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Weed Management

Bayer Crop Science

Measuring the benefit of a post-em application – 2023 trial results

Article overview

Controlling weeds in spring has a two-fold benefit. Firstly, it stops weeds taking light, water, space and nutrients from the crop, helping maintain overall yield. This is known as the direct benefit of weed control. Then there are the indirect benefits, most importantly reducing weed seed return, so there are fewer potential troublemakers next season.

Weighed against these benefits is the cost of any herbicide application and the fear it will not perform as expected due to resistance or poor application conditions. There are other control options such as hand-rogueing in late spring or patch spraying. However, the cost and availability of labour means that rogueing isn’t as attractive as 5–10 years ago and of course patch spraying has a big impact on yield – it kills the crop.

Unless the problem is so mild that rogueing is manageable or, conversely, if the weeds are so dense that only spraying it off will work, then a spring post-em is worth considering.

The question of resistance

Resistance is the number one reason for reluctance to use a mesosulfuron-based post-em like Atlantis Star or Pacifica Plus. In the worst cases, it can mean the herbicide is ineffective against black-grass or ryegrass; however, many weeds retain a degree of sensitivity. This is because the most common type of resistance is enhanced metabolism which affects plants to varying extents. It is not an ‘on-off’ switch like target-site resistance.

In practice, this means the first step is to check the kind and degree of resistance you have in your weed population. Resistance status can vary field-by-field and by weed, meaning certain actives retain efficacy on part of the weed spectrum.

The Bayer/NIAB 2021 ryegrass survey showed how resistance patterns in ryegrass are not easily predictable and that every field needs to be judged on its own merits. A key point is that previous poor control may not be due to resistance as application and timing problems may be the cause. Bayer’s testing regime in 2021 and 2022 found that 10% of black-grass samples and 58% of ryegrass samples suspected of resistance due to poor control were in fact susceptible.

In the worst cases with target-site or very strong metabolic resistance it is best to look at different options such as more chemistry at the pre-em timing, and additional cultural techniques to control weeds. Where weeds are susceptible or moderately resistant (RR rating), focus on application technique and timing to achieve the best result. These have a massive impact on the overall result and determine if the post-em is money well spent.

Find out more about optimum application technique here.

And to learn more about the best conditions for applying mesosulfuron-based products here.

We highly recommend:

  • Herbicides

    Atlantis Star

    A highly-effective herbicide for control of grass-weeds and broad-leaf weeds in winter wheat. Atlantis Star is a coformulation of three ALS-Inhibitors (HRAC Group 2) with foliar and some root activity

  • Herbicides

    Pacifica Plus

    A highly active herbicide (a combination of three sulfonylurea herbicides) with foliar and some root activity.

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