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Grassweeds 4: Active ingredients

Course: Grassweed management in cereals | Last Updates: 7th October 2015

 
James Clarke
Science and Business Development Manager
ADAS
Biography >>
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It's not what you use, but when you use it that counts. Herbicides are effective tools and there is much debate over what to use and how to fine-tune applications to get best control.

But correct timing is the most important consideration, and this is critical to effective grassweed control. This requires a knowledge of the way the herbicide works and how it is affected by plant growth and soil conditions.

What are the principles of autumn herbicide applications?

Overall smaller weeds are easier to control. They are also less likely to be affected by enhanced metabolism resistance (EMR).

Late-emerging weeds are much less competitive and produce much less seed as a result. Delaying herbicide timing too long to wait for every weed to emerge is therefore often unwise.

Some herbicides only work pre-emergence (soil applied). Others need leaf area to take them up. Foliar-applied herbicides are most effective when applied to leaves when the plant is growing.

How do herbicides work?

Different active ingredients have different ways in which they enter a target plant and kill it, and this can be quite a complex process. Herbicides can work in a range of ways but there are three ways to categorise their properties:

  • Selectivity

This is the degree to which a herbicide selects a target plant for control. So glyphosate, for example, is non-selective and acts on any green material, while some 'fops' and 'dims' are very selective even within grasses. Selectivity can also be achieved where herbicide is applied to the soil but the crop seed is protected by a depth of soil.

  • Soil or foliage-applied

This describes the target of herbicide application and hence how it enters the plant. Soil-applied (residual) herbicides are taken into the plant either as weeds emerge or via roots. These herbicides persist in the soil for a period of time, depending on moisture and temperature, and provide a level of control after application. Good examples of soil-applied, residual herbicides are propyzamide (root uptake) and pendimethalin (shoot uptake). Foliage-applied herbicides, such as "fops", "dims" or "dens", are taken into the plant through the leaf and stem.

  • Translocated or contact

This is the degree to which a herbicide is moved within the plant's vascular system to a site of activity. Glyphosate is a classic example of an herbicide which is translocated – for example from a leaf to which it is applied to the actively growing rhizome. By comparison paraquat is a typical contact herbicide that destroys just the green material it comes into contact with. Good even coverage of spray is very important for contact herbicides and they often work more quickly.

Some herbicides can fit into several categories, but the degree of importance and reliability of each varies.

What are the physical barriers to good herbicide activity?

The challenge in the autumn is that the target is often small or hasn't even emerged. Residual herbicides work best if applied to fine moist soils. If there are large clods this will create a shadow effect – the soil surface directly beneath the clod will remain free of pesticide and any weed seeds will germinate unaffected. Also, when the clod weathers down, there may be seeds within it that will escape treatment and germinate once exposed.

Moisture is another limiting factor. Some residual herbicides rely on soil moisture to carry the active ingredient to the root or emerging stem of the target weed, for example. Too much water is also a problem. Not only can it wash residual herbicides down too deep for activity, they can also end up in water courses (see Water Stewardship academy).

Weeds must be actively growing for best efficacy of foliage-applied herbicides. Their success also depends on the surface area of foliage intercepted. For blackgrass this may be a seedling of only one or two tiny leaves, so nozzle selection and spray quality will be key considerations (see Nozzle and Application academy).

What are the key timings for herbicides?

  • Pre-drilling

The aim is to encourage weed growth in the intercrop window and spray off with a non-selective, foliage-applied herbicide. Success depends on getting a good flush of weeds, which relies on moisture, low dormancy and the time available before drilling.

  • Pre-emergence

This is typically within a week of drilling and after rolling. Many of the grassweed herbicides used pre-emergence are less likely to be affected by resistance. The aim is good coverage of a residual herbicide that will control weeds as they emerge.

A good pre-emergence will take pressure off the post-emergence treatment, but success depends largely on seed-bed quality and soil moisture. You should check the label for specific restrictions, including:

  • Degree of soil cover over seed required
  • Soil type
  • Latest application date
  • Organic matter content of soil
  • Suitability of use where there is surface trash
  • Following crop and cultivation.
  • Peri-emergence

Describes applications around the time of emergence, sometimes referred to as the "green haze" stage. This is usually no substitute for well-timed pre-emergence sprays, but it can bridge the gap to post-emergence. It is too early for true post-emergence sprays and so a residual component is essential.

  • Post-emergence

The crop has fully emerged, as have many of the weeds. Foliage-applied herbicides work well if the weed is small and actively growing. Residuals, especially if not applied earlier, can be mixed in to aid efficacy or provide prolonged activity. Watch for crop and field conditions which may affect efficacy, such as where:

  • Weeds are too big, which encourages resistance and its effects are greater
  • Soils are too wet, leading to water quality and efficacy issues
  • Crops are stressed and liable to scorch
  • Crop canopy is too large and will shadow weeds
  • Temperature is too cold – herbicides work more slowly and level of control can be reduced.

How can mixing active ingredients help?

For high populations of blackgrass a mixture or sequence will be required to get an acceptable level of control in the autumn and to minimise the chances of resistance developing. You should discuss exact tank mixes, sequences and rates with your agronomist and check labels for restrictions. But in general:

  • Consider both mixtures (at same timing) and sequences (at different timings) aiming to optimise activity of each component
  • Aim to include several modes of action
  • A hard and early hit, using pre-emergence sprays, is usually a very good, and often essential start.

Golden rules

Do

  • Think timing – putting products on at the right time is number one priority
  • Maximise soil-applied (residual) herbicide efficacy and crop safety through creating good seed-bed conditions
  • Where populations are high use several herbicides in a mixture or sequence
  • Target weeds when they are small – control is easier and less affected by resistance

Don't

  • Even think herbicides until after you have considered and implemented all the cultural options
  • Wait for the late-emerging weeds – they are less competitive and have a smaller effect on seed return

Spraying

 

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