Herbicides play a vital role in integrated weed management programs. Knowledge of the mechanisms and activity of herbicides will improve the impact and sustainability of herbicides as a weed management tactic.

Types of herbicides

  • Translocated herbicides move to the site of action via the transport mechanisms within the plant; the xylem and phloem. The xylem transports water and nutrients from the soil to growth sites and the phloem transports products of photosynthesis (for instance, sugars) to growth and storage sites. It may take up to two weeks for symptoms to develop on the target weeds depending on herbicide rate, conditions and species.
  • Contact herbicides have limited movement within the plant, so complete coverage of the target is critical. Compared to translocated herbicides (for example, glyphosate), contact herbicides (for example, paraquat, oxyfluorfen, diquat and bromoxynil) tend to show symptoms rapidly, usually within 24 hours.
  • Selective herbicides will kill target weeds and not desired plants (the crop or pasture) when applied at a specified application rate.
  • Non-selective herbicides (also called knockdown herbicides) such as glyphosate or paraquat will damage most plants.
  • Residual herbicides remain active in the soil for an extended period of time (months) and can act on successive weed germinations.
  • Non-residual herbicides, such as the non-selective paraquat and glyphosate, have little or no soil activity and are quickly deactivated in the soil. They are either broken down or bound to soil particles, becoming less available to growing plants. They also may have little or no ability to be absorbed by roots.
  • Post-emergent and pre-emergent are terms that refer to the target and timing of herbicide application. Post-emergent refers to foliar application of the herbicide after the target weeds have emerged from the soil, while pre-emergent refers to application of the herbicide to the soil before the weeds have emerged.
  • Herbicide mixtures and sequential applications involve the application of more than one herbicide, usually to increase the spectrum of weed species controlled but also for resistance management. A mixture involves the application of multiple products in a single application. Where herbicides are antagonistic and cannot be mixed together in a single tank, they are applied sequentially.

Knockdown herbicides for fallow and pre-sowing control

Knockdown herbicides (or non-selective) kill all plants when used in sufficient quantities, under suitable environmental conditions.


  • Knockdown herbicides effectively kill weeds and are cost-effective.
  • Use of knockdown herbicides can improve the timeliness of sowing.
  • Use of knockdown herbicides rather than cultivation will reduce the risk of erosion, improve soil structure and improve plant available soil water content.


  • Consider the suitability of herbicide use for fallow or pre-sowing weed control by assessing environmental conditions.
  • Stressed weeds will not be adequately controlled by knockdown herbicides.
  • Overuse of knockdown herbicides will select for resistance.
  • Suitable meteorlogical conditions for spraying can be limited, especially for weed control over the summer fallow.

Pre-emergent herbicides

These herbicides control weeds between radicle (root shoot) emergence from the seed and seedling emergence through the soil. Some pre-emergent herbicides may also provide post-emergent control.

Benefits and issues

  • The residual activity of pre-emergent herbicides controls the first few flushes of germinating weeds while the crop or pasture is too small to compete.
  • Good planning is needed to use pre-emergent herbicides as an effective tactic. It is necessary to consider weed species and density, crop or pasture type, soil condition and rotation of crop or pasture species.
  • Soil activity and environmental conditions at the time of application play an important role in the availability, activity and persistence of pre-emergent herbicides.
  • Both the positive and negative aspects of using pre-emergent herbicides should be considered in the planning phase.

Selective post-emergent herbicides

These products control weeds that have emerged since crop or pasture establishment and can be applied with little damage to the crop or pasture plants.


  • Post-emergent herbicides give high levels of target weed control with the additional benefit of improved crop or pasture yield.
  • Observations made just prior to application allow fine-tuning of herbicide selection to match weeds present in the paddock.
  • Timing of application can be flexible to suit weed size, crop growth stage and environmental conditions.
  • Some post-emergent herbicides have pre-emergent activity on subsequent weed germinations.


  • Use careful consideration when selecting the best post-emergent herbicide to use in any one situation.
  • Application of post-emergent herbicides to stressed crops and weeds can result in reduced levels of weed control and increased crop damage.
  • Crop competition is important for effective weed control using selective post-emergent herbicides.
  • The technique used for application must be suited for the situation in order to optimise control.
  • Always use the correct adjuvant to ensure effective weed control.
  • Selective post-emergent herbicides applied early and used as a stand-alone tactic have little impact on the weed seedbank.
  • Choose the most suitable formulation of herbicide for each situation.
  • The effectiveness of post-emer

The most popular herbicides in the UK are:

Diflufenican and Flufenacet Mixture
Iodosulfuron-methyl-sodium and Mesosulfuron-methyl



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