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Young emerging wheat plants

Post-emergence protection protocols

With wheat drilling well underway in many parts of the country, Dr Bill Lankford, ADAMA’s herbicide technical specialist, turns his attention to post-emergence herbicide applications and how to maximise the control of difficult grassweeds in cereals.
Young emerging wheat plants

The key timing for the application of residual herbicides in cereal crops is at the pre-emergence stage – within 48 hours of drilling, but preferably as soon as the seed has been sown.

However, assuming the seed has been drilled into a well-prepared seedbed and at the correct depth for the herbicide(s) being applied, the risk of damaging the crop damage as a result of applying a post-emergence treatment should be low, with this later application timing enabling growers to extend the period of control. Crop safety is however very much weather dependent, as excessively wet conditions can amplify the intensity of the active ingredients being used. It is therefore essential to check the weather forecast to ensure post-emergence treatments are applied in safe conditions.

Stacking actives gives the best results

Trials carried out by and on behalf of ADAMA have shown that using a selection of different active ingredients from a range of modes of action gives the best possible results on difficult to control grassweeds (notably back-grass and ryegrass).

The use of as many as six different active ingredients will improve control levels where target grassweeds express a metabolic resistance to herbicides.

While the application of such a high number of active ingredients is feasible (from a crop safety perspective) at the pre-emergence stage, applying so many products can elevate the risk of crops being adversely affected.

The use of such a broad range of herbicides certainly isn’t recommended in anything less than ideal circumstances – for example when the seedbed is poorly consolidated or when adverse weather is expected. In these scenarios, it can pay to split the stack of residuals, not only in order to protect the crop’s integrity, but also to maximise the herbicide programme’s efficacy: making separate applications allows a higher total loading of the different active ingredients to be applied safely and extends the window of activity thereby improving the control of later germinating grassweeds.

Peri-em or early post-em: which is best?

There are two schools of thought when it comes to the best time to apply post-emergence residual herbicides in cereals:

  1. At the peri-emergence stage, just as seedlings are about to emerge
  2. At the early post-emergence timing: within four weeks of drilling when the crop has one to two leaves

In Dr Lankford’s opinion, the peri-emergence timing works well in trials, but in practice poses an elevated risk to the crop because it can be difficult to accurately time applications on a farm scale in conjunction with good spray conditions. As such, the early post-emergence timing is preferable, but the actual timing of application will obviously be dictated by weather conditions and other on-farm priorities and workloads.

In terms of which herbicide(s) to use at either of these timings, it is advisable to select active ingredients which weren’t used at pre-emergence. It is also important to select products based on their efficacy versus the weeds being targeted: some weeds will have successfully emerged despite the application of pre-emergence herbicides, so it may be necessary to use a contact herbicide as well as a product which will be taken up via the root system.

Towering over weeds

In terms of specific products, TOWER’s three-way combination of chlorotoluron (250g/l), diflufenican (40g/l) and pendimethalin (300g/l) makes it ideally suited to the post-emergence timing: in trials to assess the control of AMG, TOWER it is the only product I’ve seen where its early post-emergence activity is equal to its pre-emergence performance.

This is, in large, due to its unique ingredient, chlorotoluron, which is taken up by foliage and roots. Likewise, the pendimethalin contained within TOWER also boasts some foliar absorption although its primary mode of action is via root uptake. Meanwhile, the diflufenican element within TOWER provides additional efficacy via contact and residual activity.

At its full rate, TOWER provides 600gai/ha of pendimethalin, allowing it to be used after a pre-emergence treatment of 1,320 gai/ha.

An additional active ingredient in the form of prosulfocarb (as TOPSAIL) will further enhance post-emergence weed control, with 3.0l/ha in wheat or 2.0l/ha in barley/rye giving good efficacy. An alternative fourth active to consider is flufenacet which, at 120gai/ha, has been proven to give valuable additional efficacy.