Getting the best out of post-emergence herbicides for black-grass control

Controlling black-grass, once it has emerged in the crop should be considered the last resort. Before this point, every opportunity should have been taken to minimise the amount that will emerge in the crop using cultural control techniques, such as cultivations, delayed drilling, rotation and improved drainage.

Pre-emergence herbicides should also have been used before contemplating a post-emergence herbicide – they are less affected by, and at less risk of developing black-grass resistance – and will help to reduce further the population the post-emergence herbicide has to control.

Once the decision has been made to use a post-emergence herbicide to control the remaining black-grass the following four sections will help get the best from them:

Product Choice: Which post-emergence black-grass herbicide should be used?

Where black-grass is present, its control is the number one priority. Black-grass populations can quickly get out of hand due to the plant’s prolific nature, so a zero tolerance approach is key, even where populations are small.

The history of the field should be considered.  Ask yourself questions such as: what active substances have previously been used? What was their mode of action? Can you use an alternative? What other weeds are present? Which crops will follow?

Likewise where a population is known to have resistance in a significant proportion of the field population, either avoid those active substances and modes of action or ensure they are used in conjunction with other effective herbicides with alternative modes of action. 

Atlantis WG (mesosulfuron + iodosulfuron) remains the most effective black-grass herbicide at post-emergence. Bayer trials have shown on average 11% greater control than pyroxsulam-based products.

Where bromes are also an issue, Pacifica (mesosulfuron + iodosulfuron) is an alternative option for use in the spring.

Never use less than full rates of the product selected for black-grass control – the full rate of Atlantis WG is 0.4kg/ha.

Always use the adjuvant specified by the manufacturer.  Most will have spent many years developing formulations that complement and optimise the efficacy of the active substances. With Atlantis WG, many other adjuvants have been tested over the past 10 years, but none has performed with the consistency of biopower. It means while you may want to choose another adjuvant, it is likely to be less effective and a false economy – not least because every percent of control of black-grass is usually critical.

Application: How should post-em treatments be applied to get the best black-grass control?

Post-emergence treatments are contact herbicides which are taken up through the leaves of actively growing plants. It is important that weeds are growing as this will facilitate movement of the active substances through the leaves towards the target sites. 

For Atlantis WG the best conditions for application are:

  • in autumn when black-grass is actively growing and at no more than 1 - 3 leaves;
  • the leaves are dry;
  • and the weather will stay fine for at least 2 hours.

Using the recommended adjuvant biopower will optimise uptake of Atlantis WG by the leaves of growing black-grass plants.

Aim for fine to medium-sized droplets. This will increase the likelihood that the spray will hit the leaves without too much momentum, allowing them to be retained.  In trials flat-fan nozzles have proved reliable delivering a consistently good spray quality even when the target is of varying sizes.  Keeping forward speeds to 12km/hr or less will also help.

Low water volumes (down to 100 L/ha) are effective for use with Atlantis WG provided that the sprayer set up is optimised. If not, use of higher spray volumes may prove more reliable.

When tank-mixing Liberator with Atlantis WG focus on the contact element and apply it as you would apply Atlantis WG alone.

Timing: When is the best time to apply Atlantis WG for black-grass control?

Over 10 years of trials have proved that the most consistent black-grass control from Atlantis comes when applying it in the autumn, when the black-grass has 1-3 true leaves (GS11-13).

Only when the black-grass is not actively growing or the crop is stressed is it best to delay an application of Atlantis WG. 

Otherwise delaying applications provides black-grass with more time to steal potential yield and, where Enhanced Metabolism Resistance (EMR) exists, become bigger and more able to break down the active substances before they’ve taken effect.

Research shows that by waiting until the New Year, growers are likely to lose up to a tonne of yield/ha and for every month applications are delayed after that, another tonne of potential yield can be lost.

But Atlantis WG, like all other contact herbicides, is most effective when there is active growth, so if the target weeds aren’t growing, in very cold weather for example, there is little point in making an application. 

However, experience has shown that Atlantis WG works better going into a cold spell rather than coming out of one.  Consequently, if applications are made after an extended cold period it’s best to await active growth before applying Atlantis WG rather than spraying on the first warm day.

Tank-mixing: What should I mix with Atlantis WG for black-grass control other than biopower?

If the Atlantis WG is being applied in the autumn, and/or at the target growth stage of one to three true leaves, then it can make sense to apply it with another residual herbicide.

While tank-mixing several actives at the pre-emergence timing is likely to be more effective, the use of a residual herbicide at post-emergence can be useful in providing additional residual control, and to hit black-grass with multiple active substances and different modes of action.

However, there are a number of risks to tank-mixing herbicides, especially post-emergence.

One of the risks is that herbicides are carefully formulated, and by adding different chemistry to the mix, you can reduce the efficacy of the others. Mixtures can also get “too hot” for crops. And the degree of additional control gained with each herbicide added to the tank-mix gets less and less and so there comes a point when it is not economically viable.

The point at which tank-mixes become uneconomical is very situation dependent, but combinations of four of five are unlikely to be worthwhile as antagonism and the law of diminishing returns is likely to counteract the benefits.

That point is entirely dependent upon the situation - how much black-grass is present, its resistance status, its potential impact on yield and the crop’s value.  It is also important to consider the impact of black-grass on the rotation - if it is not controlled in this crop, how easy will it be to control in following crops and what impact is it going to have on their yield?

The safest of all mixtures are those that are simple (containing two products, for example) and supported by the manufacturer.  At post-emergence, a half rate of Liberator (0.3 L/ha) can be tank-mixed with Atlantis WG. This tank-mix is particularly useful where conditions are dry at pre-emergence and where black-grass germination is protracted.

Ideally any post-emergence black-grass spray should be applied without tank-mixing in any other type of product, such as fungicide, insecticide or growth regulator.

Bayer trials have shown that the efficacy of the black-grass treatment can be diminished when adding additional components to the mix. There is a risk that additives, such as wetters, adjuvants and stickers, in other products can interact negatively with Atlantis WG, preventing it from sticking to the leaves and entering the black-grass plant quickly.

This means, for best practice, Bayer advises not to tank mix any other type of product in with the post-emergence herbicide treatment, if possible.

If a tank-mix partner is required then make sure that you check the latest tank-mix guidance sheet, products which have known antagonism to Atlantis WG and will therefore reduce efficacy will not appear on this list, leaving you with the safest mixtures.    

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