demonstrates how a severely competitive weed like cleavers would take priority in a herbicide programme over a moderately competitive weed like field pansy. The control of broadleaf weeds will be based on products containing diflufenican (DFF), pendimethalin (Stomp), prosulfocarb (Defy) and picolinafen (Flight) in combination with specific grass weed products.

There are many combinations available on the market but herbicide selection should be based on the expected weeds in combination with early application.

Annual meadow grass (AMG)

When IPU was on the market, AMG could be successfully controlled up to early tillering. The currently available suite of active ingredients can give excellent control of AMG, but success will only come from pre-emergence or early post-emergence applications. Control will be reduced after AMG reaches the two leaf stage. Later, control can be achieved in wheat with Alister Flex in the autumn or Pacifica Plus in the spring.

Successful control of AMG can be achieved with pre-emergence or early post-emergence herbicide applications. These include:

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Flufenacet, e..g. Firebird/Navigate/Gorgon/Fastnet;

Prosulfocarb, e.g. Defy/Roxy;

Pendimethalin, eg. Stomp Aqua/Most Micro;

Chlorotoluron, e.g. Tower.

Integrated Pest Management (IPM)

In an era of increasing herbicide resistance and a lack of new chemicals coming on the market, we cannot rely on a chemical solution alone to control weeds. The goal of any IPM plan is to exploit all available options to control the pest (weeds in this case) and use the herbicide as the final piece of the plan.


While brome grasses are a major concern for growers, the number of reported cases of blackgrass in Ireland has increased in the last three years. It is still at low levels relative to brome, but this is a worrying trend.

Resistance testing carried out at Teagasc Oak Park indicates that a high level of resistance exists in the Irish blackgrass population, which is similar to the UK.

The key germination period for blackgrass is in the autumn, therefore the IPM plan for any grower with blackgrass should involve stale seed beds and spring cropping as herbicides cannot be relied upon for control. The most likely mechanism of spread is machinery.

Inside the gate is normally the first place where any weed enters a farm from the combine or baler after coming from an infested field. Therefore, machinery hygiene is of the utmost importance to stop its spread.

Arguably, fields with ­blackgrass should be set to grass for five to six years.

Extreme vigilance is ­required by all in the industry to ensure that the amount of blackgrass in Ireland stays at the current low levels.

With a reducing pool of chemical options available to growers, cultural control as part of an IPM plan is an essential component of weed control in cereals.

To get the most from the currently available ­herbicides, timing of ­application needs to be precise with pre and early post-emergence offering the best route to successful weed control this season.

Ciaran Collins is a Teagasc crop specialist.

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