Should you use pre-emergence residual herbicides to control black-grass?

Black grass in field large image

Pre-emergence residual herbicides are the most valuable component of a chemical black-grass control strategy, providing moderate to high levels of efficacy, even against target site (ALS & ACCase TSR) resistant black-grass.

Resistance to residual herbicides tends to develop slowly relative to that for post-emergence herbicides, so even when some enhanced metabolism resistance (EMR) is present, they can still provide useful control.

Below are five tips on how to get the most out of residual herbicides.

  1.     Active ingredient selection & efficacy
  2.     Product selection
  3.     Optimising performance
  4.     Tank mixes
  5.     When to stack

1. Which active ingredients offer pre-emergence black-grass control?

With its efficacy being little affected by resistance, flufenacet remains the most effective active ingredient for pre-emergence black-grass control, typically giving up to 80% control.

Other residual active ingredients such as diflufenican, flurtamone, pendimethalin, prosulfocarb, flupyrsulfuron and tri-allate, provide lower (5% to around 40% control) and more variable control than flufenacet when used alone.

But they can give a useful 5-10% lift when applied in tank-mix or pre-emergence sequence with flufenacet-based products.

They also offer alternative modes of action which is useful for resistance management and can improve the spectrum of weed control.

2. How should growers decide which pre-emergence residual herbicides to use, at what rate and when?

For black-grass, a flufenacet-based pre-emergence should form the backbone of the herbicide programme. It is the active substance least affected by resistance.

There are a number of products available. Liberator (flufenacet + diflufenican) is widely acknowledged to offer excellent control, up to 80%, while being gentle on crops. Using Liberator allows growers to use pendimethalin elsewhere in the programme.

Growers can follow-up the first application of Liberator at pre-emergence with a second application post-emergence up until 31 March, provided the crop has not reached GS23. This flexibility is particularly useful in a dry season, for example, and / or if black-grass germination has been protracted. This allows growers to extend the residual activity and control germinating black-grass for longer.

The second application must be at half rate (0.3 litres/ha) at least six weeks after the first which must have been applied before growth stage 13 of the crop. Typically this application is in a tank-mix with Atlantis WG.

Where there is a significant weed burden (e.g. due to poor stale seed-bed control), growers can include glyphosate in a tank-mix with Liberator in pre-emergence applications. This is potentially very risky and can only be applied within hours of drilling the crop, but can be a useful tool, if timed correctly.

3. How valuable is herbicide ‘stacking’ and at what point might it become uneconomic?

Pre-emergence is the keystone to herbicide programmes and optimising control at this point is very important for long-term black-grass control.

Adding herbicides, particularly those with different modes of action, to the ‘stack’ or tank-mix adds a few more percentage points of control, however, the law of diminishing returns applies.

With each herbicide that is added the extra control gained is reduced. A second product might add 10% more control, for example, but the third may only add 5% and a fourth just 1 or 2%.

Growers also need to be aware of the potential for antagonism. Some active substances inhibit the performance of others.

Crop safety is another potential issue. Generally, the more active substances in the mix the hotter the combination will be.

4. How should growers decide whether to ‘stack’ or tank-mix other residual herbicides for pre-emergence black-grass control?

Stacking pre-emergence herbicides may be appropriate where:

  • black-grass populations are very high despite the use of cultural controls
  • a population is known to be resistant to a wide range of post-emergence chemistry
  • subsequent application of post-emergent herbicides is going to be difficult because of the soil type and weather

Click here for more advice on selecting the appropriate tank mix partner for Liberator.

5. Under what conditions can the best results be expected and when might it be better to delay pre-emergence treatments? What are the risks involved?

For optimum efficacy Liberator should be applied:

  • onto a fine consolidated seedbed, free from trash
  • when moisture is present
  • at true pre-emergence (within 48hours of drilling*)
  • achieve even soil coverage

*seed should be buried to a depth of 32mm

Trash can absorb some of the product and clods create spray shadows in which black-grass can germinate and grow unharmed.

Like other residuals, Liberator uses the moisture to move through the soil to create a continuous layer, normally 10-20mm deep, through which the roots and shoots of germinating weeds grow and take-up the active substances.

It is rarely worth delaying pre-em applications. Even in a dry year, waiting for rain risks missing the true pre-em timing, after which the roots and shoots of growing weeds are often beyond the layer of the pre-em and are less likely to take up the active substances.

Instead if the weather is dry and predicted to remain dry, then consider delaying drilling your crop rather than your pre-emergence spray after drilling.

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