How To Get To Grips With The Growing Challenges Posed By Herbicide Resistance

A full Integrated Pest Management (IPM) strategy is needed at a time when farmers are facing the loss of some key chemicals for weed control, writes Shay Phelan

We now realise that relying on herbicides alone to control weed problems is not a practical solution.

While herbicides will continue to form a part of the strategy a more integrated approach including other techniques is needed. Herbicide resistant weeds are now common on many farms and coupled with the potential loss of active ingredients in the future means that a full IPM approach is needed.

The good news is that an IPM approach to weed control does not necessarily mean that it will be more expensive – in fact in certain situations it may well be cheaper. There are a number of components to a true IPM strategy for were control.

Good weed control is an important consideration

Good weed control is an important consideration



The first ‘tool’ in any IPM strategy is field records, knowing which weeds are problematic and in which fields or parts of fields. Having this information on a map or in a diary can help to target certain areas rather than an overall application, which can reduce costs.

The records will also be useful in selecting the appropriate herbicides ensuring that optimum weed control is achieved. You should also use records to comment on the control achieved after application.

This is very useful where you suspect resistant weeds such as poppy, chickweed, marigold or wild oats are starting to become a problem as they will help you to identify and address the problem earlier.



Using crop rotation to control weeds is as old as farming itself, knowing that different weeds germinate at different times allows out-of-crop control to be carried out, reducing the burden subsequently.

Black grass is a classic example where switching from autumn sowing to spring sowing allows control of this autumn germinating weed with stubble cultivation thus reducing the weed burden

Crop rotation also allows you to use alternative chemistry to control weeds for example controlling grass weeds in beans, peas or oilseed rape can be done using graminicide type herbicides which can’t be used in cereals.

Stale Seed Beds

Cultivating stubbles and encouraging weeds to grow again is an old technique. These weeds can then be ploughed down and buried or they can be sprayed off with a total herbicide thus providing a clean seed bed.

The popularity of cover crops under the current GLAS scheme provides an ideal incentive to stubble cultivate while sowing a cover crop. Some of the species that can be sown in this scheme can also give some weed control by smothering out weeds when they germinate.

grass weeds

Manual Control

Rogueing has gone out of fashion over the last number of years as farms get bigger and labour becomes scarcer, however it is probably one of the most important tools for reducing weed seed return for the following year.

Wild oats in particular in many cases can be controlled before harvest by hand rogueing. Don’t forget that the plant that you pass by on the tractor or combine will provide seed for the next 10 or more years. Wild oat control alone is costing most farmers in excess of €25 per ha.

Herbicide Application

Pesticides should always be the last resort for controlling any pest and herbicides are no different.

However where it is unavoidable there are a few guidelines that should be considered when using them which should ensure good control and reduce the likelihood of resistance;

  • Use the appropriate chemistry and where possible use different active ingredients.
  • Use the appropriate dose.
  • Alternate chemistry from year to year.
  • Be careful with tank mixes as this can affect the efficacy of the herbicide.
  • Make sure weeds are actively growing.

Grass weeds and wild oats

GRASS weeds such as bromes, canary grass and blackgrass as well as wild oats are a growing concern on most tillage farms.

In some cases they are the most important weed and they can prove quite difficult to control.

The starting point with any grass weed control strategy is correct identification which isn’t always easy. Quite often this is only done after the grass has set seed which is too late for that particular season.

Once the weed has been correctly identified then the appropriate control measures can be put in place.

Stubble cultivation plays an important role in controlling grass weeds and wild oats however not all grass weeds germinate under the same conditions for example sterile brome needs to be covered to break dormancy whereas meadow brome needs a few weeks light exposure to break dormancy while canary grass is generally a spring germinating weed.

Herbicide Resistance of Weeds

Herbicide Resistance of Weeds

Sterile Brome

While there are herbicide options to control grass weeds such as sterile brome in wheat the options in barley or oats are very limited so crop rotation needs to be used in order to control it. In extreme situations, where bromes have taken over the crop especially on headlands, consider removing the crop before the brome heads out to prevent seed return.

Wild Oats

When controlling wild oats in wheat or barley they again work best when the weeds are small and actively growing. This often coincides with broadleaf weed control which can complicate things a little bit.

Where hormone type herbicides e.g CMPP, Mircam plus, Foundation are used leave 7-21 days before applying the wild oat herbicide depending on the product. However, where the wild oat herbicide is applied first leave seven days before the hormone herbicide is applied. Consult with your agronomist for more product specific advice and tank mixes.

First Priority Is Weeding Out The Chief Culprits

Good weed control is an important consideration  when growing any crop if you want to achieve full  yield potential.

While there are still a large number of options available quite often it can be confusing as to what are the best choices. In truth, the appropriate herbicide choices can often be different for different fields on the same farm, this depends on the weed spectrum within each field.

The most important aspect of designing a herbicides programme is knowing what weeds are most common and which of these rob most yield, then and only can you decide which herbicides are needed.

The table below outlines how competitive weeds can be and the number of weeds per square metre to give a 5pc yield loss. Plan to control the weeds that rob most yield first and the ones that are less competitive after that. However be aware that weeds such as annual meadow grass or groundsel, which are not very competitive, can quickly increase in numbers and subsequently need control.

Each herbicide will control certain weeds but will not control others so you need to choose which herbicide or combination will give you most broad spectrum control. All the weeds controlled by the herbicide are clearly described on the label so if you’re not sure check the label.

Early control of the weeds is important as smaller weeds are easier to control and they generally can be controlled by lower doses, which is better for the environment and the pocket. The general advice for timing of the spring cereal herbicide application is as follows;

  • Spray early at the 3-5 leaf stage for successful weed control in spring cereals.
  • Reduced (50pc) rates have been successful in Teagasc trials where the weeds were actively growing and at an early growth stage.
  • To maximise the uptake of herbicides the weeds need to be actively growing for 3-4 days before spraying.
  • By delaying Broad Leaved Weed (BLW) control until GS 30/31 you may lose yield potential from early weed competition.
The french farmer fell ill after inhaling fumes from a weedkiller container. Pic. Philip Doyle

The french farmer fell ill after inhaling fumes from a weedkiller container. Pic. Philip Doyle

Shay Phelan is a Teagsc crop specialist based in Oak Park, Co Carlow

(Source – Irish Independent – Indo Farming – Shay Phelan – 11/04/2019)

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